Intermittent Rough Running Diagnosis

[tony-baroni]

Good morning! Thanks Stew for the tip, this community has a wealth of knowledge. I'd read that page at some point in the past, but didn't remember it when I was burning a chip. The offsets were correct though, since my size bin was the default Moates used in his SW.

This led me on a full search for Moates and chip burning posts. Using the moates guide, and realized I needed to use erase chip and verify blank steps before programming. Since the chip had been verified, this was a bit surprising, but that appears to be the only thing that changed which allowed it to work. Or there is something else happening that I haven't found yet.

I now have the stock chip burned and the vehicle running, with the codes cleared. It still has the problematic high idle and shaking. However, the INT and BLM values with the stock chip at idle and part throttle are much closer to 128. Knock counts are lower.

[vilefly]

Has anyone checked if the IAC valve is even moving in the real world? It should seat itself and open back up once upon KOEO conditions. Another problem is mismatched pintle styles that do not seal properly against the throttle body idle passage. They all swap. Try plugging off the inlet port to the IAC, and see if the rpm drops.....use a rubbermaid glove or something, whatever works. If no change, then remove and plug off all non-critical vacuum ports, remove the pcv valve from the valve cover, start engine, and block off the pcv vent on the other valve cover. Now block off the pcv valve grommet in the remaining valve cover with your hand, and see if the crankcase still has vacuum. If it does, then you have an intake gasket problem.

The only other thing is setting the tps to exactly .50v at closed throttle, and have the timing set to stock timing at idle (on the chip, too), but I suspect this has already been done.

[Kilroy]

Well read your whole thread and one thing comes to mind working on these vehicles, IGNITION MODULE. It will screw up your timing, intermittently cut off ignition signal to PCM. You will be chasing your tail blaming everything. The PCM rarely codes for the module. Also the trigger wheel , a hole burnt through ignition rotor causing spark to ground out through distributor shaft. But your files always show timing all over the place, IGNTION MODULE main cause for that. Also don't buy cheapo buy AC DELCO . Brand new I've seen same problem, good for a day or two same problem so you think module not the problem and its the cheapo failing. Hope this helps.

[tony-baroni]

I have an update, I've been hoping to review a scan before posting but my cable stopped working so I have another diagnosis to do!

I brought it to my mechanic, they confirmed the IAC was bad. SPR performance service has been great during this process, and they sent me a replacement valve, as well as tuning up the TBI.

The mechanic installed the IAC and confirmed that fixed it.

When I picked it up, it still had a miss, and as it warmed up and I got onto the freeway it got a lot worse. So much for paying them to help with diagnosis!

I discovered a youtuber, ScannerDanner, who teaches auto diagnosis. I watched a bunch of his videos, and worked though his basic evaluations for a miss. I diagnosed a bad spark plug wire on cylinder #5, which was also the spark plug that looked the worst. Now I know that's another sign I should pay closer attention to.

Fixing that had a really big impact on the performance. That was last Sunday, and I drove it 60 miles that day. I still felt a little something going on, but it's driveable now. I also am using the stock chip program, the shifting is much smoother. I'm curious to see how my mileage and my acceleration performance will compare.

Once I fix my cable I'll take some more logs and ask the group here what they think about the timing.

I will also look around to see what I can find for a test procedure for the ignition module. I'm disappointed in the AC delco performance spark plug wires, they don't grab onto the end of the spark plug very well. They have a lifetime guarantee, I plan to visit the dealer today with the one bad wire and see what they do for me.

[vilefly]

Ignition modules either have the 5v square wave HEI reference signal fail, or Fail to fire the coil while still output an HEI ref signal. 1% will have the Bypass circuit mess up and set a code for it.


The test for the pickup coil signal gets interesting, and can have random symptoms like rough idle, randomly dies, stumbles on acceleration, etc. The quickest test is to drive it once in a normal condition, then disconnect the "set timing" connector with the tan/blk wire, and drive it again to see if any symptoms disappear. If the idle quality does not change, either.....then the problem is not the pickup coil or a cracked magnet in the distributor reluctor. If it does change, replace the distributor or just the pickup coil if you are a gambler.