Hello everyone. ..here's my issue which im SURE some of you have delt with. When the car is first driven in the morning(open loop) the car drives perfect for the first 5min and once the engine temp reaches around 180-200 all this sudden I get low throttle stumble/studder/hesitation. If I give it more gas it takes off plus there is no issue under full throttle too. I have replaced the fuel pump, fuel filter, cap and rotor, wires(previous owner said he just did the plugs and o2 sensor), tps is new, we even used wire separators/holders to make sure its no heat on wires. The starter and battery is brand new also. I have put techron in the tank twice in the past couple weeks..the previous owner deleted/blocked off the egr..here's the funny thing...we swaped the maf with a new one ..no change(mechanic returned it).we disconnected the o2 sensor and it made no difference AND the CEL never came on either. Tomorrow im gonna swap in a new ecu and see if that fixes it...if it doesn't then my mechanic will return it(I have built a GREAT relationship with this mechanic so he let me do alot and is VERY lenient on labor...please help...this has to be remedied before I start anything else

 

Also..the previous owner stated the vette has ls1 injectors. I know the stock injectors are 22lb so I don't know why that was done or if it is causing my issue....its like its a lean misfire but hopefully someone else can chip in

 

Not sure if LS1 injectors are exactly the same or not, but when running in closed loop, the AFR is slightly leaner than when in open loop, so it can make a problem more obvious.

Here's what I would do:

Plug in a scan tool, look at BLMs, anything over 132 or under 124 should be noted and reported back here. You'll need to drive it under those conditions to see also.

While you're doing that you can also check for vacuum leaks.

The ECM operates in open loop for a few seconds when you start it when it's hot. Would you be able to restart the engine and quickly go to see if it still does it? This would basically differentiate the problem from being a closed loop problem or a temperature problem.

 

Thanks for responding. ..this site is awesome.....ok...here's what was checked today....we swapped in a new ecu to see if it was affected and the issue remained (my great mechanic lets me try new parts and returns them if needed:woohoo): we tested the fuel pressure and it was hovering around 30-32psi ...I know that's way low...then we tried it again and the guage needle literally pulsated between 30-40psi only smoothing steady to 35--38psi for a brief moment while coming to a stop then back to the fast jumping between 30-40psi....we attempted to pull codes and data log while running and the code was for lean exhaust and the data log said the o2 read lean...again we swapped in new maf and ecu to eliminate them and the issue remained(old maf and ecu now put back in)... the fuel pump and filter are new the o2 is supposed to be new and looks new ...any ideas?...mechanic want to try fuel pressure regulator........we cut the car off with the fuel pressure guage on and it immediately slowly dropped to around 21-23psi and sat there..I found that odd...I thought it was supposed to take a while not start dropping immediately

 

sounds like leaking fuel injectors fuel pressure drops

 

Today to me was great because we eliminated some main -costly- parts as the culprits...what was read by the snap-on scanner was o2 sensor was lean and a code for lean exhaust...plus inconsistent/low fuel pressure ...i thought that fuel pressure measured via the fuel guage was not affected by faulty injectors this measurment is before the injectors

 

Fuel pressure is strange it should remain steady.

Check TPS voltage it should be .54 @ base idle.

 

My apologies. .....the cap/rotor plug wires and tps are brand new and I personally watched my mechanic set the tps to .54
The previous owner deleted the egr.....the funny thing is the car is a rocket until temps reach around 180-200 degrees(probably going into closed loop) then it starts having a stutter around 2300-3000 rpm and it makes sense of being a lean hesitation...if I give it more gas it clears up and takes off...its like my throttle becomes an on/off switch

 

The EGR block off on a 89 should not create any running issue.

The fuel pressure regulator may be causing the erratic pressure readings. Pull the FPR hose, check the hose for fuel inside of it = leaking FPR. Run the engine with the FPR vacuum disconnected + vacuum hose plugged. Fuel pressure should be @ 40-43#.

If it pulls hard @ full throttle when up to temp:
Idle / light throttle stumble, hesitation @ operating temp a lean condition is occurring.
Suggest engine running & up to temp, check intake gaskets for leaks. I spray WD40
with a straw, a change in idle rpm = a leak.

 

Come to think of it i do remember my mechanic taking the vaccumm off the FPR and it was bone dry. the funny thing is the fuel pressure guage needle was frantically zipping/buzzing back and forth between
30-40psi and it was basically impossible to get a solid reading on fuel pressure and when when i made the car stumble the needle just kept jumping back and forth ...no rise or fall ....i thought vaccumm leak ...would that effect fuel pressure? ...to my knowledge it can't...and remember it drives fine for the first few minutes until temps reach 180+ then it starts....if i let the car sit for a while and cut it on and the temp is like 150 it will bdrive fine again until the temps gets to around 180-200

 

LS1 injectors will not work correctly in your car. LS1 engines only fire one injector at a time and they are what is known as "peak and hold" injectors. Your car fires all 8 injectors at the same time and they are known as "saturated"' injectors.

The coil resistances of the injectors are quite different. Saturated injectors are around 16 ohms. Peak and hold injectors are quite a bit lower than that, roughly 3 to 4 ohms.

I don't think the engine would even run with peak and hold injectors because the ECM can't handle the low resistance. I think your previous owner might have meant LT1 injectors.

 

An off-idle stumble is usually caused by a vacuum leak.

I had a vacuum leak where the PCV valve hose attaches to the intake manifold. It was OK when cold but started leaking when the temperature went up and the rubber got soft. It turned out I used the wrong size hose. I used 1/2" because I had some and it seemed to fit OK. It turns out the correct size is 7/16". This was a tough one to find and I only did find it because the vacuum leak made a whistling noise when the engine was hot.

 

I will look further into that its just soo consistant with temps around 180-200 and almost never throws a CEL and if it does it goes off after a min or two and its for lean exhaust