1991 Corvette Idle Problem

**Clem Z.** · July 13, 2009, 01:05 PM

Re: 1991 Corvette Idle Problem

Originally posted by Joseph Adelizzi (41481)

Has anyone experienced this problem with there Corvette?

I own a 1991 Corvette with a L98 engine. Since this spring I have replaced the following:
Battery, in-line fuel filter, spark plugs, spark plug wires, distributor cap and rotor, idle air control valve, throttle position sensor, adjusted and set timing to factory specfications, Front Y-pipe w/pre- cats, Catalytic converter, rear y-pipe, and mufflers.

The car runs very well on start up; however after the engine warms up there is an idle issue. The RPM's can fluctuate 2-3 hundred RPM's. After 1200 RPM the engine runs very smooth (hot or cold) yet if I am traveling on a back road and the engine is operating at less than 1000 RPM again the RPM will fluctuate. Also on a number of occasions after the car is warm it will either not start (until it cools down) or it will start and run for a short time and stall. Yesterday, the car started, I backed out of a parking spot and the car stalled. Last week the car started, I drove out of a parking lot to a stop light and the ar stalled. I will need to let the car cool down and then it runs fine again until it warms up again. When hot the car idles at around 600 RPM (but fluctuates between 600-900 RPM).

Thanks for your Help

find someone who has a GM TECH II scanner tool and knows how to use it and they should be able to find your problem. most places charge bout $80/$100 for this service and it is cheaper than just changing parts.

**Jon S.** · July 13, 2009, 03:50 PM

Re: 1991 Corvette Idle Problem

Originally posted by Joseph Adelizzi (41481)

Has anyone experienced this problem with there Corvette?

I own a 1991 Corvette with a L98 engine. Since this spring I have replaced the following:
Battery, in-line fuel filter, spark plugs, spark plug wires, distributor cap and rotor, idle air control valve, throttle position sensor, adjusted and set timing to factory specfications, Front Y-pipe w/pre- cats, Catalytic converter, rear y-pipe, and mufflers.

The car runs very well on start up; however after the engine warms up there is an idle issue. The RPM's can fluctuate 2-3 hundred RPM's. After 1200 RPM the engine runs very smooth (hot or cold) yet if I am traveling on a back road and the engine is operating at less than 1000 RPM again the RPM will fluctuate. Also on a number of occasions after the car is warm it will either not start (until it cools down) or it will start and run for a short time and stall. Yesterday, the car started, I backed out of a parking spot and the car stalled. Last week the car started, I drove out of a parking lot to a stop light and the ar stalled. I will need to let the car cool down and then it runs fine again until it warms up again. When hot the car idles at around 600 RPM (but fluctuates between 600-900 RPM).

Thanks for your Help

Sounds exactly like what my '91 did when I had some bad injectors. Pull the wire connectors off each injector, and measure the resistance across the terminals. Correct readings should be 15 -16 ohms. I would test cold, then warm up the car and test again and compare the results.

If the issue is bad injectors, you will have a few that measure below 10, at which point you may want to contact www.fuelinjectorconnection.com. Aftermarket injectors work the best, but they won't pass judging. This company sells both aftermarket injectors and original injectors.

The original Multec injectors are not ethanol-proof. As a 'wet coil' design, the fuel flows through the injector coils. The ethanol content in modern fuels deteriorates the epoxy that insulates the coils, and the injectors short out. Aftermarket injectors don't have this problem. If originality is a desire, then you should also ask them about stock appearing injector options. Another thread on the board talks about some of these that they were able to get for a ZR1. You can always replace the injectors with the same Multec part, but they are still wet-coil, and the problem is likely to happen again at some point.

**Ken A.** · July 13, 2009, 03:55 PM

Re: 1991 Corvette Idle Problem

New Delphi injectors (OEM) are now constructed to handle today's fuels.

**Joseph A.** · July 13, 2009, 04:00 PM

Re: 1991 Corvette Idle Problem

Thanks for all the info. I will try the injector testing you described Jon. If the injectors all pass, do you think it could be the o-rings that seal the injectors to the intake mainfold so you don't get a vacumn leak. If there worn out, could they also be causing this stuff to happen????

Thanks again for all the help

**Jon S.** · July 13, 2009, 05:01 PM

Re: 1991 Corvette Idle Problem

Originally posted by Joseph Adelizzi (41481)

Thanks for all the info. I will try the injector testing you described Jon. If the injectors all pass, do you think it could be the o-rings that seal the injectors to the intake mainfold so you don't get a vacumn leak. If there worn out, could they also be causing this stuff to happen????

No idea, but it doesn't seem likely to me. The other piece that comes to mind is the O2 sensor. When my car first started doing this, I followed through the diagnostic steps in the GM service manual. One of the things I discovered was that the car was running pretty good in 'open loop' mode after first start up. As soon as it warmed up and switched to 'closed loop' mode, it would run rough. That discovery led me to the O2 sensor. I replaced it, and it seemed to work fine for a few days, but then it started acting up again. It was then that I learned about the injector test. The O2 sensor didn't solve my problem, but it may be something to consider - although I'd definitely test the injectors first.

Originally posted by Ken Anderson (10232)

New Delphi injectors (OEM) are now constructed to handle today's fuels.

Ken - that's great info. I wasn't aware of that. Do you know if the new Delphi injectors are visually close enough to the originals to pass flight judging? Any idea on cost? When I did check on OEM injectors, I was told they were about $150 each, but I don't know if that was for one of these Delphi injectors you describe. I went with the Bosch Design III injectors, and was able to get the entire set for less than the cost of two OEM injectors - but originality wasn't my goal.

**Joseph A.** · July 13, 2009, 08:34 PM

Re: 1991 Corvette Idle Problem

Thanks Jon, I will test the Oxygen Sensor. Im sure those injectors for Delphi Ar going to be expensive.

**James W.** · July 14, 2009, 06:25 AM

Re: 1991 Corvette Idle Problem

Originally posted by Joseph Adelizzi (41481)

Thanks Jon, I will test the Oxygen Sensor. Im sure those injectors for Delphi Ar going to be expensive.

Just as an FYI, I just replaced the injectors on my 90 ZR-1 with Delphi injectors made up by Jon Banner (Fuel Injection Connection). They are nearly identical to stock and perform excellent! Cost was well below stock injectors, which won't last very long in today's fuels. If interested, contact jon at fuelinjectionconnection.com. he's very knowledgable and a great guy. He really worked with me to get the right appearance and function on my 90.

Jim

**Mark M.** · July 14, 2009, 07:28 PM

Re: 1991 Corvette Idle Problem

Joe, I had a problem just like yours, I had the car checked and it turned out to be a bad injector, as was mentioned before, if you disconnect the bad injector ( once you find the right one)it will idle better. I ended up replacing all eight injectors. Its a lot of work just to get to the injectors on the L-98, there are bolts that are hidden behind the intake manifold ,once you get the intake off you are home free, but getting to that point isnt easy.If you do the job yourself, make sure you don't forget the clips that come with the new injectors. In the end the car runs great now, I'd put money on your problem being the same. Good luck. Mark

1991 Corvette Idle Problem

1991 Corvette Idle Problem

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