Help! 1991 L-98 Runs horrible after warming up!

[aftershk]

Hi everyone,
After pulling my 1991 out of winter storage this year, I noticed it running rough when in gear after it has warmed up, when I put in neuteral it would smooth out.
I thought that possibly the gas was bad and ran it low, filled her up with 93 octane and even added a bottle of octain boost, again, runs good cold and now worse than ever when warmed up.
I brought it to my mechanic, he changed fuel filter, did some regular maintenance like cooling system flush, oil change and then checked all wires plugs and finally did a smoke test and said that throttle body was leaking and needed replacement? I trust him 100% but car has only 38000 mile on it is pampered more than my wife, just ask her! and was just wondering if it's possible it could just need a good cleaning or if his diagnoses sound correct?
I appreciate your help!
Thanks, Paul

 

[boomdriver]

Hi everyone,
After pulling my 1991 out of winter storage this year, I noticed it running rough when in gear after it has warmed up, when I put in neuteral it would smooth out.
I thought that possibly the gas was bad and ran it low, filled her up with 93 octane and even added a bottle of octain boost, again, runs good cold and now worse than ever when warmed up.
I brought it to my mechanic, he changed fuel filter, did some regular maintenance like cooling system flush, oil change and then checked all wires plugs and finally did a smoke test and said that throttle body was leaking and needed replacement? I trust him 100% but car has only 38000 mile on it is pampered more than my wife, just ask her! and was just wondering if it's possible it could just need a good cleaning or if his diagnoses sound correct?
I appreciate your help!
Thanks, Paul

I'd get a second opinion...thats too low milege to have worn out a T-body shaft and make an air leak of any size....sounds to me like hes grasping at ....whatever....cause he don;t know and someone else told him that the t-body can have air leaks.

egrm PCV, AIR system, all that is ok? fuel pressure is good and stable? been timed?

 

[Hib Halverson]

It's possible the TB is faulty, but considering the mileage on the car and that the symptom shows-up only once the engine is warmed-up, I'm going to guess it's not a throttle body issue.

Does your mechanic have a scan tester? If so, tell him to look at serial data once the engine is in closed-loop to see if the system is having trouble controling idle.

Do not use octane boosters to solve idle problems. Some of them, mainly ones which use MMT, will damage cat convertors. Many others, offer no practical improvment in octane when added to premium fuel.

 

[keremarsan]

in closed loop ( enters in close loop, I think , after approx 167 degree fahrenheit ) engine uses voltage signals from O2 sensor ( s ) . until 89 there is one O2 sensor on left exhasut manifold , maybe there is 2 or 3 for year 90 and 91 of L98 ....

First to check is o2 sensor in my opinion if rough running when hot...

kerem

 

[aftershk]

1991 L-98 Runs horrible after warming up! (problem located)

I would first like to say Thanks to those whom replied tomy post, I appreciated the advice.
I finally was able to get another opinion from a mechanic who is more familiarwith Corvettes, He found that after checking some of the suggestions I received,the problem appears to be with the fuel injectors.
He tested the resistance on all and found that 4 were half of what they shouldbe and the car was running rich, he suggested that I replace all 8 which seemslike the right idea, everything else checked out fine except for questionable Oxygensensor which he replaced.
Does anybody have a suggestion to what injectors I shouldpurchase to replace the original equipment?
Once again, I appreciate the help.
Paul

 

[WRECKER88]

Talk to Jon at Fuel Injection Connection.

Fuel Injector Connection

 

[aklim]

I would first like to say Thanks to those whom replied tomy post, I appreciated the advice.
I finally was able to get another opinion from a mechanic who is more familiarwith Corvettes, He found that after checking some of the suggestions I received,the problem appears to be with the fuel injectors.
He tested the resistance on all and found that 4 were half of what they shouldbe and the car was running rich, he suggested that I replace all 8 which seemslike the right idea, everything else checked out fine except for questionable Oxygensensor which he replaced.
Does anybody have a suggestion to what injectors I shouldpurchase to replace the original equipment?
Once again, I appreciate the help.
Paul

Which 4? Are they all on one bank or different banks?

 

[aftershk]

Which 4? Are they all on one bank or different banks?[/

Which 4? Are they all on one bank or different banks?

I'll have to find that out? would 4 on the same bank mean something? I'm a cabinet maker not a mechanic. Thanks

 

[aklim]

I'll have to find that out? would 4 on the same bank mean something? I'm a cabinet maker not a mechanic. Thanks

Yes, if everything on one side or the other is faulty, I would be curious. So if every injector failed on the driver side or passenger side, I would wonder why it became that way.

If you screwed up 1 cabinet, oh well. I'm sure you screwed up 4 cabinets in your time. So, if the 4 came in a row, we have a systemic problem. 4 throughout your life, probably not.

4 injectors on the driver or passenger side could be a broken wire, bad ECM or fuse. It would also make me wonder why. OTOH, if they are 3 on one side 1 on the other or 2 and 2, it probably was wear and tear.

I would send them off to Jon at FIC and have them tested just so I know

 

[aftershk]

Corvette Running Great!

Just a follow up, After replacing all eight fuel injectors with new Bosch 22lb Fuel Injectors, and cleaning the throttle body, the car runs like it never did before!
The power difference is incredible! Maybe I can put a few miles on her before putting her away for the winter!
Thanks Again to everyone for the advice!

 

[John Robinson]

If the bad injectors were all on one side I have a couple of thoughts

1. I don't know but if this is a bank fired engine then it would stand to reason that the fuel would be metered to which side was first to fire. Therefore putting the very first of the old gas down one bank and fouling the injectors.

2. If the injectors were fouled randomly check the firing order to see if they follow a pattern in the firing order like all in a row. If so the same problem of the old gas going there first and plugging the injectors.

 

[BigJimZ16]

If the bad injectors were all on one side I have a couple of thoughts

1. I don't know but if this is a bank fired engine then it would stand to reason that the fuel would be metered to which side was first to fire. Therefore putting the very first of the old gas down one bank and fouling the injectors.

2. If the injectors were fouled randomly check the firing order to see if they follow a pattern in the firing order like all in a row. If so the same problem of the old gas going there first and plugging the injectors.

John, both good thoughts, but he said that the resistance of the injector coils was half of what they were supposed to be. This would seem to point to an electrical issue rather than a mechanical one. Just my $.02...

 

[jwardg]

I had a similar problem on my 1991.Except with my 90/L98 it would not only run rough in drive, it would eventually shut off.
I had a local mechanic look at it . Ended up replacing the fuel pump, fuel filter, relay, ECM and 1 injector. Seem to have worked. I drove the car for a day or so.
Filled it up with premium gas and Chevron Techron Gas treatment as recommended. Drove the car home. Next time I started the car, the same symptoms returned but this time even worse.
Did some research on my own. Measured the injectors, 5 out of 8 gave low resistance readings. GM maintains the position that Multec port fuel injectors should not be cleaned. Some of the cleaners contain high percentages of methanol and other solvents that cause damage to the injector wire insulation. Word to the wise...be careful about using fuel additives and injector cleaners. Premium grade gas should be sufficient to clean and run the car as designed. Surf the web for GM service bulletin number 91-312-6E May 15, 1991( Multec Fuel Injector Cleaning Information. ) I ended up replacing all eight injectors. The car is a work in progress but running much better.

 

[boomdriver]

First of all, I am shocked that a reputable mechanic or shop would replace a single fuel injector. WHY? it cost the same in labor to do all 8 and the other 7 ARE going to fail seeing as how its a 20 yr old engine....amazing. A set of contemporary inj from FIC that are the latest design and have no neg feedback cost $200 including all the gaskets and o-rings for the rails.

Next, did anyone read the flash codes?
I'd be willing to bet that most of that work done was not necessary. Sounds more like a MAP issue. Those do fail.

Regarding the MuleTic fuel injectors...they will show a bad Ohms reading thats an indication of a weak or slow injector response to a signal. But, because this motor is "bank fired" the speed of the inj is not as critical as other models. It'll live with a slow (even a few) injectors since they are spraying gas when the intake valve is closed, open, halfway...whatever.

What does happen is the elec coil in the injector is able to short out either thru a cracked case or it shorts internally to the inj body where the signal gets stuck...leaving the inj stuck wide open or unable to open. If its stuck OPEN, that usually means the whole bank is stuck open and the fuel will pour into the cylinders and fill the crank case with gas. I've seen 2 gallons of liquid in a pan and raw gasoline pouring out of the exhaust. This condition is not intermittent....it happens and it stays like that until repaired. The inj body can crack and allow the coils to throw their positive charge signal to the intake manifold. I have seen the arc jump from the fuel injector to the intake. Very common with stock injectors.

What CAN be intermittent is a bad fuel regulator. The plunger sticks, causes flooding thru gas entering the vac line. May not do that the next start up.
So far the ONLY complaint with the Bosch-III fuel injectors is that some cars will have a longer cold crank time...possibly because the B-III are not leaking fuel into the cylinders to help prime, I don;t know. Once warmed, even after several hrs they fire right up. Let it sit overnight and get stone cold and it has to crank over for 3-5 seconds before it'll think about firing. Once lit, runs great.

I agree about the cleaners. Many are just some form of solvent...even paint thinners. Toulene, Xylene, MEK, Alcohol, etc..
Most of these solvents will do damage to older O-rings and some plastics. They dry the rubber then it cracks, breaks and leaks. Vac seal is lost, fluid seals leak under pressure.
Your injectors have a screen in the top that you cannot service so the cleaners will try to dissolve any gunk in there. They usually can;t. If you want to clean and lubricate your fuel system, plastic parts and rubber pump impellers, add a bottle of Marvel Mystery oil to the gas. You WILL notice an improvement within 1/4 tank of driving. The MM helps dissolve gum and lubes everything from valves to injector nozzels.