I have a nice low mileage (59k)1994 Corvette with an LT1 engine. It’s a garage kept nice clean red on black convertible. I have a problem that seems to be getting worse. Several months ago my car started idling funny. The idle speed was normal but about every five seconds it would get a slight stumble. Then the idle speed got faster, around 1000 rpm in park and would still slightly stumble. Then I went to accelerate fast one day and it bogged completely out, if I drove it with normal acceleration it was fine. Now the idle seems more erratic. I have not received any check engine lights or anything. I was told the 94 model has OBD 2 connections with an OBD 1 system so I’m not sure what code reader to even try. The Opti Spark Ignition was replaced about 15 years ago but it only has about 8,000 miles put on it since then. I messed it up when I accidentally got some antifreeze on it flushing my radiator, smoked it. I never wash the engine and I don’t see any leaks now. It started running rough and backfiring back then, a little different than what I’m experiencing now. The fuel filter has been replaced. I’ve tried injector cleaner additive. I’m not sure if there is a scan tool or something that could help pinpoint the problem and I don’t trust taking it to a Goodyear type shop with a 20 year old mechanic that does not know what an Opti Spark system is. I don’t have an independent mechanic yet since traditionally I’ve done my own repairs. I was a dealership mechanic in the early 80s but l lost touch with the newer technology. I don’t want to start swapping parts like the throttle position sensor, coolant sensor etc., because that could add up to a lot of money and not fix the problem. I thought about starting with the throttle position sensor however, still undecided. I really don’t want to swap out the out the Opti Spark ignition again because that’s a tough job. I’m not sure if it’s an electrical problem or fuel injector problem. I’m hoping some of you experts can guide me on where to begin to diagnose the problem, is there a scanner tool that would work on this car with OBD 2 connections? Should I try to check for engine codes through the dashboard even though I’m not getting a check engine light? Do my symptoms sound like a throttle position sensor or anything that commonly goes bad with these cars? I know it could be one of a thousand things. I don’t know who to trust to work on it and I don’t want to just start throwing parts at it any advice would be great thanks!!

 

The Opti serves two functions. One (the optical side of the unit) is to furnish camshaft position, velocity and acceleration data to the ECM. The other is to distribute the spark to the appropriate plug wire when the ECM triggers the module to open the primary circuit to the coil. If the optical side is misbehaving, the ECM should throw one or two codes, which should trigger the CEL. If the distributor side is misbehaving, you'd almost surely be experiencing misfires, crossfires, backfires, etc. Based on the symptoms you describe, I seriously doubt the unit is misbehaving. I'd look elsewhere -- starting with a fuel pressure gauge, taped to the windshield during a test drive.

If you don't have the factory service manual set for your car, I'd strongly encourage you to get one. Given that you were a mechanic "in a previous life," you should be able to make great use of the information contained, and learn to properly diagnose such failures as this one.

Good luck with it. Let us know how it turns out.

BTW, your car looks quite familiar.





Live well,

SJW

 

injectors or fuel pressure check ?

 

You have a lot going on @Tmlee2002 and you have the right plan, don't just start throwing parts at it. Proper troubleshooting is key. I agree with @SJW With no codes being thrown I would look at fuel pressure, pretty easy to check and a needed investment in a good fuel pressure tester with a long hose when owning a C4. May also consider pulling a few easy to reach spark plugs and see how they look. A really easy thing to do is wait until the sun goes down, turn out the lights in your garage, start the car and let it run with the door up and see if you spot any arc'ing around the spark plug wires or plugs. Making it as dark as possible will help make any arcs easier to spot. Let us know what you find.

 

Thanks, I will check into a fuel pressure gauge. Funny, I tried the dark room check already, no sparks anywhere that I could spot. Years ago I did that and saw a spark plug wire sparking off of the block, a neat way to find an ignition problem. The OBD 1 with an OBD 2 test terminal doe’s not help things. My uncle has an OBD 2 tester but I’m afraid to plug it in, I am afraid I’ll short something out because the pins are different. I don’t understand why GM did that.

 

I agree with SJW: I think it's not likely the Optispark causing the issue. I would also bet it's not related to fuel, or at least the idle issue isn't (the bog on heavy acceleration might be).

Question: does the bad idle occur regardless of coolant temp, or does it only happen after the car warms up?

 

The ALDL thing all GM cars that use programmable ECM/PCM use the 16 pin OBD2 style connector. OBD2 is 96 to present. GM usss OBD1 1980 Oldsmobile 260 Cutlass and in all models 1981-95. Light trucks depends on GVWR. Scanner wise you need a scanner that does GM OBD1 and 2 or and OBD1 with the correct ALDL connector and cartridge for atleast GM 94-5. EBay usually has tech1s and Snap On MT2500s for sale. You want to be sure they com with the correct cartridges.

Car will display flash codes. Corvette Central covers it in the DIY Section and there are likely YouTube videos. Opti cap and rotor could be a cause they serviced separately. Access to a "Sun Scope" could be helpful. Most of the diagnosis in FSM regardless of system requires a GM tech1 scanner.

 

Honestly the LT-1 isnt that special of an engine also found in the Buick Roadmaster, Chevy Impala and a couple of others IIRC.. shouldnt be a big deal for any decent shop to hook up to a scanner and pull out codes. Same with any other work needed on the fuel system or ignition system.....

 

check the sending unit/fuel pump in the fuel tank. takes about a 1/2 hour to pull it. there is rubber fuel line in there, could be cracked and sucking some air, lowering the pressure at the regulator. also check the screen on the pick up tube.

 

You can consider to replace fuel pump and tank gasket if you haven't done it before. Car is old pump too. And it had worked hard in the last 27 years. For about 50$ or less and one or two hours work you will get a nice job.

​​​​​​Also broken or almost gone pump doesn't make any code for obd.
When replaced mine I've solved a little stall problem I had on mine 1995. when I was stomping on throttle at low rpm high gear car was hesitating. Two years later pump died. This to say that wear is slow process

Most likely no a steady flow of gas was send to the engine

​

 

Thanks, I have changed C4 fuel pumps before and I remember it not being that hard of a job. I will definitely follow up on your advice.

 

Well done

Let us know if this will solve your problem