I started this issue in another thread but it got confusing and didn't really go anywhere. I've made a little bit of progress but am stuck again, so I'll try to make everything clearer here.

First: I have reason to believe the distributor was damaged. The connector was plugged in backwards, and the engine was cranked with the connector like this. I don't know where the pins on the connector go, so I wouldn't know what systems would be damaged. This led me to replace the ICM, and the condition improved quite a bit. The engine is much smoother and much more willing to rev, and the engine gets hotter before starting to have issues. Also, the car would not idle without EST plugged in before, but now it does. Described below is what it's doing right now, with the new ICM:

Now it runs great until the engine gets hot. When its hot it doesn't want to rev past 2k RPM, it'll just shake the engine pretty violently and occasionally backfires from the intake.
I've checked oil pressure, base timing, verified TDC, checked TPS, CTS, IAC, MAP, all of that. Everything is working as it should. I have tunerpro and a cable and everything checks out on there, nothing looks out of the ordinary.
When I unplug the spark advance, the engine still runs a little rough around and above 2k RPM, but it'll rev past it just fine. It hesitates a little right around 2k but if I hold it for a second or so it'll climb up past it. The car idles fine both with and without the EST.

I'm suspecting other parts of the distributor might be damaged. I'm not really sure how to check it and I don't know what parts would cause this issue. It's noticeably better with the new ICM, which leads me to believe the ignition system is the culprit. If you have any other ideas I can check over those, but I've gone through most everything else on the engine.

I hope this is clearer than my last post, if anyone has any suggestions I would appreciate them.
Thanks

 

My understanding from following the other threads you started...the ECM is not going into closed loop. CEL code(s) would be a good place to start. Until you get it to go into closed loop, there's not going to be any long term fuel trims that may help clear up your issues as it's running off a default base "limp home mode" tune.

 

Alright, I’ll take a look at it when I get the chance. If the O2 is unplugged the SES light should be on right?

My SES light is not on even with the sensor completely disconnected. The ECM is getting a steady value around 450. From past experience driving the car using part throttle for a long time trips an O2 code.
I’ll try and follow the wire back and see where it goes

(there are no codes stored on the car at the moment, I haven’t reset them since the problem began)

 

I'm having a lot of trouble getting the O2 thing fixed. It's wired up in a real strange way, it looks like someone intentionally forced it into open loop operation, and I also have very little time throughout the week to work on it.
It's been like this as long as I've owned the car though, and it makes me wonder if it would really cause a problem.

The problem developed a little recently, so now the car doesn't run any better with the EST unplugged. It also won't hold an idle with it unplugged. I don't know if that's helpful at all or points anywhere.

(There are no CEL codes)

 

Alright, I know it's been forever. I don't want to start another new thread so I'm keeping it here.
I've been too busy to dig into the wiring on my corvette but finally today I got in there and I think I got the O2 sensor wired up correctly. Unfortunately I can't check it on TunerPro at the moment because I only have a desktop, I was borrowing my dad's laptop but at the moment that is unavailable. It'll be a few days before I can get the scanner on it.

The idle is way way smoother now. Before I could kinda hear the engine stumbling at idle and it was vibrating a little, although the actual RPM was staying steady. Now the car doesn't vibrate at all, and the engine is running unbelievably smooth. However, the problem is still there. Once you rev it up, it'll start shaking and won't get past 2k RPM.

 

Didn’t see mentioned but were injectors ohmed cold and hot ?

 

when u scan it it should telegraph from fuel trims and o2 running very rich or lean.

 

I have ohmed the injectors and they were in range cold and hot. I also used a socket extension to listen for the pintle clicking, and didn't find any dead ones.

I should be able to get the scanner on it tomorrow and take a look at fuel trims.

 

Just throwing this out there: I had an issue with rust debris in my gas tank that wouldn't show symptoms until I drove it for a while, then it would stop revving past 3k. Then 2k. What was happening was driving the car was causing the debris that were settled around the bottom of the tank to plug up the strainer and starving the motor of fuel. If I turned the car off for a few minutes the debris would settle back down but then the problem would start again.

I would definitely check your fuel pressure while the problem is occurring. It could be anything but it would at least be nice to rule out a fuel problem.

 

I didn't get to check the fuel pressure today because I got caught up with some other errands I had to get done.

But I did get a datalogger on the car and can confirm the O2 sensor is now working.
In closed loop the BLM went up to 160
The car is idling better than ever (in closed loop)

From what I understand this means the engine is super lean?
I've heard this could be caused by a misfire. This is all following an electrical failure which makes me suspect other electrical components not working properly.

I'll still check fuel pressure and such first, though, just to make sure everything there is good.

 

They're Bosch III's, reman from injector connection I bought 4 or 5 months ago.

I don't think I can trust my multimeter. I'm gonna get a new one and check again. (It's a cheap $10 multimeter and is showing signs of inconsistency)

 

What brand of ICM did you install? This is really important. Most any other than GM and maybe Davis Unified are garbage out of the box. That was 20 years ago. Those might even be questionable now. Might find an original looking one in a salvage yard. And the grease of course. The cheap ones fail at operating temps every time.

 

It's AC Delco
I just got it from RockAuto
And I did remember to put plenty of grease on it

 

They're yellow. I've been running these injectors with no issues for several months. If it was the injectors I doubt it's because they're the wrong ones

I don't have a fuel pressure gauge, I'll have to rent one and will update y'all on what readings I get

 

When you rent the fuel pressure, it would be great if you could tape it to the windshield and check the pressure while the car is driving under load. And I apologize if I'm stating the obvious but just make sure before you do that, that all the fittings are tight and nothing is leaking. I kept having trouble with mine leaking every time I'd prime the fuel system until I just went and got a good one. (Had bad luck with two of the Harbor Freight ones).

Anyways once you know how the fuel system is acting, you'll have a lot more clarity on the situation. You'll get to the bottom of this. Keep us posted!

 

Blm 160, maximum additional fuel being added to compensate for the extra oxygen your O2 is sensing.

This can be caused by a restricted fuel flow. Sometimes a weak pump, sometimes a plugged fuel filter. If the fuel injectors were egregiously plugged up or too small (the yellow bosch 3’s are good from what i have found, in the 22lbs version for 86-91 L98)….you simply have too much unburned Oxygen because not enough fuel is being delivered to combust all the O2.

Or, you could have a vac leak. Which would be adding too much unmetered air, the ecm calls for more fuel, trying to get back to stoich. Usually, this will also be accompanied by a higher idle, and, if you look at iac counts, they will be close to zero (closed).

Ive also seen where the exhaust manifold gasket is pooched, and the exhaust stream actually draws air into the exhaust and it then gets read by the O2 and tricks the ecm into thinking it is too lean…leading to ask for more fuel…yaddah yaddah.

 

I got a fuel pressure gauge.

Everything seems to be good
Priming the system is about 42PSI and holds,
Idling is around 38PSI or so both cold and hot
Revving the pressure stays pretty consistent right under 40PSI, even while the engine refuses to rev further.
I unplugged the FPR vacuum and pressure when up to 45PSI. Tried revving like this and no change.

I didn't put it under load because the car jerks so hard when I do that it feels like I'm gonna get whiplash, and it backfires so much it sounds like a really loud popcorn machine.


I don't believe there are any vacuum leaks, that was one of the first thing's I looked for. I only checked with carb cleaner around the intake gaskets, but I'm pretty sure it's all sealed up.

BUT I do suspect there might be some flow restriction from the injectors. I recently replaced intake manifold gaskets to solve a bad vacuum leak and may have partially obstructed the injector ports with RTV when I set the manifold back onto the block. The timing of this happening would make sense as well.
When I get the chance I'll pull the fuel rail off and see if there are any obstructions, or damaged injectors.

 

It was permatex right stuff, it does say sensor safe. It's got a brand new O2 sensor in it as well.

 

I think I found the issue

I took the intake and fuel rail off today and there is indeed RTV partially blocking most of the injector ports, some nearly halfway. I'm not sure how I'm gonna clean those out as I'd like to avoid taking the manifold off, but it'll have to wait until tomorrow anyway. I'll update when I get this done.

 

that is a first for me. I hope that’s your issue, it is easily solvable.