Frustrating drivability issue...Help LT1 experts...shade tree mechanics

Hello everyone. I'm attempting to chase a driveability issue I'm having with a 96 RMW. 60K original miles, bone stock car.

The car starts easily/quickly. It idles smoothly, and does not have a miss at any speed. At part throttle, from a dead stop the car wants to die. (Feels like its starving for fuel). If you push the pedal to the floor it takes off and will pull to 100 MPH no problems. If you apply the pedal lightly it hesitates and stumbles.

I have driven the car with a fuel gauge taped to the windshield. (42 to 46 PSI steady with vacuum pulled from the regulator). I then proceeded to change the following parts in the order listed: Fuel filter, plugs, MAF, MAP, TPS, new GM Opti. I have driven it with the knock sensor unplugged and plugged in...no change!

I'm ready to douse it with gasoline and set it ablaze.

Any ideas? I'm all ears...thanks in advance.

Do You have a way to scan for other parameters? I.e O2 mV or iac counts ?

No codes? How does it run with the MAF unplugged?

[quote="Big Wagon Guy"]The car starts easily/quickly. It idles smoothly, and does not have a miss at any speed. At part throttle, from a dead stop the car wants to die. (Feels like its starving for fuel).

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If you push the pedal to the floor it takes off and will pull to 100 MPH no problems.

That would seem to rule out fuel pump and filter -- it would be worse, not better at high throttle settings

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If you apply the pedal lightly it hesitates and stumbles.

So the problem is only at light throttle settings? How light is light? Just touched? 1/2 way to the floor? Is there any difference between normal operation and a just-started cold engine? Under hard acceleration ("Power Enrichment Mode" in GM parlance), the O2 sensor feedback is overridden, similar to the open loop mode that occurs before the O2 sensors heat up and start functioning. If the car runs OK when cold or under hard acceleration only, then maybe this points to an O2 sensor problem.

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I have driven the car with a fuel gauge taped to the windshield. (42 to 46 PSI steady with vacuum pulled from the regulator).

Do you mean you disconnected the vacuum line? If so, how's the pressure with the line connected?

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I then proceeded to change the following parts in the order listed: Fuel filter, plugs, MAF, MAP, TPS, new GM Opti. I have driven it with the knock sensor unplugged and plugged in...no change!

MAF and TPS definitely would have been on my list of suspects. Have you checked the Opti vent system for proper operation? Particularly the air inlet hose at the intake duct? If the inlet hose is plugged, vacuum will build inside the opti and that can cause problems with cross-firing. GM had a TSB about that. EGR operation is another possible culprit, although I would think that would also affect idle.

It would be worthwhile to scan the live data feeds for anomalies. The sensor itself is only part of a circuit. The signal that makes it back to the PCM also depends on the wiring, connectors and supply voltage. The scan tool will tell you the results of the entire circuit as the PCM sees it.

When I first got Dusty (94 RMW), it had a similar issue, in that it was hesitant under mild or mid throttle but would take off strong at WOT.   After it being misdiagnosed by a professional shop ($100 fee, and a brand new fuel injector from the general $180 in 2007) I still had the problem.  Then went through all the vacuum lines, plugs, wires (a total PITA without taking off the water pump), only to wind up at the opti.    Once the the cap was off I could see the issue immediately in that a hole had burned through the thin metal plate in the back and it was arcing through on #7 firing.

That said it was running on 7 cyl (and the gas mileage reflected the 12.5% drop as well...12 mpg in town) but the computer was pretty good at masking the miss (I had an 89 ford van with the 5.0 HO that lost a cylinder and you really tell that was missing).

My recommendations would be make sure there isn't a misfire,
Then go through all your 02 sensors and bench test them (if you can get them out), check vacuum lines.

As far as the misfire goes a 96 With OBDII should detect that.

Since you have not mentioned the spark plug wires or the coil or the ICM I would at least check them and make sure they are in good shape. Perhaps swap with a car that does not exhibit the issue.

With a 96 you have OBDII. (Unless you have changed the PCM)

If you have an android phone and some time you can get a pretty inexpensive and powerful scan tool setup.

The Bluetooth readers now go for under $10 on [You must be registered and logged in to see this link.]. Read the reviews and don't use the included software disk. They tend to be filled with malware. They are cheap Chinese junk but your chances of getting one that works are pretty good. Just read the reviews.

If you have an Apple product they have WIFI adapters as well. They are usually under $20.

I have tried several of the applications out there and Torque so far has been the best. It works very well and I have diagnosed several cars without issue. The free version will let you do some things but since I use it all the time I purchased a full license. The dashboard function is really nice to see what is going on. I have it setup so if I need to I can have a Tach and temperature gauge that read real numbers. Along with Throttle position Vacuum and O2 sensors.

With a scan tool. It should let you see the raw data from your OBDII interface. Test the TPS and the O2 sensors. Check that the TPS is a smooth progression through it's sweep. As stated by others the wiring may be at fault and this should give you a good idea if that is where the issue lies.

-D

In reading the comments I noticed that I did not provide all the pertinent data.

Car is bone stock, OBD2. No codes present. Tonight I swapped the coil and ICM. No change.

The fuel pressure is with-in spec with the vacuum connected and without. (I'm confident its not a fuel problem).

I installed an new Opti vent and harness when I did the Opti. (All have about 3 KM on them).

Car runs the same with the MAF unplugged/plugged in.

The problem does not seem to be temperature dependant. The car will do it with-in a second from starting or after reaching operating temp. The issue is notable with minimal throttle application. (Think just pushing on the pedal enough to start the car moving).

A friend with a GM Tech 2 will be over tomorrow to scan the car. I'm leaning towards O2 sensors or something funky with the PCM. I will post an update after we have scanned the car.

I have owned a couple LT1 B cars and this is a new one for me.

Thank you all for your time and suggestions. I really appreciate your taking the time to share your experience and suggestions.

Only does this in gear? Have you checked the EGR? Unplug it and manually pull vacuum on it and see if there is a change in RPM.

Unplugging the maf puts it in open loop. That takes the O2 sensors out of the equation. Pcm failures are exceedingly rare.

An EGR fault is more likely.

Unannunciated misfires on 96s are not unheard of.

Well after changing virtually every sensor on the car and installing a new OPTI...it was the EGR solenoid that was the culprit!

Learn something new every time you work on these things.

Thank you all very much for your assistance and advise.

JP

WOOHOOO!!!! I FINALLY got one right!!!!

Glad you got it fixed.

How did you find that the EGR solenoid was the problem? Did you unplug it and the problem went away? Or what?

Hello Joel. It was the posts from Jason and Superior Finish that lead me to the EGR. I pulled the vacuum off it, pushed on the EGR plunger/diaphragm with my finger and finally unplugged the solenoid and the problem went away.

I was 99% sure it was an electrical issue. The car acted just like it was starved for fuel. (Yet had perfect fuel pressure).

I wonder how many Optis or fuel pumps have been changed when in fact the EGR was to blame?

Big Wagon Guy wrote:

I wonder how many Optis or fuel pumps have been changed when in fact the EGR was to blame?

Opti's are the devils work,. they have been known to cause all kinds of problems ,. from leaky tires,. flat batteries,. radio stations not coming in properly,.

when in doubt on an LT1

change the opti,...


then mail your old ones to me,...