aboodsalman

Hey guys,

I have a 1992 corvette and am having some problems with it. When I first drive it its fine for the first 3 minutes or so. However, after that whenever I accelerate any faster than a prius I can hear the engine choking and the revs hesitate. I thought these were typical optispark issues but I've changed the following all to no avail...
- Distributor (Optispark)
- Coil
- Knock Sensor

Im a bit lost as to what to do and what the problem is. Does anyone have any ideas?
Thanks

dizwiz24

sounds like optispark.

they work fine when cold but once warmed up, thats where you get the hesistatiom and stuttering.

that said it is possible plug wires , a weakening coil, etc. could be the issue but i bet it is the optispark

Bwbob

Fuel presure?

Klondike

That is my thought as well.

It kind of sounds like you have a fuel pressure problem. I'd check the pressure at the port by the fuel pressure regulator. If it drops off when you throttle it up, you could either have a plugged-up filter or a weak pump. Also today's fuel with the 10% alcohol is not kind to the original Multec injectors. Jon from "Fuel Injector Connection" is a great friend of this forum and has a wide selection of good injectors and advice on installation. Of those three things, I think the filter is the most difficult to change. The injectors are maybe an hour and a half job and the fuel pumps on a C4 is one of the easiest jobs. (Seems like everything else on a C4 is a buried bitch to work on.)

If you changed the original Opti, I hope you didn't toss it yet. The original sensors in those are the best and they are getting hard to come by. Unless it's completely dead or the bearing is worn and wobbly, a cap and rotor usually will fix them right up. The replacement Opti's are hit and miss on quality in the electronics department, but can be made as good as the OEM if you have a good original sensor to transplant into them. Just clean the lenses with isopropyl alcohol and a q-tip and they are usually still in good shape.
Keep us updated on your progress and symptoms and there will always be somebody that has 'been there' to help.

JD Shredds

As stated above, this sounds like a fuel delivery problem. Look into the fuel system.

Ratboy

Ohm out the injectors. I think they should all be 15 ohms. I'm sure theres a thread here with the exact values, but primarily you want them all within an ohm or so.

yd328

The ignition control module could also break down, they are mounted to the front of the right cylinder head and require heat sink grease when mounted. I agree with checking the fuel pressure, it takes one potential problem out of the picture. Any problems idling or is it just under load?

Gary

MAKenna

https://www.corvetteforum.com/forums...osis-help.html For what it is worth I have been having a similar issue. This link is to my thread to show you what I have done so far and what was suggested FWIT. Maybe you will find something helpful. I have my back in the shop letting them have another crack at diagnosing this similar issue.

dizwiz24

the dichotomy of c4 model/owners becomes apparent on these posts:

l98 owners (familiar with the injector problems - that were fixed in the lt1 models) blame fuel delivery iSsues

lt1 owners - aware of optispark pitfalls know of the optispark problems that can cause the same issues

auggy

Odds are it's an ignition problem. Since the ignition control module is not very expensive($13 to $130 on Ebay) you should replace that. I am surprised you did not replace it since it should be located( I think ) directly above the coil at the right front part of the engine. If you are located in the MD. area. I will loan you one to try.

FAUEE

Agreed, check your ICM. Many people don't put thermal paste on them and they overheat. Sounds like the issue here.

JD Shredds

Yes and no. I'm an LT1 owner pointing to the fuel delivery. LT1's can have injectors and other fuel system issues. I've seen this exact symptom from bad fuel injectors on an LT1.

Also, the OP said he replaced the Opti. So there's that.

aboodsalman

Thanks for all the responses guys. I also replaced the fuel filter and fuel pump (both brand new), but i still had the same problem. I took it to the dealer, and they checked it out and said that the idle fuel pressure is 40 PSI. I'll see if i can check out the ICM and see if that makes a difference.

JD Shredds

Injectors? I've seen bad injectors cause this exact symptom.

dizwiz24

replaced the opti with what, a genuine ac delco NOS optispark (with mitsubishi sensor) - that we all know arent avail anymore ?

or did he get lucky and find an old man selling a spare nos acdelco optispark, in some tattered old box, on ebay ?

or replaced it with a rebuilt in china unit like what is most commonly sold and many are known to be broken right out-of-the-box (as i had this miserable experience once ) ?

aboodsalman

Latest update:

I just finished cleaning the injectors and that also didn't change the problem. Yes, the problem is just when I accelerate not when idling. And I can accelerate all I want for the first 3 minutes of starting the car but after that is when the problem starts. That's why I'm not convinced its a fuel problem.
The replacement opti was a Chinese one off ebay, although reviews from real-sounding customers looked good. But even so, my waterpump is fine and there is no evidence of dripping or anything like that which could damage the opti.
Also I did put some thermal paste on the ICM.

PLRX

Aaron Keating

It's probably your opti. The chinese replacements are known to be garbage out of the box. I'm not even fooling with an opti replacement for my Formula, I'm buying the torqhead 24x conversion next month.
Made it all of a year with the car before I had an optifailure with that car. Makes me glad my vette is a Real V8,(Gen1) all the more.

It COULD be your fuel pump however, and if you haven't done it, I highly recommend you change it. It's 20+ years old and is probably weak to start with. Once the pump has been running for a while it builds heat, that it cant fully shed...then it starts working intermittently giving you the shudders under acceleration (where you are sending more load to the pump). It's like a 15 minute job at most, and race tronix makes a hell of a 255lph kit for our cars.

FAUEE

Dumb question... but did you make sure all the plugs are good and tight, and going to the right cylinders? It's really easy to have the back 2 cylinders on each side switched. It'll still run, and will idle normally, but will not accelerate very quickly. They're easy to reach and swap, might be worth a shot...