Hello all, I have a 91 L98, a month ago it started missing and hestitating bad. it wouldnt pull past 101. closed course in Mexico etc etc. no pre cats and revs easily in park, slight fuel smell in exhaust. I did a load of reading on here and started with fuel.... 44lbs key on, 36 at idle, momentary dip in pressure at WFO then back to 44. searched some more, thought ICM replaced that tonight and it is 90% better but I still have to change the rate of throttle somewhat to get it to accelerate. I went to 125 tonight and still pulling when I ran out of room. after an hr and a half of driving around, the Idle miss returned. Im thinking tps and a tuneup but cant do that til payday, yes Im poor. any ideas?
Please dont say search, Ive searched and read a ton of information that wasn't about a 91, its so hard to know what is important and whats not. Thanks in advance John K1

 

Check the ohms on your injectors while the motor is hot

 

Thanks Sheepster for the quick reply, Ill go buy a meter this weekend.
Later John

 

I never ohmed injectors before since I usually send them out every few years for testing. Maybe you have a clogged injector or two or three? Won' hurt to send them to FIC for testing and cleaning.

As to the TPS, make sure that the current is steadily increasing as you engage the throttle.

Also spray a fine mist of water around the wires in a DARK garage to test for spark leak.

Use a propane torch to check for air leaks in the intake around the gaskets or hoses that might be rotten.

Have you cleaned the TB lately? If not, clean it out and remove the IAC housing completely and make sure it is spotless.

 

Have the same problem on my 91. I cleaned out the throttle body, this improved the low end idling but still not 100% and smells rich. No codes.
I suspect injectors so have fitted a set of Bosch III's and a fuel filter. Not run it yet as we have 6" snow but will report back when I get chance to test it.

 

Still lots of things to test before reaching in your pockets...

#1 IMO would be the electrical.
When you replaced the ICM you got a dramatic improvement...so that means you're headed in the right direction.

Go thru ALL the power source connections at or near the battery. If you have some separate wires for positive that are not part of the big pos battery cable, clean all those. Some cars have a jumper post, some don;t. Where ever these source wires are, they are key to clean power to the ECM and the engine control devices. I have seen a dirty connection cause misfiring and the sluggish performance that you describe. Clean the contact, good as new. These also have the fusable links inline hidden under the battery. The links are ok, but thats how you know which pos wires are control circuits.
Grounds...
The bundle by the oil filter is equally as important as the power wires...even more since the ECM controls everything by ground, not power. Power is always ON in a vette...things are managed by the ground being completed or not. If those connections are dirty, same result. Poor performance.
Pull all the engine harness plugs and look for corrosion. You WILL find some plugs that have the green/brown scale growing on the contacts. Clean and reassemble with silicone grease to seal out moisture.

When you tested fuel pressure, was the gauge needle steady as a rock? did it move smoothly when RPM changed? if it had any flicker during rev up that indicates a bad filter of sock. If the gauge was slow to recover or keep up with the rpm thats a tired pump. The gauge shows fuel being used...the needle behavior shows the health of the system when you know what to look for.

TPS..get the wire diagram from the FSM and test the range of the potentiometer. It should be a smooth increase from .54v at idle(closed) up to near 5V at wide open. ANY dip, flat spots or rough, inconsistent readings mean the TPS is worn out.

o2 sensor anyone? has that been changed or looked at recently?

Assuming there are no codes present, there are only a couple things that can ruin performance and not set a code...fuel system and exhaust. You say the pre-cats are gone, what about the main cat?

When you pull the o2 sensor, start up and see if there is any exhaust blowing out the o2 hole...IF there IS< your CAT is clogged and needs to be replaced. The gas smell is a clue...there should not be any gasoline smell from the exhaust if the cat is working at all.

Just to be safe, pull the dipstick and see if there is any gas smell there. If there is, you may have a shorted injector OR a bad regulator thats flooding the engine.

Also, check to see if the EGR is sticking. If its even just slightly stuck open thats enough to kill the higher end of performance. You almost have to pull it to see the pintle and the seat and to know for sure that its working freely. EGR is a very useful part of the engine control system but it CAN cause lots of issues if it fails or the system gets damaged. For all the good it does, it can cause equally as much grief. Hard to diagnose in many cases.

Use the clues....gas smell, spark, etc.

If you are getting a flooding condition or just way too rich from a bad inj, there will be gas in the oil. Stop running it until this is resolved...that extra gas ruins the lube for that cyl and will cause great damage if it keeps running.

If flooding is not the issue, the injectors ohm test ok, then run a high quality inj cleaner thru the thing. Even a pt of Marvel Mystery oil in less than 1/4 tank of gas will help to loosen sticky, dirty injectors and let them spray again.

So, wire harness & grounds.
exhaust system for cats. (2-3 psi is the limit for back pressure) not much!
fuel system & inj for shorts, poor connections and worn regulator or injectors.

You should be able to find something if you go thru these things.
Good luck

BTW
best deal on a cheap but working volt meter is at Harbor Frieght. They have a home/DIY model DVM for under $10. It works, does voltage, ohms, and a variety of other tricks. Nice little folder instruction booklet written in Mandarin, Spanish and English. Best part is when you stick it in something that fries it, the cost of replacement is nothing to cry over...