All;

I’m more than a little confused with an issue with my ‘93. About a month ago, we were driving the car, and it started missing heavily, wouldn’t let me drive more than 5 mph, and finally stalled. Got the car to start easily, but with a fairly heavy miss. Managed to get the car home, and just before I arrived home, the miss wasn’t nearly as bad, but still could be felt. The only code set was for an O2 sensor failure.

I replaced the O2 sensors with new OEM oxygen sensors, but the misfire continued. Not too long before, I had seen a code 16 set, the Opti looked terrible, so since I was already looking sideways at the OptiSpark, I took the plunge, disassembled the OptiSpark, and found a failed distributor cap seal, evidence of heavy carbon tracking, and green terminals inside the cap, and rust, indicating a moisture issue. I purchased an MSD distributor cap and rotor (one of the vented type), cleaned the OptiSpark as best I could, reinstalled, and couldn’t get the old OptiSpark to hit a lick. So I replaced the OptiSpark with a Petris Enterprises unit. The plug wires looked terrible, the previous owner hadn’t done much more than slapped wires on the car, then used zip ties, electrical tape, and allowed them to rub on pretty much everything. I found numerous damaged spots on the wires, so I replaced the wires and plugs with OEM (Delco) parts, and spent most of the day routing the wires properly, and replacing the spark plugs. The previous owner had installed Autolite APP 606 plugs.

The water pump looked fantastic, no signs of leakage, and since I’m on a very tight budget, decided to reuse the water pump. There have been no signs of leakage since the pump was reinstalled.

So, I’ve replaced the OptiSpark, plug wires, and plugs...the car starts easily, but there’s still a noticeable miss at all RPM ranges, from idle on up. There are no codes being set. I discovered that my ‘88 IROC actually uses the exact same ignition coil, so I “borrowed” the coil from my IROC, and this also made no difference. I’ve verified for the umpteenth time that the plug wires are seated properly at the OptiSpark, and that the wires are connected in the correct order, and routed properly.

At this point, the only thing I’ve not been able to eliminate is the ignition control module...it it possible a failing ICM could be allowing the engine to run, but badly? The car starts easily, shows mileage around 27MPG on the highway, and is running at normal temperatures, and shows no codes or issues with my OTC 4000e scantool. My experience with other GM cars is usually the ICM either is working, or isn’t...but it does appear that the ICM on the car is original to the car...I have 98,000 miles on the car.

Any suggestions would be helpful...

 

You will not get a code for misfiring on OBD.
Could be ICM,fuel pressure,injector,running rich/lean ( i would look at long term fuel trim -13% +23% would be too rich or too lean ),excessive spark retard from knock sensors,cylinder compression,stuck/leaky valves,worn cam lobes,bent pushrod,worn rocker arms,broken valve spring.
I hope i covered most possibilities and i wish you luck in finding the cause.
I was lucky,mine was misfiring when i took it out this spring and found a chaffed spark plug wire and replacing them took care of it.
Hopefully someone more knowledgeable than i can help you out.

 

MaineJim has given you good advice. I had a very similar problem with my LT4. I used my tech 2 to troubleshoot the misfires. But it didn't point me to the problem as it showed almost all the cylinders were missing.
I replaced the Opti with a Petris too which helped but did not completely fix it. Changed out the ICM and that didn't work.
In the end it turned out to be my fuel pump that was new and defective. It was a new Delphi, it had the proper fuel pressure when running but would not hold pressure when the ignition key was turned off.
So apparently I had pressure but not enough volume.
I replaced the pump with a Deatchwerks pump and my problem was solved.
Unfortunately misfiring is difficult to diagnose. Check your fuel system next and also make sure your injectors are ok.

 

Thanks for the pointers...I’m not showing knock retard..in fact, I forgot to mention I replaced both knock sensors with new Delco parts, because I drained the block completely, and had to get aggressive with the old sensors in order to remove them. BTW, the OEM knock sensors are getting hard to come by..I actually had several orders canceled by sellers, and ended up getting my two from two different sources.

I bought a nice fuel pressure test set the last visit I made to Summitt...sounds like the smart money is on making sure I didn’t miss anything unusual with the scantool, and check the fuel pressure.

 

Are you seeing any minor backfiring through the intake?

 

That’s the interesting thing...no backfiring, even when I go full throttle. I did have minor full throttle backfiring before I began work on the car, but that seems to be completely absent now, after having replaced the opti, plugs, and plug wires. It almost feels like I have a dead cylinder...which makes me now wonder about fuel pressure, and injectors...but it seems unlikely all of that would have hit at once.

 

probably not the ICM, usually when that goes car is dead, crank no start. Make sure all of your connectors to the opti, coil, and ICM are in good condition. If you have have a timing light it's a cheap way to tell if you're getting the proper "pulse" from the distributor. At idle you should see a nice steady pulse about every half second or so. When my opti went bad the timing light was flashing like the car was running at 3000rpm even though it was idling. This was on 3 cylinders, 2, 4, & 5 if I remember correctly.

 

That’s a great idea..I have a nice inductive timing light I bought a few years ago to replace the antique SunTune light that was destroyed by flooding...I learned my lesson about storing stuff in the crawl space under my house.

 

I may have found the problem...the suggestion was made that there may be an issue with fuel injectors...a stethoscope shows all 8 are opening/closing, so I decided I’d keep the engine running, and pull the power to each injector, and see if there was any change in the idle. All 8 showed a drop in idle, and idle quality, but #3 showed some, but very little change.

I’ll do some additional testing, but I have a suspicion that #3 injector is partially plugged, and creating the misfire...fortunately, it appears that pulling the fuel rail on an LT1 is ridiculously easy...the fuel rail runs around the back of the plenum, and it looks like pulling the 4 bolts holding the fuel rail to the manifold is all that’s required to pull the injectors.

If #3 is simply partially blocked with junk, could it be as simple a fix as pouring some good fuel injector cleaner in the tank?

I should explain why I’m trying to spend as little as possible on this..the past 18 months have been like a bad soap opera. I bought the Corvette in January last year for my wife. I have an ‘88 IROC-Z she really likes, but she couldn’t get the hang of driving a manual car...so I had the idea of finding her a sporty automatic car to drive on nice days.

5 weeks later, she was diagnosed with Early Onset Alzheimer’s disease, and due to problems driving (she totaled our Equinox 2 weeks after buying the Corvette), she can no longer drive. She’s deteriorated now to the point she can’t even tell you what the year is, and a few weeks ago, she told the doctor I was one of our sons...we’ve been married 30 years.

In January, I had a heart attack, fortunately not severe, and just last week, 20 of us at work were laid off. Finding a new job is going to be difficult, as my wife now requires 24 hour monitoring (she’s staying at home with me)....and most employers aren’t too interested in hiring an older guy that has limited availability.

I had decided several weeks ago to sell the car, as I really can’t justify the continued expense, and losing my job this past week has only made the need more urgent—so, I’m trying to spend as little as possible, but still fix this right, in order to get as much as possible for the car when I sell it...let’s face it, no one really wants to spend much on an old car that has a drivability issue, so I’m trying to fix it, in order to sell it. I hate to see it go, but that’s life sometimes.

 

Sorry i dont know how to properly clean injectors,haven't looked in to it yet since i never had any trouble with them.
My '92 has 135k miles and if they ever start acting up i''l just replace em all.
I'm sure someone here can guide you on the clean up.
Hope that cures it and i wish you and the wife the best.
Sucks to sell the car but hopefully you will get it running well and get decent money from it.

 

I’ve ordered a Delco OEM replacement from RockAuto...it will be here Tuesday.

In different circumstances, I’d replace all of them, but all signs point to #3 as being the problem child. Fuel pressure is good, and I’m still setting no codes. I was shocked t find how easy access is to the injectors on the LT1...much easier than on my ‘88 TPI definitely.

 

Quick update...loaded up on fuel injector cleaner, hoping that since the injector acted as if it were partially blocked, cleaner might do it some good. Driven the car about 300 miles since putting in the cleaner, and the improvement is surprising. There’s still a very slight miss that disappears every minute or so when stationary in gear for about 20 seconds or so, and a slight miss when lightly accelerating, but once above about 15 miles per hour, it’s hard to tell there’s a problem. The car will cruise effortlessly at 80 MPH without issue, and at WOT, now that the OptiSpark, plug wires and plugs have been replaced, it pulls harder than it ever has in the 18 months I’ve owned the car. Honestly, I had no idea how badly the car had been running prior to replacing the ignition.

I’m still going to replace the suspect injector when it arrives from RockAuto on Tuesday...but I’m extremely surprised at the dramatic improvement I’ve seen over the past several days from the fuel injector cleaner. I’m fairly confident, based on what improvement I’ve seen the past few days that replacing the injector should completely resolve the issue.

 

Good news on the improvements.
Hopefully it will be 100% after the injector replacement.
Out of curiosity what did you use for a cleaner?

 

I used the Lucas Deep Clean Fuel system cleaner...it didn’t appear to do anything at first, until about 150 miles. I’m now running pretty much 100% gasoline, and while the idle isn’t perfect, it’s still impressive considering the cleaner is all I’ve done to it so far.

 

Ok,thanks and hopefully the injector will make it 100%.
Let us know after you replace it.

 

#3 injector was the problem child. Replaced the injector with a new OEM (Delco) injector ($75.00), replaced the O-rings on the injectors, since RockAuto had the GM O-ring kit for only $8.00, and put it all back together. It’s running like a new car.

Seems a shame to get rid of it now...it’s running better than it ever has. After having replaced the OptiSpark, which was bad, new plugs and spark plug wires, and now the bad injector, I’ve discovered that the LT1 makes quite a bit more power than I thought it could.

The OptiSpark definitely was failing, and I’m very happy with the Petris opti...having it vented like the later OptiSpark units is nice peace of mind that there shouldn’t be any further problems with the ignition.

Thanks to all for your good suggestions, and advice, they were very helpful!

 

Great news and yeah the LT1 makes plenty of power.
I even get 28mpg when taking it easy on the gas pedal,my driving here is almost all highway,no traffic lights and few stop signs.
Question now is,are you going to sell it?

 

Yes, I’m going to have to sell it. I’ve done quite a few things to it over the past year and a half, because I really intended to keep the car...in fact, I’m waiting for the LCD connector from Batee to come in, because the LCD display started acting wonky with all the potholes on the roads these days. I figured stuff like the miss, and the LCD display really ought to be sorted out before selling—I’ve seen way too many Craigslist ads reassuring prospective buyers “it only needs _____”. Most of the time, that’s complete BS.

http://batee.com/corvette/dcrg/servi..._lcd_conn.html