I’ve got a 92 corvette that’s been sitting around for 10+ years that I’m gonna sell. The only problem I have left is a bad miss. I have checked fuel pressure but that’s about all. The miss will come and go and is more likely to happen under a load. Sitting still it dosnt seem bad, but sometimes it gets so bad it seems like a dead miss and is almost undriveable at times. Before the car was parked and sat the motor ran perfect. All help is appreciated because I can’t just run codes on this like a newer car and don’t wanna replace the optispark when it could be something else.

 

At night when it is dark pop the hood with it running. Look for the plug wires arcing on metal. Simple free test. Depending on how bad the wires are will be how bright the arcs are if any.

 

There could be a lot of causes, which are unlikely to be the opti, but could be. You can absolutely run codes on this car, and you need to. Chances are you will get useful info. Also, tell us if the problem is present right at cold startup, or does it only appear after the car warms up? If no codes come up, then I'd probably look at the coil being on a slow-death trajectory since you say it happens more under load. I think that most other likely suspects, including a bad opti, will show up in the trouble codes and/or can be narrowed down to bad sensors if the problem only happens when the car is warmed up (and goes into closed-loop mode).

 

Never thought about that but I’ll try it

 

[QUOTE=MatthewMiller;1604585641]There could be a lot of causes, which are unlikely to be the opti, but could be. You can absolutely run codes on this car, and you need to. Chances are you will get useful info. Also, tell us if the problem is present right at cold startup, or does it only appear after the car warms up? If no codes come up, then I'd probably look at the coil being on a slow-death trajectory since you say it happens more under load. I think that most other likely suspects, including a bad opti, will show up in the trouble codes and/or can be narrowed down to bad sensors if the problem only happens when the car is warmed up (and goes into closed-loop mode).[/QUOTE. the problem dosnt seem to be on just start up, for example…… I put Techron injector cleaner in it because car had not been started in a couple of years and drove it around, parked it for a couple of days. The next time I got in it I started it and let it warm up for few minutes and then drove the hell out of it around block a couple times and it never missed a lick…… So I thought problem was solved….. but no it was just one of the occasional times it runs good.

 

You got my question backwards: I'm asking if it ever happens right after a cold startup. I realize it happens once the car warms up. However, some of the sensors on the car aren't used by the PCM until the engine coolant reaches full operating temp. The oxygen sensors are one example. If the car runs fine cold but then starts running badly when warmed up, it could be a clue that you have an issue with one of these sensors rather than, say, an ignition part. OTOH, if the car sometimes runs badly right from the get-go after a cold start, we can rule those sensors out. IME it takes anywhere from 5-10 minutes of driving to reach a warm enough coolant temp that the PCM goes into closed-loop mode.

 

Those symptoms could also indicate that the MAP sensor going bad. I had one failing in my DD Buick and it would miss under loads and then progressively missed more and more. It should show up when you pull the codes if the MAP has failed or is failing.

The suggestion by topfuel67 makes sense and is a good idea to check. With your engine running after dark open the hood and watch for arcs. My C3 looked like a Tesla Coil light Show under the hood. Spark Plug wires are not what they used to be unfortunately. I use the MSD Super Conductor spark plug wires and my last set lasted less than a few months before I saw serious arcing and the power not getting to the spark plugs.

You might consider running a bottle of the BG 44K Fuel Injection system cleaner through the engine as it can help remove old deposits and can clean the injectors a bit. On my C4 whenever I notice any miss the first thing I do is runs a bottle of the BG 44K through the gas and see how much that helps. BG is one of the better fuel injector cleaners out there, it is used by many local mechanics and dealer mechanics as well. They make a more complete system that consists of three cans, it has the 44K and a Spray Throttle body cleaner and a can that gets sucked up by the vacuum system.

It is normally a good thing to start at the "simple things" and check them out before replacing the ECM or OPTI units. Having access to a good OBD1 Scanner might be of great help here.

"In the Open Loop mode the ECM is getting data from the following sensors: Intake Air Temperature sensor, Coolant Temperature Sensor, Throttle Position Sensor, MAP of MAF. The ECM will also receive distributor reference pulses and then trigger the fuel injectors.

For the engine to be able to enter Closed Loop "Three conditions must be met before the computer switches from Open Loop, as in Warm Up, to Closed Loop.
1. The Oxygen sensor must be switching back and forth around 450 mV, from above 600mV, to below 300mV.
2. The CTS (coolant) must be warm enough, typically above 158* (F)
3. A certain amount of time has elapsed on the Computers timer."

Referenced: Corvette Fuel Injection & Electronic Engine Management by Charles O. Probst, Bentley Publishers
By the way the above mentioned Book is an "awesome book" to add to your collection! It really helps you understand how the systems work together and covers all the engines used between 1982 thru 2001 in the Corvettes.

 

That post above by ctmccloskey is really informative, and that book sounds great. Somehow, I never knew about that when I owned my C4.

Also: Bravesc10, why did you name your car Miss Optispark?

 

How old is the gasoline in the car now?

 

I’ll get ahold a obd 1 code reader then

 

it’s got a full tank of topped of fresh fuel in it which I’ve drove and let run enough that I’d say the old fuel is almost gone or very diluted.

 

But also no there dosnt appear to be a difference in cold and warm though

 

Just for the heck of it, pull codes in History.
Jump A and G on the ALDL with a wire or paper clip. Then on the LCD it will give you H codes, History codes.
I say try this as the other way to pull codes doesnt go as deep, but is another testing of the system
I would also do as others, check condition of plugs , wires etc as this is basic troubleshooting.
Opti are a joy to troubleshoot, lol. 92/93s werent very good at surviving engine washer or leaky water pumps... Lots of people have had misses due to water getting the opti and ruing the signal.

 

Okay, then per ctmcloskey's post above, you should be looking at "Intake Air Temperature sensor, Coolant Temperature Sensor, Throttle Position Sensor, MAP of MAF." I think I'd look really hard at both temp sensors and the MAP sensor, since this problem isn't completely consistently present (temp sensors) and seems to get worse with load (MAP). But again, you really need to pull the codes first.

 

So I can use a paper clip the same way as a obd 1 scanner correct?

 

Yep.

 

https://tech.corvettecentral.com/201...trouble-codes/

 

When it's running poorly pull over and unplug the MAF sensor and see if it clears up. Been there a few times on various GM vehicles. I think you still may have bad gas in there.

 

Ok c12 is the only code I got, which means nothing right? I’ll try unplugging mad tomorrow and I’ll try to drive it some more and put more fresh fuel in

 

I believe C12 is no reference pulse which you would get if the engine is not running.

May I suggest removing all the old fuel? You will be sure the fuel is not the problem, Since it runs like it has bad gas, I would want to be absolutely sure it is not the problem, since the car certainly had bad gas in it from the get go.