richardgharris

I removed the egr valve and tested it by seeing if it holds vacuum which it does. It did not however close all the way to allow no air through when closed, I could blow a bit air through it when closed. I cleaned it but it looked petty good! The solenoid I tested by putting 12 volts to it and heard a click, blew air through it and it passed air freely when opened. The car is doing different things now. It is very sluggish at this point and feels like it is running without all eight cylinders. I am not sure were to go next? Since it does not feel like it is running on all eights at this point, I may check the plugs next or at least a couple to see how they look?

richardgharris

I replaced the plugs today. I decided on NGK brand. I am not finished with the job as of yet. The plugs are in and torqued at 12ft/p. The plugs looked pretty good that were removed. All the wires were AC/Delco and looked like new. Number 1 cylinder looked to be not firing well, had allot of carbon on it. All the other plugs looked good. I did a compression test on 1,3,5,7 and 2 and 4. They all were over 180 except number 2 which was 170. Since 6 and 8 are impossible to get the compression tested on I decided not to test them. Also the fact that all the others were all over 170. I can put to rest the thoughts of having an major internal problem with the engine. I will check all the connections and put the boots on the plugs tomorrow and see were we are with a test drive. After this repair if there is no change I am back to the fuel system being the problem! As always any thoughts are welcome!

Rex Ruby

Years ago I had a miss that drove me nuts and I replaced a lot of parts until it was fixed. Turned out to be a bad ECM.
It's either fuel or spark. Since you have you have good fuel pressure I'd lean more toward spark. I know the PO did a new opti but that doesn't mean that new opti is good. There are lots of reports of new OEM units being bad and aftermarket ones being bad too. If you aren't throwing a code 36 (opti) I would replace the ECM.

richardgharris

I am leaning toward the ECM or PCM? Today I rechecked the fuel pressure this time under load with the gauge taped to the windshield. Remained good 36 to 40 ish all the time, held pressure after shut down. I feel a bit like Alexander Bell when he discovered the Telephone! He failed 10,000 times but learned what did not work which was brought him closer to the solution!!!! Before I have the PCM rebuilt I am going to check to see if the cats maybe blocked. I am going to removed the O2 sensors and see if that helps. Once I have done that I may clean the throttle body. It seems to be coming down to the PCM. What is puzzling is there are no codes being kicked and it runs great for about a three minutes before the problem starts???? After that fails to fix it I am going remove the PCM take it apart look for obvious burn out, remove Prom reassemble reinstall and see what happens. Then I will have to replace the PCM either rebuild mine or buy one repaired. I am hoping someone can just repair mine so I can put it back in without having to have it flashed? Off to the garage!!!!!

richardgharris

Well I am 90% sure I found the problem. Clogged Catalytic converters. I am not 100% sure but with the O2 sensors removed it ran smoother, had no pinging, no backfiring, and no stumbling! I did not get on it because of the added noise. When I put on the AC it bogged a bit but I think the added load was too much for the tiny holes I created by removing the O2 sensors. Tomorrow I will start to remove the exhaust system, disassemble it and inspect it. The mufflers and pipes are in real good shape on the outside. we will see how they look on the inside?

richardgharris

Removed the whole exhaust system today, what a job for an old man! The Cat brackets above the cats were the hardest part. Was not sure it could be done but I removed them. My question is this:

I expected to see blow out cats but I did not see any evidence of it. The screens were at both ends of each cat so they could not be inspected on the inside. My question is there a way to tell if they are good or bad? I have to make a decision soon so I can get this thing back together. My other question is, what are good replacement cats for the 1994 corvette. The flowmaster 2010013 and 201112 look like a direct fit.They are around 400. Are there any others that are good and may be cheaper? Thank you.

car addict

I don't really know. Would think flow of air through converter would be tip off, but just guessing here. Suggest you contact forum member VikingTrad3r who recently replaced cats on a C4.

Corp Z

Have you checked/changed the Ignition Control Module and the coil? Also try cleaning checking the Maf?

richardgharris

They were changed out by the dealer before I bought the car with no change in the performance
.

obijohnkenobe

If it runs good in open loop, and not good at all in closed loop, then it has to be a sensor. I'd look at the O2 sensors on the car... the ECM gets mixture information from the O2 sensors and adjusts on the fly in closed loop. If the car runs well in open loop but not closed loop, then the only two things that can change are fueling and timing.

Before I would keep throwing parts at the car (expensive and no guarantee of fixing it), I'd get a data connector from the OBD1 port and run a data log to my laptop. This will cost you well under $100 and you can see what goes sideways when the engine warms up... and that would be a real clue to where the problem is. This cable from 1320 Electronics will work with a Windows 10 laptop and TunerProRT. Or you can go to the same site and get the Bluetooth model that will work on your phone or tablet.

Stop guessing, get some data. It will be a lot cheaper.

richardgharris

I might be on to a possible cause for the performance problem. I will discuss it here but also start a new tread. When I removed the plugs and changed them I noticed number 1 cylinder was black with carbon, not ash like all the others. That being said I went out last night and started the car to remove the H64 code again. As I was looking over the engine in the dark I noticed number one or two spark plug wire sparking, arching to the exhaust manifold heat shield. It got worse on acceleration. I then went to look at the receipts of the work that was done on the car. Even though they had replaced the opti at the dealership I could see no were that the ignition wires were replaced. I believe the wires are original. My question is this. If number one and or two is misfiring the O2 sensor would pick that up as a rich or lean code 64. Send that code off to the computer and computer would change the fuel mixture and the car would run like crap. Is this a possibility?????? I ordered new wires but they will not be here until next week so all I can do is think about the possibility that I have found the problem!