Spark retard and poor fuel mileage when towing

I bought a camper last spring and have been towing it many times this summer with my '95 Caprice stw.The camper weighs about 2900 punds loaded and is about 90" wide. I have a 2.93 diff, and tow with drive, not overdrive. Speed is usually 50-60 mph.

The car is otherwise stock, but has 2,5" catbacks. The cats and o2-sensors are new. At first trip the fuel mileage was pretty bad at 11,8 mpg. I changed the spark plugs that had about 25 km on them, as well as the fuel pressure regulator. After that, I've got about 13,8 mpg or worse during the summer.

Engine pulls otherwise well, but when I start to give it throttle when climbing a steep hill, or when leaving an intersection, I'm getting knock retard. I also sometimes hear an occasional ping when starting to give it throttle at low rpm.

I haven't seen any overheating. Thermostat is a 180 deg gates, the plugs are AC delco #14 rapidfire, which should have heat range of 3 (the hottest delco being 5). I tried to find intake or exhaust leak, but didn't find any. I've been datalogging with datamaster, and can't find anything being wrong except the knock retard. It's there also when not towing, when depressing throttle quickly, but it's milder. BLMs climb to 130 and sometimes over when cruising.

I tried to bring WOT-enrichment TPS%:s down, which helped a little bit when towing, but the knock retard is still there. I've also lowered the fan kick in temps, but that of course isn't helping while towing down the road, since the engine is not overheating.

When not towing, the engine feels very healthy, and I've never noticed any hesitation. Without camper I get usually about 20-21 mpg when driving to work, one way trip being 18,7 miles.

Something must be wrong cause I'm getting the noticeable retard...
What can cause this condition? What should I check/do next?
Thanks for any advice!

Have you changed you knock sensors? You will get retard when it knocks. Another option is going to a higher octane gasoline, just to test the knock qualities. The gas mileage for towing does not seem all that bad. You are almost doubling the weight you are moving.

I haven't changed knock sensors. But I'd think it's really knocking, not just fooling sensors, since every now and then I've heard a 'tic' sound when it happens... Or can something in the retarding process itself cause the sound? I think it's been only a single sound before it starts to slow the engine down.

I started to use 98 octane gas, which is similar as 93 in America (different standards), but that didn't help.

The system may be doing what it is supposed to. I am not one of the designers, but when you get knocking, unless it is compensated for and stopped, you will have holes in the tops of the pistons.

I have the same thoughts as Fred. The system is detecting a knock or a ping (they are very similar but not the same), then retarding the timing to protect your engine. And 14 mpg isn't too bad for towing that weight in hilly country at varying speeds. You might do better than that at a steady 55mph on a fairly flat freeway, but those seem to be less common in Europe than over here.

There are a couple of possible causes of "knock". The first of course is timing too far advanced for the octane rating of the fuel. As you say, Europe lists RON while over on this side of the pond PON (RON + MON / 2) is the standard rating method.

Your equivalency is only approximate and may vary a few points under real-world conditions, especially with modern fuels that contain varying amounts of things like ethanol. According to some research, ethanol increases gasoline's RON rating but has little effect on MON. A 95 RON ethanol blend may only have a MON of around 81, which would equate to a PON of only 88!  

The other cause of "knock" is pre-ignition -- ping -- where a hot spot like a carbon deposit ignites the fuel before the spark occurs. The hot spot could even be the spark plug itself (that's what spark plug heat ranges are all about -- they refer to the plug temperature, not the combustion temperature).  Small changes in timing will have little effect in this case, the timing needs to be retarded so far that the fuel has all been burn by the time the spark occurs, so if your PCM is successfully preventing knock by retarding the timing a small amount, that should rule out ping. When you say there was no difference, did you actually use a scan tool to confirm how much spark retard was being applied in each case, or are you just going by the mileage?

Finally, while the engine coolant temperature may be 180°F, the combustion chamber temperature is much, much higher -- even in a brand new engine in perfect condition. The function of the EGR valve is to lower combustion chamber temperatures by injecting a little recirculated exhaust gas into the mix. So if the EGR system is not 100%, those temperatures may rise, which potentially could add to hot spots. Do you have to pass emissions testing over there? Higher than normal NOx levels on your last test could point to the EGR circuit, since NOx rises with combustion chamber temperature.

We pull our 3000lbs trailer with our 93 Chevy caprice. (350 tbi) and are getting around the same mpg. (but there is no "knock") I agree, try a higher octane rating first.

The LT1 has a higher compression ratio than the LO5, so I would expect it to knock more readily on the same octane fuel.

Fred Kiehl wrote:

The LT1 has a higher compression ratio than the LO5, so I would expect it to knock more readily on the same octane fuel.

 Compression ratio is only part of the equation.  Combustion chamber design is much more of a factor, particularly with regards to temperature across the surface, quench area, and so forth.  Compression ratio differences alone are not much of an indicator particularly when the combustion chambers are so differently designed.

Are you sure you didn't go the wrong way with your plug changes?  GM picked the stock plugs for a reason.  

When you are watching Datamaster, how often is EGR enabled while cruising?   I've noticed that my RMW, if I'm traveling at higher speeds under heavier throttle, will not enable EGR.   EGR is great for lowering combustion chamber temps. Towing in D may put you in an rpm range too high to enable EGR as well, but I'd have to look at the stock constants to verify this (just going off of memory here).

Also, watch for Cat Protect becoming enabled.  This will definitely kill your mileage, but is a safety feature to protect your catalytic converters.  I've found that extended spirited driving even without towing will enable it.  

Have you tried cleaning out the engine with Seafoam?  I tried it on a '70 Mustang I was sure was full of carbon and that thing smoked for a good hour after I restarted it.

Can you post up your datamaster logs?

I don't think your fuel economy is off.  

Joel has some good points wrt EGR and cat protection.  Without knowing your mileage, I was going to suggest getting a Motorvac treatment.  You want to ensure your car is running optimally, so cleaning the maf resistors with contact cleaner or maf cleaner is also a good idea.  They get pretty dirty after a while.  Make sure you don't physically touch the pellets with anything like a q-tip.  As it is, your BLMs seem a bit high (lean) at cruise IF you haven't tuned it for lean cruise. Not sure what your BLMs are doing the rest of the time.

We can't guess your altitude and temp, or your car's mileage.  But but when towing uphill, you're putting a huge load on the engine.  Your iron-headed LT1 shouldn't need to change spark plug heat ranges, and was designed to run on 87 Octane.   If you're still getting that much KR with the higher octane, you have to look elsewhere.  The delco 41-906 plug is listed as a 4 and the 41-943 from 1996 cars is listed as a 3 so the rapidfire's heatrange is fine.  However, they used to recommend not re-gapping rapidfires, though I believe that's no longer the case.  If you did not regap it, ensure you have the proper gap.

I would test the KS to ensure proper operation: [You must be registered and logged in to see this link.]  Keep in mind a faulty PCM is extremely UNlikely.

Also, confirm you have the proper LT1 180 T-stat, not just a SBC 180 tstat.   You don't mention whether or not you have the tow-package.

Upon initially reading, the first thing that popped into my head was get rid of the Rapidfire plugs! I personally don't believe in all the gimmicks of the spark plug companies, they are just trying to get you to buy their plugs, stick with the basics when it comes to spark plugs. I've personally changed out more non-OE recommended spark plugs to correct more problems than I can count. This includes Ford, Chrysler, Asian and European engines. (I know some of you guys use the NGK plugs and they work, that's great!) I realize the rapidfire is an AC design plug, but I feel they just wanted their piece of the pie when it comes to gimmicky stuff. I've tried them and they went into the trash shortly thereafter. The plugs created a spark knock/ping/detonation that wasn't there previously to the plug change. If you consistently tow with this vehicle you may consider dropping one heat range from the recommended heat range.

The second thing that came to mind, is the EGR system working? I have a tendency to believe it's working to some degree. From what I've read, it does not seem to happen unless you are towing, right? Maybe the EGR passages are starting to clog up and not allowing enough recirculation of exhaust gasses through when you are working the engine hard. As mentioned above the exhaust gases actually help cool the combustion process and allows more spark advance during the periods of operation.

Don't mask the problem with higher octane fuel, eventually the higher octane fuel won't be able to "correct" the problem any longer. Then what do you do? Find the problem and fix it.

Many possibilities, one solution.

great advice on the plugs, really loose timing chain,?
These motor cleaners are in my opinion, not good for the cats,
I just rig up a temp hillbilly water injection setup.

gmtech wrote:

Upon initially reading, the first thing that popped into my head was get rid of the Rapidfire plugs! I personally don't believe in all the gimmicks of the spark plug companies, they are just trying to get you to buy their plugs, stick with the basics when it comes to spark plugs. I've personally changed out more non-OE recommended spark plugs to correct more problems than I can count. This includes Ford, Chrysler, Asian and European engines. (I know some of you guys use the NGK plugs and they work, that's great!) I realize the rapidfire is an AC design plug, but I feel they just wanted their piece of the pie when it comes to gimmicky stuff. I've tried them and they went into the trash shortly thereafter. The plugs created a spark knock/ping/detonation that wasn't there previously to the plug change. If you consistently tow with this vehicle you may consider dropping one heat range from the recommended heat range.

The second thing that came to mind, is the EGR system working? I have a tendency to believe it's working to some degree. From what I've read, it does not seem to happen unless you are towing, right? Maybe the EGR passages are starting to clog up and not allowing enough recirculation of exhaust gasses through when you are working the engine hard. As mentioned above the exhaust gases actually help cool the combustion process and allows more spark advance during the periods of operation.

Don't mask the problem with higher octane fuel, eventually the higher octane fuel won't be able to "correct" the problem any longer. Then what do you do? Find the problem and fix it.

Many possibilities, one solution.

When the manufactures start using these MAGICAL sparkplugs from the factory is whan I will start using them in my car.

Wow, Your millage is great! Towing that weight. I have 2.56:1 axle on my Wagon. I have towed 2900 Lbs. with 12MPG. But no pings or knocks. I also use 93 U.S octane fuel.

Thank you all for your responses!

Egr is enabled every now and then when towing. But I'm a little confused here; isn't it supposed to be disabled at high load (i.e. when the knock occurs) anyway?

I found only one log that's been recorded while towing. I noticed, that cat protect is enabled for some periods of time, especially when driving a little faster, so that definitely affects the mpg. It's enabled every now and then even when not towing. Thanks for that notion, hadn't been thought of that before!

Now I also noticed on the log, that when I start to get on the throttle, I see some spark retard even if the knock count isn't increasing. So I guess it shows also the preventive burst knock retard there, and I should concentrate on the points where the knock count is increasing.

I haven't tuned the engine for lean cruise, only tried lowering the WOT-enrichment rpms to prevent knock. Which didn't help.

If we trust the previous owners maintenance record, I have the correct LT1 180 T-stat.

We have annual inspection here with emission test, but I only remember that it passed... I've towed on flat highways at steady speeds as well as on hills and small roads and the mpg actually stays pretty much the same. Of course the speed tends to be a little higher on better roads...

I had the same thoughts about the plugs. I'm not 100% sure, but think there wasn't any knock or slugging down when on the first trip prior the plug change. Mpg was worse then, but maybe because of the fuel pressure regulator. So maybe I try AC 41-943 plug from 1996 cars next.

I think I'll fabricate cat delete pipes before next summer, and remove cat protection. I think cats are just a stupid restriction there when towing, and will probably die in that use sooner or later, protection or not.

Ok, a little update.
My EGR seems to be faulty. A week ago it threw a code 32.

The solenoid sees to be working all right. I felt suction on the hose going from solenoid to EGR, while I grounded the brown wire while engine running.

The EGR wasn't moving a bit, and I couldn't move it by hand. I tried to move it by using a long bolt as a lever, and it suddenly started to move very easily. After that, when I moved it by hand when engine was running, the engine almost died.

But vacuum via the solenoid couldn't move it any noticeable amount, and activating the solenoid affected running very little.

So it's pretty clear, that the EGR valve has a leak and needs to be replaced, right?

edit:
Removed my stupid rant about external torx screws.
EGR valve is retained by ordinary hex nuts. Nuts are mounted on studs that have smaller external torx heads, to help remove the studs if necessary... I guess there will be enough room to pull EGR backwards after just removing the nuts.

A little update.. Last year I wrote about the EGR. I swapped it for a new part, but continued to have problems with idle. I changed the spark wires and distributor cap and rotor. That didn't help much, and finally I noticed the new egr was faulty. I got the old egr moving, changed it back, and it started to idle fine! No code 32 after that.

I swapped the rapidfire plugs for AC 41-943 's meant for '96 cars.

I disabled cat protection, but didn't have time to get rid of the cats yet.

Before this summer I also swapped the 2.93 gears for a 3.42 set.

Finally towed the camper last week. It ran fine and strong. I didn't notice any knock or lugging at any circumstances. But the fuel mileage was very bad again at about 11,7 mpg. Didn't matter if I drove 50 mph with 3rd or 55-60 mph with OD.

So despite the new ignition parts, no knocking, and the lack of cat protection feeding in additional gas, I got much lower mpg than the last time. Can the gears do so much? I think it's kind of weird. Many people have reported much better mpgs with larger campers.

Anyway, my first problem to fix now is rear suspension; my air shocks broke down during the trip and the ass end started to hang really low. Hopefully I'll be installing new shocks and springs this week.

The 3.42 gears can make that much difference in high speed driving when switching from 2.93s. It cost me about 2 MPG when switching 2.93 to 3.23s in regular highway driving without towing anything.

Your fuel mileage is about right for towing. I tow a 1700 lbs pop up camping trailer from time to time and a 1100 lbs flat bed (with a huge metal gate/ramp) and I get about 12 mpg. Granted the LT1 is not as efficient as your motor.

Have you disabled burst knock in your tune ?  If not you should disable it.
what kind of gas are you using (ethanol% / Octane) ?
What brand of O2 sensor are your using ?  There are some brands that are known to cause issues with these cars, namely Bosch.
Do you have a scan done while towing ?  It would be good to see your LTFT/BLMs and O2 mV while towing.

Being that you have changed the gears and clearly have access to tuning, did you correct the speedo/odo for the new gears ?  Just asking because your gas mileage dropped by pretty much the delta of gear change. Also, obviously, ensure the tune is set for the correct revs/mile (tire diameter). Also, ensure you are calculating the fuel mileage properly by filling to the same point and using actual distance traveled and volume of fuel registered at the pump. Very obvious points that somehow people sometimes manage to miss.

I have a set of cat-delete pipes I can sell you, but it might be exorbitant to ship them to Europe ?  I'm just guessing based on some of your comments.  Add your location to your signature. PM me if you are interested in the cat delete pipes.

Again,  if you are calculating using US Gallons, your mileage doesn't seem outrageous for towing a 7.5 foot wide, 2900 lb trailer that may or may not have the aerodynamics of a sheet of plywood.

You should keep cat protect enabled until you actually remove the catalytic converters. GM put that functionality there for a reason. You may perhaps melt them by doing this.

Nice people here, thanks for the replys!

I haven't messed with burst knock yet.

I changed the speedo and think it's at least very close to accurate.

Good point about mileage dropping by the same factor as gear ratio increased. The engine turns faster now at a given speed, which at the same time lightens the load (the amount of fuel/air mixture per revolution is smaller), so I figured the mileage drop should be smaller than the increase in engine speed. But it's possible there's nothing to worry about. Probably pretty small things can affect mpg with a load like this. And at least I'm happy it didn't have knock anymore.

Tuning and logging is a little difficult now, as the screen of my laptop is broken; only small portion of screen is working anymore... But I try to log the next time I'll be towing.

Yeah, I have to enable cat protect or lose the cats.

andy caprice wrote:

Good point about mileage dropping by the same factor as gear ratio increased. The engine turns faster now at a given speed, which at the same time lightens the load (the amount of fuel/air mixture per revolution is smaller), so I figured the mileage drop should be smaller than the increase in engine speed. But it's possible there's nothing to worry about. Probably pretty small things can affect mpg with a load like this.

I think his point was that unless you changed the programming to match the new final ratio, the odometer would be lying -- making it appear that the mileage was much worse than it really is. I have a tow-pack with the lower gearing and despite the higher revs, the (unloaded) highway mileage is quite comparable to a non-tow-pack car -- 23 to 24mpUSg on dry, smooth pavement with the cruise control set at about 65mph and properly inflated "All Season" tires.

Yes, I understood the point about speedo error, his comment just led me to think about how much the mpg had decreased.
Yeah, I don't think that my mileage has gone much worse in normal driving after the gear change. Haven't made enough highway trips yet to confirm, though.