my gf's 94z has been acting funny lately. Say shes driving 40 and needs to speed up, if she lightly accelerates you can feel the car surging and hesitating, but if you mash the gas pedal it picks right up and accelerates fine. She does have a cel every once in a while for the egr valve. Also it only seems to do it once the car has been warmed up and driven for a while, I don't think its ever done it cold. any suggestions?
Thanks
-Rob

EDIT: Forgot to add that when the car is warm and I come to a stop, the rpms fluctuate and the car acts like it wants to die.

 

Rob, the low rpm stumble is becoming a very common problem with LT1's as they get older. Do a search for stumble/hesitation. I myself have the same ghost I'm still chasing. Could be any of a dozen or more things.

 

ya, I saw the thread, it was like 90 pages long lol. I didn't have time to sit there and read thru all those pages.

 

Replacing my EGR helped the same problem on my car. It still has a slight stumble though. I am going to try reducing the hole size of the EGR exhaust port, because it "feels" like the engine is being overly exhausted.

 

When i cruise around 40-50 and give it a little bit of gas nothing happens and if i continue pushing the pedal it will eventually kick in and really shot the rpm's up. I did a datalog and found the O2's are showing .004 for a while so it is the fuel pump turning off when it isn't needed. I guess this is normal and wouldn't be anything to worry about if thats all thats happening. Mine isn't a surge when i give it a little gas but a feeling of a lack of fuel or zero power.

 

a little update: I completely cleaned the throttle body, cleaned the iac, installed new gaskets, etc. I removed and cleaned my egr valve- it is bad, wont hold when vacuum is applied. I capped off the vacuum hose going to the egr. I installed a new fuel filter also. I still have the same problem. I'm thinking its the opti. The reason I believe this is because I was watching the rpms with my autoxray and when it starts getting jumpy at idle I saw the rpms jump all the way up to 6000 rpms on the scanner. I'm guessing the optical sensor in the opti is going bad and giving the pcm incorrect rpm readings. Is there anything else in the car that would give a false rpm reading?
Thanks
-Rob

 

You're on the right track - the scanner shows a spurious rpm signal because the bearing is going out on the opti - and that causes the computer to chase the rpm around. Replace the opti and it will be fixed.

 

the only thing is, it only acts up when the car is warmed up. It runs great when I first fire it up. If it was the bearing, wouldn't it do it hot or cold? Or do you you think the heat just makes it worse to the point it throws off the computer?
Thanks
-Rob

 

The high resolution pulse is used after the engine enter closed loop, which requires it to be warm. Mine did the same, and several others I have repaired did the same.

 

ah ok, gotcha. Thanks for the input. I know when the opti went out on my 94 TA it just died, no warning.
-Rob

 

Mine runs fine in open loop too, then once it warms up I get the poor throttle response. Time to check my opti too it seems.

 

Every opti/cap-rotor problem I've ever had with my car has shown this symptom. Runs fine when cold until it gets heat-soaked. That's when the stumbling/backfiring occurs.