707vette

Here we go with another lt1 misfire. A little info on car...94 corvette, a/t.
Came into shop with an intermittent dtc 36. Driving down road, once in a while there was a skip to engine, setting code. Also had some detonation under high load/lower rpm.
Traced dtc 36 to intermittent internal opti problem. (msd unit, about 4 years old). Customer decides to change water pump at same time( not a bad idea.) New opti (msd) and new water pump, and we're up and running again. Or so I thought.
About 20 min after driving around, trying to figure out detonation issue( which by retarding timing on msd unit seemed to take care of), the new water pump let go. Pulled water pump, and opti, opened it up and was still dry inside. New water pump on, same opti, and thats when it got fun.

On startup, cold or hot, getting erratic misfires. pulled plugs(ac delco), looked ok, went with new tr55 plugs... new set of 8.5 msd wires routed correctly. New injectors( also swapped back old ones with no change). I've tried a new coil, and new icm with same results. The strange thing I'm seeing on scanner(as well as tach), is rpm spikes. I actually pulled both injector fuses and cranked car...... steady at about 150 rpm, then a spike up to 2-3k rpm. I have 12 volts switched at dist. connector, good ground, and 5 volts on each reference signal. According to alldata, I have EMI on reference circuit, and all it asks me to check is dist ground and routing of secondary wires. Fuel pressure is around 48-50 with adjustable regulator, new fuel filter, cleaned maf sensor/(runs same unplugged)

Distributor connector, coil connector, and icm connector all look good(no corrosion or broken/bent pins. i'm on 4th msd opti unit now, and talking with their tech line is pointless. Everything seems to be pointing to faulty signal out of opti, but i find it very hard to believe that 4 in a row are bad. Ive also tried a pcm from a 96 with no luck.

Not sure what the hell Im going to do next, any suggestions would be greatly appreciated. Thanks

antfarmer2

Is that fuel pressure a bit high?

pkincy

With vacuum hose detached spec is 41-47 although you would like 43.5. With vacuum connected it will be 5-7 psi lower.

I missed something....After the water pump died you have done 3 new MSD optis with no good results? Because with all you have replaced it sure sounds opti related.

Did you misspeak when you said you put a 96 pcm in? The 94 is OBD1 and 96 is OBD2. 94 only fools you with an OBD2 connector pinned out to an OBD1 pcm.

Given all you have done trying a known good 94 pcm makes sense.

Also replacing all electrical connectors to the icm and the opti makes sense also, no matter how good they look assuming you have a donor car around that is running well.

The other thing to check is a short to ground in the reference voltage. Both 5 volt and 0.5 volts. I inadvertently pulled the AC pressure sensor once and let it flap around in the car and every time that hit something metallic it would short out the reference signal. It wasn't until I scanned it and found a wonky TPS voltage that I realized the TPS was on the same reference circuit as the AC sensor.

Good luck with it.

scotth48

I thought the 94 still used aobd1 connector with the 95 having the obd2 connectorbut still working as a obd1. 96 for the first obd2.

I had a problem like this on my 93. Replaced the ignition coil module as the can overheat as the thermo grease dries out underneath it, along with a new coil.

antfarmer2

Nope the 94 has ob1 and a ob2 connector

707vette

Thanks for responses.... I have fuel pressure set on high side, but lowering to factory specs doesn't change anything.
The closest year pcm i have laying around is a 96, and it ran exactly the same, minus having codes for transmission and crank sensor, I'm still waiting for a good running 94 or 95 to come in so I can try pcm.

Almost thought i found it this am......i grabbed and bent wires on pcm side of distributor connector and the spikes seemed to stop. So, i got rid of connector temporarily, but after reconnecting, still seeing rpm spikes on scanner.
I'm a little suspect of icm connector, so I'm on to finding a pigtail harness for that.

I've thought coil and icm, but ive tried multiple coils and two different icm's with same results. Theres no tach filter on car right now, but i cant see that as being an issue.....or could it??

kimmer

I was having some of the same symptoms a while back and I tracked it to a bad connector to the icm, ordered a new pigtail and it was fixed, good luck.

707vette

So, after repairing bad connection at icm green wire to tach wire and installing new connector for distributor, rpm spikes are almost none. ordering a new icm pigtail as we speak.

so, I took it for a spin, and what do you know? the pinging is there with a vengence.

Its pinging under medium load, pretty badly. driving down the road my knock retard is steadily pulling about 4 degrees with no audible knock. When i make it ping, it seems to be pulling timing, but if I play with my foot on gas, i can keep it pinging.

Fuel pressure is good, new ngk tr55 plugs, no carbon buildup. coolant temp around 190 all day. it will ping after just starting car, (cold/open loop)so dont think its heat related. car has new injectors, fuel filter. I tried a knock module from a 96 and it went crazy with knock count just sitting at idle. driver side seems a little leaner than passenger as ltft are higher....

before all this low resolution craziness there were times it wouldnt ping at all, other times very badly.

going to try another maf sensor tomorrow and see how it goes

kimmer

Pinging like that can be caused by a bad egr.

Red 91

I had a bad knock sensor. You could pull them both and with a vom tap each and note the MV output of each one. If one has lower output with the same tap it could be your problem. Good luck!

707vette

Thanks for replies...
Egr valve is new, and working properly.
Ive checked both knock sensors, and they both respond to tapping block, also tried new one with no change. only strange thing i noticed is one of knock sensor wires has bare spot, but doesnt look like it was making contact with anything. I'm going to wait til icm pigtail harness comes in just to make sure i dont have any timing issue with the erratic low resolution... will update after new harness/connector installed

707vette

Ok... So I have a new icm pigtail, as well as new distributor connector installed and I"m about to burn this thing to the ground.

Still having erratic misfires, iac trying to compensate. I did get code 36 about 4 times yesterday, cleared it and came back, scanner showing no high resolution signal.... (chart leads to faulty distributor).

After about 4 starts with code, code went away on its own. sometimes when its misfiring the low res signal stays fairly steady, but im unsure if high res signal is erratic as well, since my tech 2 only shows yes or no for high res signal. still seeing rpm spikes/low res glitches on scanner....

Anyone else have any suggestions? I'm at a standstill

antfarmer2

Is the pcm still off the 96? Did you use heat sink past when you changed the icm?

707vette

No, I have the original 94 pcm on there right now... still havent had a 94 or 95 pcm i can try.... The 96 pcm was doing the same thing with the exception of throwing crank sensor codes

707vette

And yes, Ive spaced the icm as well as new heat paste, also tried a couple different icm's off good running cars.

antfarmer2

The 96 has a crank sensor yours does not what did you use for spacers on your icm?

pkincy

Signs are pointing to carbon tracking internally on the OptiPuke. I know it is a Pita the first time you change it but once done it is really pretty easy. I say that but I have changed my 93 F Body 5 times and have not yet changed the 39,000 mile 93 Corvette as it still is going strong, so I had a lot more room to get at the Crank Balancer and could get a pulley puller on it pretty easily. It is just a bit tighter with the crossmember on the Corvette.

And my practice has been to do a new water pump since that has to come out first, and also new wires as they all will come off the front and you can get them on a lot easier.

Good luck.

707vette

Everything seems to be pointing to corrupt low/high res signals from opti.....

But its the 4th msd opti i have on there......Hard to believe 4 in a row doing same thing, but i havent tried a different manufacturer yet...

Thanks again for all responses, sometimes it helps to get different perspective on it, especially after I've tried almost everything i can think would cause this

antfarmer2

Very well could be the opti. If you put non conductive spacers on the icm I would try takeing them out and clean then heat past there.

707vette

antfarmer2.....I just used washers. And I'm aware of why it threw codes, was still misfiring though...Not sure of any other differences between the 94 and 96 pcm except for crank sensor and trans codes coming up....
Don't want to waste money on a pcm until i have a 94 or 95 i can substitute