Engine surge and stall

the engine in my '92 that i just finished putting back in has a bit of an issue. it starts up and idles nice, revs up in park/neutral no problem and sounds good. once its been running for 5min (ish) it develolps a heavy surge, giving it throttle does have some effect but you really need to caress it eventually no amount of throttle play will cure it and it just stalls. wait a couple seconds and she starts right back up and does it all over again. the issue is the same with the engine hot or cold. no check engine light

whats been done in the last 4 months:
engine re and re
lt1 cam swapped in
headers
headgaskets
new valve springs
valves relapped
new ignition module
wires, cap and rotor
new gaskets throughout
a/c delete
AIR delete
No cats, just mufflers.
rebuilt tbi
new 4 wire o2 sensor
cold air intake
fuel pressure bumped up (stretched regulator spring a bit and tang moved up)

at first i thought maybe a fuel pressure problem, now im thinking tps perhaps?

heres a video, it had a real abrupt stop here and it kinda scared the sh*t outta me:

what's the exhaust smell like? rotten eggs = clogged converter = back flow of exhaust = car stall.

No cat. just have headers into mufflers right now

Did you have the chip burned for the cam upgrade? Are you using the original injectors? What is the fuel pressure? You stretched the FP spring...how much, and what do you mean by the tang moved up? A shim "washer" the thickness of a quarter is about all you need to max out the fuel pressure. You should use the injectors from a 91-93 9C1 LO5 with the LT1 cam. They flow more, and do not require a substantial pressure increase, even with the LT1 cam. I am running the 9C1 injectors on mine. If you can get the fuel lines from a 91-92 Fleetwood with an LO3/5, or a 93 FW, they have a Schrader valve port, and you can measure fuel pressure. They bolt up to the B body perfectly. I have a set of each, and would part with a set for $50 plus shipping.

The video sounds like a fuel pump failure, or pump hose failure. It is probably a fuel pressure variation if it does not throw a code. The ramping up an down is what mine did when the pump hose in the tank was cracked. You should consider putting the original size spring back in the fuel pressure regulator, and use 9C1 injectors. I also have a complete injector tower with injectors that I removed from a 9C1 for $75 plus shipping. I also have a 9C1 computer available. I put it in my car, and it ran the engine with similar parts. The only differences I have are a larger 46mm TBI body, and shorty headers instead of full length ones. Mine idles at 550 RPM, and does not have any issues. I have an adjustable fuel pressure regulator, and am running about 13 PSI. I put a pressure gauge adapter between the line and the TBI, but I would recommend the fuel line swap. I had to reshape the pressure line to make it fit (read, royal pain). The FW lines bolt in without modification.

Check the spark plug gaps again.

Stock tune right now. With stock injectors. If you have some 9c1 injectors i might be interested i gotta do some more research though. Unfortuneatly no fleetwoods in any yards around here. My fuel lines at the motor have been replaced anyway with differnt bends and fuel injection hose for the return line. The Sparkplugs i double checked when I put them in. I forgot a seal at the bottom of the regulator assembly. So I re re the assembly and am testing now. I tend to agree with you about the pickup line in the tank as rare as that problem may be my car seems to be acting like yours was. Really don't feel like dropping the tank.... Rrrrrust.

*edit

Seal didn't fix it.

If I hold the pedal at a consistent rpm, say 1500 it will slowly start to lower unless I apply more gas. If I hold the pedal at a consistent rpm and quickly let off it will either stall or come very close to it. Me thinks its deffinately fuel pump/pickup hose problem

I should also mention I'm under a 1/4 tank which is the lowest I've ever let it go.

Sounds like something is having trouble compensating. Like a sticky IAC valve or similar. Do you have a scan tool? Maybe watch the data stream for a similarly rhythmic value.

I did not read all of the replys......so forgive me if this was mentioned already.

Vacuum leak

My Bro in Law had this happen to one of his cars and after spending hundreds of dollars on re-mapping his computer, paying a mechanic to look it over, tearing the engine apart, swearing, wasting a weekend....you get the point. Turned out to be a small crack in a vacuum line. Eventually it would lose pressure and stall out or would die. If we let it sit for 10-15 minutes it would start like nothing ever happened......and then a couple miles down the road it would start all over again.

Check some of your lines, this could be a quick 25 cent fix.

Cross your fingers

buickwagon wrote:

Sounds like something is having trouble compensating. Like a sticky IAC valve or similar. Do you have a scan tool? Maybe watch the data stream for a similarly rhythmic value.

wouldn't i get a check engine light though? i unplugged the tps for a second just to see what would happen. there was a noticeable change in idle quality, i did this before the surging started back up. i'll take a look at the iac tomorrow.

Bobloblaw wrote:

I did not read all of the replys......so forgive me if this was mentioned already.

Vacuum leak

My Bro in Law had this happen to one of his cars and after spending hundreds of dollars on re-mapping his computer, paying a mechanic to look it over, tearing the engine apart, swearing, wasting a weekend....you get the point. Turned out to be a small crack in a vacuum line. Eventually it would lose pressure and stall out or would die. If we let it sit for 10-15 minutes it would start like nothing ever happened......and then a couple miles down the road it would start all over again.

Check some of your lines, this could be a quick 25 cent fix.

Cross your fingers

nope. vacuum lines checked out good.

If the O2 sensor reads lean (possible with the original injectors), it could slowly close the IAC, trying to richen the mixture, and in doing so shut down the engine. The symptom you have in the video does not lend itself to that as an issue. If you have exceptionally high fuel pressure it could compensate for the smaller injectors, although the computer may not be happy.

I have not tried to run mine with a stock wagon computer, so I do not know how that would act with the mods. The only issue you should run into is lack of fuel at the top end. The idle should be stable even with the stock computer.

If you pull the lead off of any sensor, you should get a SEL. A fuel or vacuum issue will not throw a code. You can squirt some water with a couple of drops of dishwashing liquid in it over the engine where there might be a vacuum leak. Along the head to manifold joint, and on the lines.

You can get new tank straps from Amazon and a couple other places. I may be able to get you a set of rear strap bolts.

benn wrote:

wouldn't i get a check engine light though?

Not necessarily. The check engine light only comes on when a device's parameters exceed a certain threshold, and the capabilities of the OBD1 system of 1992 was not particularly sophisticated compared to today's cars. It is possible that the underlying problem is not severe enough to set a code, particularly considering you have made fundamental changes unanticipated by the designers.

For example, you have installed a 4-wire O2 sensor, where I believe the original was a single wire? Perhaps this introduces some sort of ground loop issue that affects the voltage seen by the ECU (I don't know that it does -- just speculating on a possibility here). A ground loop might even affect the voltage seen by other sensors. So long as the O2 sensor makes some sort of response, the system would figure that the voltage is valid and not set a code. Out of curiosity are you dead certain which wire is the O2 sensor ground wire? Any chance you connected the output wire to the sensor ground circuit?

Unplugging sensors might be a good way to isolate the type of problem without a scan tool. Fred may well be correct about the symptoms, and certainly has more experience with this sort of modification than I, but the IAC is used to control the idle speed, not the mixture. Mixture is controlled by adjusting the fuel trim values (which establish the injector pulse width). Given that you say the car starts and idles fine for the first few minutes, I think I would start with the O2 sensor, to prevent the system from entering closed loop mode. I'm not saying that is the problem, but it is a very quick and easy check. If the idle does not stabilize then you have ruled that out as a possibility.

Similarly, unplugging the IAC might give a clue to the mechanism involved. If the idle stabilizes, that suggests the speed variations were caused by the ECU's attempts to control the idle speed, rather than a mixture issue.

Of course, there could be a non-electronic cause too. A fuel pressure gauge and a vacuum gauge might be called for.

The ground for the computer and O2 sensor at the thermostat housing may be faulty. Check that as well.

I have seen the same symptoms on a hot start as well. I have not figured out what causes it, but if the engine is turned off and restarted, it clears. I do not get a CEL when it happens, so it must be a fuel issue of some nature. I almost appears to be a viscous resonant cycle that gets worse with each swing. It only happens when the system is in open loop, within a minute of startup, when hot, after sitting for at least 1/2 hour, and no more than an hour and a half (an example would be going to lunch and acting up when leaving). I replaced the O2 sensor, and it made it better for a while, but the symptom has returned. Both of my cars tend to do it, and that is really weird, because one is a stock setup, and the other is the modded one.

It seems the more I play with the pedal. The more load I put on the engine, the faster the surging will start. I let it just sit there and idle yesterday and it got up to operating temperature and idled there for 10 min or so before the idle speed slowly lowered about 100 rpm to 400ish and began to surge

Vacuum leak...major one? That's what it looks like to me....

Well last winter I had a HVAC issue. Turned out to be the main vacuum feed line for HVAC system replaced that and is still working good to this day. Brake booster line was checked and is good. Air injection was deleted and the vacuum port was plugged off. Vacuum line from fuel canister purge system is intact with no leaks. New pcv and new vac line aswell so that's good. Then there's the vac line at the back of the TB which goes to the maf. I checked it and its also  good. So that accounts for all the ports on the throttle body. I did delete my egr and got a block off plate. I can't hear any vacuum leaks from it but I'll double check

Would it be helpful to get a video of the spray pattern?

benn wrote:

It seems the more I play with the pedal. The more load I put on the engine, the faster the surging will start. I let it just sit there and idle yesterday and it got up to operating temperature and idled there for 10 min or so before the idle speed slowly lowered about 100 rpm to 400ish and began to surge

More load = more exhaust heat = faster warm up of the O2 sensor = enters closed loop mode sooner. I'd really be curious if there is a relationship between entering closed loop and the onset of the idle surge. Do you have a scan tool?

No I don't. I was gonna see if one the performance shops has an aldl cable I can data log with.

4 wire o2 is a heated o2. 2 white wires are +ve and -ve for the heating element, black is the signal wire, and grey is the signal ground.

i made my own harness, heater ground and signal ground are soldered together and use the same ground, the +ve side of the heater wire is spliced into a switched 12v source. and the black is plugged into my original o2 signal wire.

I'll bring a scan tool with me Saturday.

Sounds like a Bosch O2 sensor (grey, black and 2 white wires) in which case you have them at the right polarity. However, I wonder if maybe you should separate the grounds. I wonder if there could be some small voltage difference between the ECU reference ground and the O2 signal ground due to the heater current. The O2 sensor normally fluctuates between 0.2 volts and 0.8 volts, so it wouldn't take much to skew the voltage seen by the ECU.

buickwagon wrote:

I'll bring a scan tool with me Saturday.

Sounds like a Bosch O2 sensor (grey, black and 2 white wires) in which case you have them at the right polarity. However, I wonder if maybe you should separate the grounds. I wonder if there could be some small voltage difference between the ECU reference ground and the O2 signal ground due to the heater current. The O2 sensor normally fluctuates between 0.2 volts and 0.8 volts, so it wouldn't take much to skew the voltage seen by the ECU.

that would be awesome.

i'll seperate the grounds, that kinda makes sense, still with a surge as extreme as mine youd think the threshold would be surpassed, especially considering something as finicky as an 02. for what its worth its an ntk 21054 sensor

Before separating the grounds, try unplugging the O2 sensor. If it continues to surge, then the O2 sensor was not the cause.

Okay. I split up the grounds. Took the signal ground to the the thermostat ground and the heater circuit to another ground point. Started up car. Got a check engine light for the first time after putting the motor back in but, No more surge. I was able to drive it around the block. Still seems to have a bit of a bog. Didn't really get on it though. Then my abs light came on??? Pull into my garage pull the codes with a paper clip I got:

22 tps sensor signal low
23 intake air temp sensor error (low temperature indicated)
24 vehicle speed sensor error (open circuit or no activity) my speedo hasn't worked for a while

42 ignition bypass circuit error (after market column and lock cylinder)

Codes 24 and 42 seem odd to me. The car hasnt had a working speedo for a long time. And the column and lock cylinder were replaced 10 years ago after the car was stolen. Yet I've never gotten these codes before. Weird. Could the abs light have something to do with code 24? Vss

The bog may be inadequate fuel from the injectors.

EST circuit can cause code 42.

Code 24 could be your VSS buffer, or a misadjusted park/neutral switch.

Find a separate ground for the O2 sensor that does not interact with the sensor system. The heater may find a ground through other systems if the ground is faulty, and cause all kinds of mayhem.

so move the sensor signal ground away from the thermostat ground?

i moved the heater ground to the firewall

Code 42 is set if the EST bypass is either open or grounded. A code 42 will prevent the system from entering EST mode -- the timing will be fixed at the base timing of the distributor. Is there any chance you have left the bypass connector disconnected after setting the base timing? Or neglected to clear the codes after having reconnected the bypass?

Codes 22 and 23 both rely on a good sensor ground. 22 will set if the TPS signal voltage is less than 0.2 volts above ground. Conversely, 23 will set if the IAT sensor voltage is high (as in a poor ground). This seems mutually exclusive, except that the grounds of the IAT and the TPS are tied together and both are supplied with a 5v reference signal. If the signal ground was interrupted, then I think the TPS signal would see the difference between the 5v IAT reference and the 5v TPS reference instead of the difference between the TPS reference and the ECM ground. I would have a very close look at the sensor and ECM grounds.

The VSS circuit does not seem to be connected to the same sensor ground, it is tied to the engine ground. This may be coincidental to the other codes, as the speedometer has not worked in some time, but as long as you are checking grounds you may as well check the body to engine ground and also fuse #13 (which powers the VSS buffer).

The ground strap from the back of the engine to the firewall was loose. Maybe that's my issue?

Quite possibly. Clear the codes and see if they come back.

BTW: not sure if you have an FSM. I'll bring one of those too. Got a photocopier?

Yep I got a FSM! Thanks though.

So here's an update. I rewired my o2 sensor grounds and the surging and stalling has stopped! Thank god. But theres still some bugs to be worked out...

Buickwagon came by my house Saturday to pick up some spare interior parts and stuff for his wagon and did a little data logging with mine. The bug I've yet to work out is a bog between 1500 rpm and 2500 ish. This problem was occurring before I did the cam swap and engine re and re. It was wrongly diagnosed as a stretched timing chain. My chain was pretty stretched but unfortunately not what was causing this bog.

Some culprits me a buickwagon narrowed it down too
-IAC valve
-TV cable out of adjustment
-fuel pressure issue/regulator
-injector pulse
-vehicle speed sensor??

Well since Saturday I checked the TV cable and it seemed to be adjusted right but I followed the FSM adjustment procedures anyway and readjusted. Didn't fix the problem.

Haven't gotten a chance to to check out any other culprits but I did get my hands on a fuel pressure tester. Luckily the fuel lines at the engine have been partially replaced with fuel injection hose so I can just splice in a "t" and check the FP.

I put ??? Around the vehicle speed sensor because I'm not sure what possible problems could occur with an in operational speed sensor. We know the computer isn't getting a speed signal because the scanner didnt get a reading.

Another interesting little tidbit. My engine light came on after about a 30 min drive around town. Came home to pull the code. Got 43 EST code. This is after I reset the codes from the last time. There was a noticible power loss when the light popped on.

Also idle is very low in city driving. Stopping at red lights it seems to idle around 400ish rpm.

The dark blue wire from the ESC module goes to the knock sensor, the brown wire toes to an engine ground, black goes to the ECM for spark retard signal, the pink and black wire goes to the ECM ignition input, and to the ignition through fuse 13.

With the above, I would check the knock sensor, and the engine ground.

According to the manual, "Code 43 means that ECM has detected a low voltage on CKT 485 (spark retard signal) for longer than 4 seconds with the engine running or the system has failed the functional check.

Engine ground is located behind the ac compressor/belt tensioner/air injection bracket correct?

i have about 11 psi of fp. still bogs at around 1500 rpm. under heavy load the CEL will come on at the same time with code 43. gonna check my esc wiring out...

Code 43? Well that's a new one for you. At least the fuel pressure is OK -- it maintains that 11psi under load?

43 means the system failed to detect a knock signal under heavy load. The traditional test of the system is to tap on the passenger side manifold or block in the area of the sensor and watch the scan tool for a "knock" indication. You don't have a scan tool, so that test will be difficult! And your exhaust is so loud I don't think there's a snowball's chance in h-e-double-hockey-sticks that you could detect a knock under load unless the piston came out and tapped you on the shoulder.

So, it could be an issue with the base timing -- ie: no knock occurred because the base timing is far too retarded. That might also explain the hard start, and is worth checking if you haven't already. The base timing must be set with the EST by-pass wire disconnected, to prevent the system from automatically advancing the timing and throwing off the adjustment. If it was set with the EST by-pass connected, the timing light would reflect the base timing + programmed advance instead of just the base timing alone, therefore the base timing would be significantly retarded.

Assuming the base timing is OK, then the knock sensor could be faulty or disconnected or the ESC module could be faulty. Or it could be a wiring issue; both the knock sensor and the ESC module rely on a good ground connection.

Do you have a chip in the computer for the mods you made to the engine? The fuel map may be incorrect for the mods.

dont have a chip yet fred. I wanna get this issue solved first. my cars been doing this since before i did all this engine work, i never used to get a CEL though. When it happened it would just bog without any indication of what was causing it, i always thought it was a fp problem

when it bogs hard under load (when it down shifts) it sounds as though it changes timing, exhaust note sounds really odd and there is just no power to be had at all. if i let off the gas and continue to cruise i can manage okay, but there is still a big lack of power.shut the car down and start it back up, cel is out and i have good power again. im starting to lose my patience with this thing ive been chasing this problem for over a year. at least now im getting a cel. hopefully thatll shine a light on the situation.

HOLY NUTSACK!

I think I figured it out!! I was checking out the wiring coming from the ESC. Tested CKT 486 which is the ESC ground and got notta. I hooked up a jumper leed to it and grounded it and took it for a long test drive with heavy loads (uphills, heavy acceleration) and didn't get any bogging or cel.

I'm happy. Now to get a proper ground for it.


Oh and fix code 42 for the est lol It seems to be intermittent. I'll get a cel light for it once in a while. Sometimes it'll turn on and off during the drive. Bypass connector is connected.

Yayyy!!! Congratulations.

benn wrote:

im starting to lose my patience with this thing ive been chasing this problem for over a year. at least now im getting a cel.

Don't get discouraged: just think of all the edumacation you've received for your trouble. If you went to college to learn this it would have taken twice as long and cost a fortune!

Took you guys long enough,I KNEW that was it all along! :lol:Congrats guys!

...so new thermostat, new water pump (In the past year) my car isn't exactly overheating but it gets VERY hot. No boiling over or anything though. Its almost like the thermostat isn't opening. If I I'm doing slowish cruising in town. Light traffic a few red lights my car will climb up past normal operating temp slowly. If I giver sh*t and mat it for a second the temps will sometimes drop back down as if the thermo stat decided to open up.

Defective thermostat? Oh and my upper rad hose is cold

Cold upper hose suggests no flow, usually a thermostat issue. Drop the t-stat into a pot of hot water. Continue to heat while monitoring the actual temperature until the t-stat opens. Make sure it's not installed upside down.

In extreme cases, a plugged radiator core won't flow either. Then you are relying exclusively on the heater core to cool the engine.

Waterpumps rarely fail to move water. When they fail it's usually the seal then the bearing.

Before you rely on a factory gauge, thats maybe giving u kerniptions, see if you can borrow, beg, buy, a hand held pyrometer,.

The gauge in this orange car runs at the lower end of the scale which is between 185-190ish measured at the t stat with hand held, by the time the water gets to 205 the gauge is over 1/2 way
its almost up to the red by the time both fans kick in,.at 215 or so,..
the gauge was giving me heart trouble when i first got the car,.
it sat and idled in neutral at the border crossing for over an hour with the gauge almost at the red zone, but didn't boil over,.
So IMHO don't let the factory gauge give you stress till you know how its calibrated,.
YRMV
NWEOI
PDCC

blahhh the ongoing sega!!!

POS chrome timing chain cover is puking oil like crazy at highway speeds. good news is the underside of my car is rustproofed. bad news is i ran it Dangerously low on oil!! no idiot lights and still had pressure according to the gauge. luckily i pulled into my driveway in time. went to move it into the garage and it cranked slower then normal. added a bit of oil and all seems to be back to normal. really hoping i didnt F*** anyhting up to bad. i know a sbc can take alot of abuse but pulling the dipstick and seeing almost nothing after driving your car is not a good feeling.


when does it get easier?

benn wrote:

blahhh the ongoing sega!!!

POS chrome timing chain cover is puking oil like crazy at highway speeds. good news is the underside of my car is rustproofed. bad news is i ran it Dangerously low on oil!! no idiot lights and still had pressure according to the gauge. luckily i pulled into my driveway in time. went to move it into the garage and it cranked slower then normal. added a bit of oil and all seems to be back to normal. really hoping i didnt F*** anyhting up to bad. i know a sbc can take alot of abuse but pulling the dipstick and seeing almost nothing after driving your car is not a good feeling.


when does it get easier?

I am told when you get filth rich and pay someone else work on your cars.

So since I fixed the rest of the codes the following was bound to happen...

 verse 13-6 in the "holy book of cars will be bi*ch" says another code will replace them.

And it is, code 44 lean condition. I have 11 psi idling, I can't check the pressure while driving. New o2 as you guys know and new fuel filter. I'm hoping this has something to do with me not having it tuned yet. I contacted TBI chips a while back to no avail. Again, really hoping its not the FP. Could my mild cam (stock lt1 cam) cause this? Also rebuilt TBI and new fuel screens on injectors.

Verse 13-7 says TBI sucks

TBI fuel pressure should be between 9 and 13, so you are OK there, at least at idle. Pinch off the return line (IIRC your lines were replaced with rubber lines? Don't pinch off the stock hard plastic line!) If the pressure climbs, you probably have enough capacity at the pump, although there is a chance that the volume through the filter is restricted when calling for more power.

Code 44 indicates that the long term fuel trim has exceeded 150 -- that is to say, the system is compensating for a lean condition by adding more fuel than it thinks it should have to for a given condition. The upper limit is 256, so there's still some room left for additional compensation after the code is set.

Given the modifications to your cams and exhaust system and given that you are still running the stock injectors and fuel map, I think it is entirely possible that the code is a red herring. If the engine is breathing better, more fuel is required to keep the F/A ratio at 14:1. Since the injectors are stock, the only way to get more fuel into the stream is to increase the injector pulse width. As the ECU can only recognize the increased pulse and knows nothing about the increased air flow (not having a MAF sensor on the TBI) it assumes something is wrong and throws a code.

Another possibility is an exhaust leak in just the wrong place. O2 sensors can be funny about exhaust leaks. If outside air is drawn into the stream before the sensor, the O2 sensor reading is skewed, the ECU thinks the engine is running lean and starts compensating to get the O2 sensor voltage back to it's happy place.

This sounds like the '82 vette sitting in my garage with 22K on the odometer original, with a new water pump, ignition system (plugs, wires, coil, cap. rotor) and new injectors and a TPI fuel pump with rebuilt throttle bodies, the only thing I can do is check for vacuum leaks between the upper and lower parts of the intake manifold (very likely with the Ceasefire, I mean Crossfire Injection) and reset the fuel pressure to compensate for the stronger TPI pump. (PITA, no way to check fuel pressure, plus special line needed to read pressure BETWEEN the two throttle bodies, no valve and special tool needed that no one makes anymore.)

The other thing I got for OBD-I applications is a ALDL-USB plug from Red Devil River on EBay (they do Fiero stuff and GM OBDI) It was about $60 shipped to my door to really read what's going on in that OBD-I computer

buickwagon wrote:

T
If outside air is drawn into the stream before the sensor, the O2 sensor reading is skewed, the ECU thinks the engine is running lean and starts compensating to get the O2 sensor voltage back to it's happy place.

I find it interesting and perplexing when folks add this bit to a diagnosis,..

I was under the impression that exhaust is pressure,. therby leaving little black marks everywhere when there's a leak,.

I,m not sure really how its supposed to draw air in,...

If the O2 is adding fuel because it thinks its a lean condition,. maybe it is a lean condition,..

perhaps it has a vacuum leak?

Yup, vacuum leak is another possibility. I didn't delve into that since Ben has already gone over the engine looking for leaks pretty thoroughly. However, having ridden in his car, I know an exhaust leak would be difficult to identify by sound alone.

And yes, the exhaust is under pressure. Mostly positive pressure, especially when coming from a big V8 4 stroke. But under certain circumstances that depend on variables like rpm and the specifics of the exhaust system, sometimes there can be negative pressures too. If the exhaust leak happens to be in the wrong place at the wrong time, brief puffs of air can be drawn in to the exhaust stream.

A well designed exhaust system can actually be tuned to time negative pressure waves to occur at the manifold at a desired rpm to help air flow. This is most noticeable with a 2-stroke engine, where the power band tends to be fairly narrow anyway. A momentary negative pressure is timed to scavenge the exhaust out of the cylinder initially, followed by a positive pressure pulse to retain the incoming fresh charge as the piston finishes closing the port. However, even 4-strokes can benefit from the scavenging action of a properly tuned exhaust, especially in applications were the engine is normally run at a fairly constant rpm, such as aircraft engines.

Here is a nice, easy to understand, explanation by a motorcycle exhaust manufacturer:
[You must be registered and logged in to see this link.]