My '92 Coupe (LT1 Automatic, no mods, 71K miles) has an intermittent problem that so far has eluded the fine mechanics at a very reputable Chevy dealership. Once hot, the engine will miss badly, sometimes. This is especially noticeable at idle, with the voltage dropping to about 10V for a split second as the rev's dip. It is also barely noticeable at steady highway speeds, feeling like running over small expansion strips. At its worst, the engine will stumble badly under acceleration and be very reluctant to rev. No codes are set as a result of this.

This happens randomly over time, with as much as a day between occurances (this is a daily driver). For example, I drove to another town about 100 miles away, not a peep all the way there. On the way back, ran terrible the whole way.

Alternator is a newly installed rebuilt GM, also have replaced the water pump and opti in the last 6 months (burst upper radiator hose took the opti out, water pump was a "why not now" replacement). Intake manifold was resealed and a missing exhaust manifold bolt was replaced (not on me, I am not the original owner!). Also have a new serpentine belt (another "While you are at it" with the opti replacement).

The car runs very strong when the problem is not present, and gas mileage has been the same as before the problem started. I have also tried different brands of gas (all 93+ Octane) to no avail.

Dealer has checked the opti, plug wires, TPS, O2 Sensors, EGR was cleaned out and checked, no detected vacuum leaks. Latest test was disconnecting the TCC as they thought the TC was locking. No luck with any of these.

Other than going back over everything for ECM grounds and vacuum leaks, are there any other common (or not so common) causes of this type of behavior?

 

This sounds like a bad connection somewhere or a bad ground.
With the drop in voltage I would look at the [ositive distribution block under the battery. Sometimes the battery breathes on this area and can corrode the connections.

 

I'm wondering if the drop in measured voltage is a Result of the RPM falling when it "misses" at idle - not the Cause of the miss. I'd be trying a different ignition coil next, then looking hard at the knock sensor. Driving it around with a scan tool hooked up (while it's acting up) Should find what's going on - as long as it's "upstream" of the secondary ignition circuit!

 

Hi,
Have you tried a seperate ground from motor to frame. Helped on some other cars. It's cheap and has helped a lot of people out. Eratic running, can be caused from an eratic, missing or corroded ground. Might be worth a shot. You could temp it at first, and if helpful, make it more permenant. Best of luck ,lbreen

 

My 1990 six-speed Corvette started an intermittent problem similar to yours. It got worse and worse over a period of about six months. Finally, the engine would run only 10 minutes and then cut off and wouldn't start up again for hours--sometimes not even until the next morning. A mechanic discovered that the computer had gone bad. He replaced it, which I could have done, if I had known to do that. The heat from the engine was causing the computer to malfunction. When the engine would run for only 10 minutes, he would take the computer out and put it in a freezer for an hour and then the engine would run for 45 minutes before it shut down. Your engine could have a problem unrelated to the computer, but I wanted to share this with you for your consideration. -- Eric

 

Looking around town this weekend for a new set of plug wires. I figure the originals have 13yrs and 70K miles, so it won't hurt even if it doesn't fix anything. Keeping a diary of the symptoms and its going back to the dealer next week to reconnect the TCC and investigate some of the items listed here.

I will post results, good or bad.

 

Injector's coil failing when hot
Just adding to the list

 

I just joined - and I notice that since yesterday, you noted your plug wires. Your original post said mechanic 'checked them' and I had this same experience with my 92 years ago, spent a lot of time and money chasing it down. Keep top quality and new-ish plug wires on at all times, they got hot and though maybe test ok, they go bad. I've never had a hiccup since keeping mine in good shape and making sure they're routed properly.

Curious to hear your story of how the new wires work or not.
dw

 

OK, several weeks have passed and I have not updated - I apologize. Number 4 plug wire was replaced, that took care of the rough running.

Obtained cable for OBD1 to USB, and DataMaster software. Still getting drop in voltage from ~14 to ~12.6 during braking for stop after long run and approximately every 45 seconds at idle. No consistency however, may go away for several cycles and then return. This lasts for only 0.1-0.2 seconds with no other odd readings in data.

SES light caused by occassional EGR system failure during prolonged steady cruising. This may occur as infrequently as every two weeks. I am going to ignore it for now, as the EGR system appears to be functioning normally, according to DataMaster, the vast majority of the time.

I am going to put the hunt for the voltage drop on hold. I don't have the time to allow the wire by wire examination that might be required to find it (and it is not causing any other symptoms that can be detected).

I appreciate all of the suggestions posted here. If it ever fails dramatically enough to be found, I will come back to this thread and post the cause.

 

When this happens, I'd say that your battery voltage is the 12.6 that you are seeing. I know that you have a fairly new rebuilt alternator, but I've known of lots of rebuilt alternators having quality issues. I'd get the alternator checked. Don't just have a simple test run, get it tested with EVERYTHING running. Turn on the a/c, lights, wipers, stereo, everything. It might also be a weak battery that the regulator is having problems with when recharging, but I'd guess that the alternator you replaced was not replaced with a 100% good unit. So when it is hot or under a load like with the a/c accessory running, or the brake lights all on full and rpms down, it cannot put out the needed power.

 

I would also be checking to see if there is a short to ground in the taillight system, which could be as simple as a bad bulb or socket.

 

I would think that a short to ground sufficient to drop voltage that much should cause a taillight fuse to blow. 12.6v is battery voltage. Even at idle, the alternator should be putting out 13v or above. Below 13v I always check the alternator under load. So far, it has always been a bad alternator.