Hey, all. I have been having this issue for a few years and just started digging more into it this year (car sat for a few years). I had already ordered parts from the last time I was dealing with issues and have since replaced/done the following:

• Intake manifold gaskets, oil pan gasket, timing cover gasket and all three seals in the timing cover
• Opti (it took a bath last time I did some work so I had a shop check it out after the car started running rough and they said that was the issue) - I took it apart and inspected it and it was in bad shape
  • I used the guy on eBay that rebuilds and sends you a video of it on a scope
• ICM - not AC Delco - plan to have it tested
• Wires - found some burned so bought a new set and completely re-routed based on some other installs
• Cleaned the plugs and confirmed gap at .050
• Water pump
• Flushed the cooling system entirely
• New radiator
• Injectors rebuilt with all new seals/o-rings
• New MAP sensor seal on order - it's broken down and probably not sealing well
• Checked spark and getting spark to all 8 - I don't know the strength of the spark
• Checked fuel pressure - 40 psi at prime, 32 psi running, 41 psi with FPR vacuum line disconnected, have not taken out and checked pressure with a load on the engine
• Checked for leaking FPR and looks OK - Aeromotive

Yes, I have poured some money into it but it did sit for a bit and had some oil leaks that needed fixing. I'll probably replace the rear main seal over the winter.

The SES light is on so I have a cable on order and will be downloading scan9495 to get this stuff worked out. One thing I noticed (noted above) is the seal on the MAP sensor is shot so I just ordered one today and it should be here next week. I did a quick run through on the vacuum lines and some are a little rougher than others and probably could use replacing. Any recommendations on where to get these for a 25 year old car? I also plan to further inspect the injectors and fuel pump (Walbro 255 lph), I suspect something going on with fuel because it takes three to four cranks of the key to get it to start. I also plant to check out the EGR valve based on threads I've looked at.

Well that's what I have for now. I'll post the scan file as soon as I have it. In the mean time, I'll check back here for suggestions. I know this is not something new for these cars based on what I've read but I won't be shelling any more cash out until the issue has been root caused.

- Thanks

 

How fast does the fuel pressure drop from 40 PSI after the pump primes and shuts off? Fast loss of pressure is a common problem as the cause of extended cranking at startup.

Your idle/no vacuum line fuel pressure is at the lower limit of the GM spec. The Aeromotive AFPR is often a problem. I would never use or recommend.

To clarify, you have a problem with misfires at startup, but not when the engine warms up. But after 20-30 minutes you have an erratic idle? Does “erratic” include misfires, ot just uneven RPM? And this problem existed for a few years, until you stopped driving the car. And even after all those new parts, the problem is still there?

 

Fuel pressure drops to zero after prime immediately, I assume since it's Aeromotive. I put it on in my younger days thinking it would be beneficial with my ported and polished heads. The FPR doesn't seem to be failing the test but, based on what I've read recently, it's quickly moving to the top of the list as suspect parts.

I hear a misfire in the exhaust and it is all the time. The erratic idle kicks in around 20-30 minutes after running and the revs start going a little crazy on their own. It will eventually die after being erratic for long enough. At least it did the first couple times I ran it.

I stopped driving it a few years ago because it was dying after it would warm up and I started on a kitchen project so car fun went on hold. I recently installed all the new parts I had ordered and this is the latest on it. I guess it's gotten better in that it doesn't seem to die so quick now but it has a miss. It seems to have gotten better the last few times I've started it but it's definitely not where it should be. It's worth noting that it does have old gas in it.

 

OK, here are the log files. I scanned during the erratic idle also, it started sooner today. Thanks for your help!

 

Log file 190623 during erratic idle.

Thanks - Scott

 

I'll take look at the logs tomorrow. But just glancing at all three of them, you have DTC 36 for the high resolution pulse signal from the Optispark distributor. In theory the engine can run fine without that signal, with a small increase in spark scatter that you might never notice. But in practice, it can cause erratic idle and misfires.

Clear the code (pull PCM BAT fuse in under hood box for 30 seconds), wait until the engine is cold to start it, to see how fast and when DTC 36 comes back. Could be the result of heat soak affecting the optical module.

 

The long term fuel trims are really messed up, and in open loop, the PCM can't use the STFT's to do additional correction. Bank 1 (left) is running too rich, Bank 2 (right) is running too lean, but the LTFT is maxed out at 160 (+25% fuel).

I noticed both fans are running at 140*F. Do you have a switch?

Do a more complete log so I can see what happens when it goes into closed loop. Here's my guideline. Note - do not rev the engine while it's warming up.

DATA LOG GUIDELINES

Do not reset PCM. Do not remove a battery cable. Do not pull the PCM BAT fuse.

Start COLD engine. Do NOT turn on A/C or Defrost. Turn off traction control (ASR / TCS) if vehicle equipped.

Leave the accel pedal alone. Do NOT rev the engine while it is warming up. Leave it alone.

Allow engine to idle for at least 4 minutes. Should be long enough to warm and up put PCM in closed loop.

Start driving car for about 10 minutes. Vary the accel pedal position and RPM. Make at least one WOT pull from a roll up to at least 5,000 RPM.

Mentally note (or have a passenger write down)when the engine exhibited the problem you are chasing.

Stop driving, let it idle for about 30 seconds, without touching the accel pedal.

Finished.

 

Ok, thanks. I'll run through the test sometime this week, probably tomorrow.

 

I just realized I have 5 different people I'm simultaneously trying to help (4 here, 1 on another LT1 site) with data logs at the same time. I'm going to move this to "LT1 Based Engine Tech", so all the data log requests on this site are in the same forum. Please be patient if I don't get back to you right away. These can be very time consuming.

 

OK so I finally had time to run the full data log and my car is starting to act up much quicker now. I was not able to complete the data log to the full extent. Attached is what I was able to get. The second one is after I pulled back into the garage and reset the codes. I thought maybe I found a loose plug wire before the run so I wanted to reset after the run to see if anything changed.

One thing I noticed is the tach will jump around and the AIR pump and fans cycle on and off even before I start the car sometimes. The car did sit for a while before I started noticing these issues so I may just have something as simple (not so simple to track down) as a chewed wire from a critter.

Thanks for your help.

 

I think this is a new record for the most trouble codes I have ever seen - 11

16 - Opti Low Res (stored, since if active, fuel system us turned off)
18 - Injector Fault(s)
34 - MAP Sensor Low Voltage
36 - Opti High Res
41 - ICM Open Circuit
42 - ICM Short Circuit
46 - PASS-Key fuel enable signal from theft deterrent module
48 - MAF no signal
51 - PCM EPROM ERROR
55 - Fuel Lean Monitor
67 - A/C Pressure Sensor

Looking at your earlier logs (4 of them) 3 of them only had DTC 36 (Opti Low Res), and the 4th only had DTC 34 (MAP Low Volts) and DTC 36. The PCM is suspect at this point. This is rare, but in the past week someone had a faulty PCM that was causing the ICM to fire the spark plugs any time the ignition key was turned to "RUN". Most of the current codes have to be stored, because several of them, if active, would not allow the engine to run.

The extended cranking is the result of the rapid loss of fuel pressure after the pump primes. You indicate the pressure drops from ~40 PSI to 0 almost immediately. That may or may not be the Aeromotive AFPR. Could be the check valve in the pump. You indicate that you have seen recent negative feedback on the Aeromotive. I've been warning against it for 20 years and it continues to be a problem.

The erratic idle and surging looks exactly like a problem I discovered with the MSD Opti a few years back. The RPM reading shown in the log instantaneously jumps up several thousand RPM (as high as 5,000+ RPM) for an instant (1 frame of data). but the throttle hasn't moved (still at 0%) and the MAF flow reading hasn't changed. Has to be a faulty signal from the Opti, or a spurious high voltage induced pulse in the wiring from the Opti to the PCM. Unfortunately, when the PCM sees the RPM spike, it does what it is supposed to do, and opens the IAC valve up to prevent the engine from stalling if the throttle blades suddenly closed. The IAC keeps getting opened up more and more, and the idle RPM elevates and wanders all over the place.

When I showed the problem to MSD, they initially blamed the IAC, but they finally came to realize it was their Opti cap and it was a production problem that apparently existed for a couple of years. They were providing new (or rebuilt??) caps, or complete Opti rebuilds to owners with the RPM problem. I know you don't have an MSD, but the cap is suspect. What I believe is happening (MSD would never tell me the whole story) is the cap is allowing arcing of the internal wires that route the button contact inside the cap to the plug wire terminals on the periphery of the cap. The optical module or maybe just the wiring, picks up the high voltage induced pulse from the arc, and screws up the signal going to the PCM.

You might want to contact the ebay "opti doctor", show him the data log (the long one, not the short one) and my theory, and ask him what he thinks. He is rebuilding the optical module as I understand it, so it is probably not a problem he caused, but it might be interesting to see what he thinks of my theory.

At this point, clear the codes (pull "PCM BAT" fuse in under hood box for 30 seconds), run the engine again, and lets see what codes return.

 

OK. Here is my latest log file. I didn't drive it this time, I just pulled the fuse for 30 seconds, plugged it back in, fired it up and ran the scan. DTC 36 came back as soon as I started it. It ran ok for about 10-15 minutes then starting the erratic idle issue. I let the scan go for a while to capture as much as I could. There were a few other codes thrown but they didn't hold, they were only read at one time increment each then cleared.

I also took some cell phone video I'll try to attach later.

I'm going to send the eBay opti guy a message this weekend along with my data log to see what he thinks about your theory.

Thanks again!

 

It's idling smooth.... everything is perfect...... until the scan time reaches 676 seconds. Then it goes crazy, with the RPM jumping all over the place, as high as 7,106, a fraction of a second after it was indicating 1,000 RPM. Throttle doesn't move, MAF doesn't show a major change.

At about 964 seconds, did it stall and you restarted it 9 seconds later?

When it starts the erratic idle, is the tach needle jumping all over the place..... like from 1,000 to 7,000 RPM, or is it indicating the engine is still idling at a fairly constant RPM?

This is a bit different than the earlier problem I detected with the three MSD Opti's. The PCM is not responding by raising the IAC counts. But the injector pulse width instantly drops down.from 3.5 mSec to 1.5 mSec when the erratic RPM problem starts. It's like someone flipped a switch. MAP also drops from 33 kPA to 28 kPa. I would think with the erratic idle, MAP would increase.

See what the Opti doctor has to say. The DTC 36 definitely shows there is a problem with the Opti.

 

Yes the tachometer starts jumping all over the place when the rough idle starts. I don't recall it going all the way to 7K but I suppose I could have missed it.

Yes it did stall once and I restarted it.

The eBay opti guy has acknowledged that MSD stopped selling the caps for a bit. He actually does rebuilds with the MSD caps (I should have been more clear) and said he has had some issues but it's rare, apparently one guy had arching right through the cap to the water pump. He offered to rebuild mine again and send it back to me. I was really hoping this wasn't the issue because now I have to tear it all back apart but at least the opti guy is working with me and I have some solid input from you. It seems like this is the best next step. Please let me know if there is something more I should do to root cause.

Thanks!