Ttime

I certainly don't want you to throw parts at it. It could be anything to be honest. You will need to diagnose everything that can cause this issue. It would be nice if some sort of code would pop (CEL).

LouisvilleLT4

The shop manual says there's no service interval for the fuel filter, so the only time you would ever need to replace it is if you fuel gets contaminated. I'm checking mine soon, but only because I am suspicious that someone poured sugar in the tank.

BlackbirdZ07

Ok, it started acting up again yesterday. Cutting out on the freeway. Both times it did it (yesterday and about a week ago), it was only after about 20 miles when everything had warmed up and it was also above 80* outside. I have a 160* thermostat, and the car was running at about 190*. When it was cooler outside and it did NOT have any problems even after driving a while, it was only running at about 175*. In other words, I've had the stalling and cutting out ONLY when the engine temp is above about 185*.

SO, I'm starting to think it's temperature related, although I still haven't driven it enough in different temperatures to be sure. Still no codes, and the temperature sensor seemed to be working perfectly. I had a problem with the ECM a while back that was temperature related, and replaced the ECM with a remanufactured unit. That was a few years ago, and I know they're not that reliable. Therefore I suspect the ECM. I touched it after driving and it does get pretty hot from being in the engine compartment. I'm going to try to find a hairdryer or some way to warm it up and try to test my theory.

ittlfly

Ryan..I was reading through your post again. I agree with you that it is probably heat related...and I noticed that you said that you had r & r'ed the coil and coil module awhile back. Did you use fresh grease on the back (against the flat mounting plate) to prevent heat sink? Those suckers are known to fail with load and heat. Just a thought....

BlackbirdZ07

Steve,

Yes, I did use the grease that was supplied with the new coil module. I actually replaced the old one a while back because the car stalled once and gave me a code that I traced to the tach filter and coil module. I replaced the module more as a preventative step and I kept the old one which should work ok for testing purposes. Couldn't hurt to switch the old one back in and see what happens I guess. Now I just have to find it! Thanks!

cerino2000

Man, this really really really wreaks of the opti and that is where my money is at.

STL94LT1

BlackbirdZ07

I don't disagree, but it'd be nice to have a way to be more certain before I change the opti and waterpump and hope for the best. Seeing as how there are no codes (opti usually sets code 16) and the opti is fairly recent, I'm just not that sure.

cerino2000

On my 94Z, you described the symptoms to a T. No code either and a bad distributor/cap is sensitive to heat.

ittlfly

Do you have an Autoxry or other data gathering device? YOu might leave it hooked up to the ALDL when your driving. When the car acts up again, you could take a "snapshot" of your sensors to verify they are all with range. That would also tell you what the ECM is 'seeing' from them at that time.
FWIW..the car will run with a code 36 from the opti...even if it is a soft code.

BlackbirdZ07

I'm in the process of getting an Autoxray. I know it can monitor sensors, but what about the opti? Is there any data from the opti or related to the ECM that would be helpful?

On the code 36... isn't that something that would be stored in the computer? I didn't get any codes except that code 72. Also, what's a soft code?

ittlfly

You'd need another type of scanner to monitor the opti output. I was suggesting the Autoxry to be sure all your sensors (and what the ECM was seeing) weren't malfunctioning.

A soft code will register with the ECM but will not keep the SES light "on". It is an intermittent code and may be purged from the ECM after a certain # of start cycles without the problem recurring. Have you tried to pull codes right after the car acts up? You may very well have a opti problem but throwing $ is at best a hit and miss proposition. A fuel pressure guage taped to the windshield (and a short ride) would help eliminate FP problems.

BlackbirdZ07

Ok, still giving me the same problems, but only when it's hot so it's definitely temperature related. Drove around with the Autoxray connected when it acted up. Everything looks normal EXCEPT that when it cuts out or dies, the SPARK ADVANCE goes to 5 degrees (from a normal of 20-25 degrees at idle) and the DESIRED IDLE goes from 662 to 1387 RPM. Actual idle RPM does not change.

Does this mean anything to anyone? To me it could be anything... just symptoms of an optispark problem or possibly the ECM doing something weird with the spark advance and killing the engine.

dpoelstra

I am having this exact problem with my '95 and am listening intently. Mine has thrown the code:

"DTC #15: Engine Coolant Temperature Sensor Circuit Voltage High."

twice after symptoms. My temp goes to 230 (normally 197 to 201) with a rough idle before the "bucking" as you describe. I noticed for the first time today the digital temp bounce momentarily from 215 to 145 a few times as these symptoms were raging.

I think I am going to change out the Temp sensor and see if that does the trick, but am very interested in more diagnosis. Sounds a little too common for there not to be a good answer. Don't think it's fuel.

dpoelstra

Come to think of it, my analog guage has a "flutter" sometimes. Looks like a loose ground or something, and I never thought much of it. Sometimes, especially when cold it just kinda acts crazy, the stabilizes as the heat builds. Cound this be a sign of a bad sensor, and a bad sensor causing the problem, or is it just all unrelated????

Hmmmm.....

BlackbirdZ07

Sounds like the coolant temperature sensor is causing your problem. In page one of this thread, member LouisvilleLT4 said his coolant temperature sensor failed and caused his car to do the same thing. Coolant temp was all over the place just like yours. Looks like you probably found your problem.

I'm not so lucky. My coolant temperature has never wavered, so I think my problem is something else. It's supposed to be 90* here this weekend, and I'm gonna go out and drive the car until it dies for good so I can figure it out! I'm tired of this intermittent BS. Hopefully by the end of the day on Saturday I'll need a towtruck!

dpoelstra

Changing the ECT completely eliminated my problems. I changed the sensor and the plug, and they both looked fried.

The sensor must somehow effect the analog guage, since the guage "flutter" that is always had had also been eliminated. I did not do anything but the sensor and the plug.

Since this is a quick, easy and cheap repair, I would suggest changing the ECT, and see what happens.

LouisvilleLT4

While we're still talking about this, I'll mention that I recently changed out my ECT sensor and my car is still having this problem. It seems, however, that I can trigger the problem by wiggling the wires where they go into the sensor. So for anyone with this problem, be sure to diagnose your wiring well before you go replacing sensors.

So anyway, I bought a new pigtail and am in the process of figuring out how to splice it in without adversely affecting the circuit's resistance. Any advice is appreciated.

dpoelstra

I changed the plug and sensor at the same time. Both looked fried and there were missing peices off the plug.

The new pig tail should come with about a 12 inch lead, so just cut and splice. I used butt connectors, but really should solder the connection and heat shrink it. i just didn't have any solder and heat shrink, so I will eventually redo that connection.

Easy and basic.

BlackbirdZ07

I'm happy to report that I fixed the problem. Hopefully this entire thread and the solution will help others who may have this problem in the future.

After checking all the obvious and not finding any problems, I gave up and took the car into a local dealer with a good Corvette service guy. They drove the car with a Tech 2 connected, and got the car to replicate the problem. However, it wasn't doing it badly enough to show them anything on the Tech 2. They were fairly certain it was either the coil or the optispark, although both of those parts were relatively new. Well, the opti is an expensive part that takes a lot of work to replace, whereas the coil is cheap and easy. I figured I'd take a chance and replace the cheap and easy part first and see if that would do it. So I bought a new AC Delco coil at the dealership and put it in myself.

The Corvette gods must have been smiling on me because that seemed to fix it. I drove it about 60 miles yesterday in temperatures approaching 100* here in Phoenix, and the car had no problems whatsoever. Not even a hiccup. So I'm confident that the coil was the problem and it's now FIXED.