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Well, I'm stumped. Can't seem to figure this rough idle problem out. The engine surges between 400-800rpm and at times sounds like it's gonna stall. When I point a timing light to it with EST connected, the timing constantly jumps from 12-18 or higher.- replaced ECM- replaced dizzy- replaced injectors (Delphi 22lb)- replaced IAC- tested MAF- tested MAF relays- tested knock sensor- tested TPS- checked for vac leaks, none found
- checked ECM fuse, looks good- checked 2 spark plugs, one in each bank, they look ok to me - ran the car with the oil pressure sensor disconnected to see any difference, none- checked to see if harmonic balancer had slipped, it hadn't - tried all kinds of initial timing- all cylinders are firing, all header primaries get hot equally The sparkplug wires are all new (8mm). The wire from the ignition coil to the dizzy is older, 8.5mm. Not sure if that will cause any issues, I wouldn't think so. Ultimately, I set base timing at 8* btdc, and increased the minimum air screw until I got the rpm around 800-1000 in park on a warmed up engine. This also put the TPS at 0.63v. With this setting, the engine seems to run smooth and not surge. The car is noticeably more responsive when driving as well. Any thoughts what it could be? Am I missing anything?
The added timing probably helped with the responsiveness. GM went pretty conservative on the spark timing with these TPI engines.
On your rough idle...
Does it do this when cold? Or just after it warms up to a certain point.
What happens if you disconnect the O2 sensor? (you'll get a trouble code, but it'll force the car back into open loop).
It seems to idle rough at any temp. There are a few times where the RPM is stable at 750-800rpm for a few seconds. Then goes back to rough. I'll add, with the minimum idle screw adjusted to allow more air, it seems better. But I don't think this is a fix, just masks the problem, I think.
For the o2, I know it works when I data log with tunerpro. And it seems to idle rough in open or closed loop.
Big caveat to the log though - I have a 165 ECM, so the only way I can log data is with the 10k resistor, which affects my timing. Mind you, with the timing affected by the 10k resistor, the car drives like a bat out of hell lol, but I think some of the data may be inaccurate because of it.
Good point about testing the coil and the wire's resistance. I will check that. I haven't checked fuel pressure this year tbh, but last I checked, it was 43psi I believe.
The BLM from the log seems to indicate running lean, but I'll tell you, this thing guzzles gas, so to some degree, I don't believe it. Spark plugs looked ok as well. At least to me they did.
Your BLM is almost maxed out... hitting the mid-150's.
The times I've encountered that has been due to an ignition issue. In my case, a broken spark plug wire. Sends the BLMs up into the mid-150 range.
You end up with incomplete combustion which leaves a lot of O2 left in the exhaust, which the O2 sensor detects and reports back to the ECM, which then sees a "lean" condition.
So it starts adding fuel (in your case, a lot of fuel) trying to compensate and bring the AFR back to stoich.
Definitely check your coil wire and coil. Weak spark can cause incomplete combustion. Also, go back and check your ignition wires on the driver side. Make sure none are out of family on the resistance. I know you said they're knew... but check them for completeness.
The other thing to verify is exhaust leaks. If O2 is somehow leaking into the exhaust stream from the outside, it'll fool the ECM into thinking you're running lean. So counterintuitively, the high BLM has the net effect of you running rich potentially.
Your BLM is almost maxed out... hitting the mid-150's.
The times I've encountered that has been due to an ignition issue. In my case, a broken spark plug wire. Sends the BLMs up into the mid-150 range.
You end up with incomplete combustion which leaves a lot of O2 left in the exhaust, which the O2 sensor detects and reports back to the ECM, which then sees a "lean" condition.
So it starts adding fuel (in your case, a lot of fuel) trying to compensate and bring the AFR back to stoich.
Definitely check your coil wire and coil. Weak spark can cause incomplete combustion. Also, go back and check your ignition wires on the driver side. Make sure none are out of family on the resistance. I know you said they're knew... but check them for completeness.
The other thing to verify is exhaust leaks. If O2 is somehow leaking into the exhaust stream from the outside, it'll fool the ECM into thinking you're running lean. So counterintuitively, the high BLM has the net effect of you running rich potentially.
This all makes sense. I'll have to first check the ignition coil like you mentioned, to rule in out. I might just replace all the spark plug wires next year, to be sure I can verify.
I considered the theory that o2 is getting into the driver side header. I'll have to do a smoke test to confirm. But you'd think if this was the case, the engine would run pretty smooth in open loop since the ECM is "ignoring" the o2 sensor. Whereas, it seems mine idles rough in open loop as well lol. Theoretically if I disconnect the o2 sensor, I should be in open loop all the time and should notice the engine running smoother.
It also doesn't help to "fool" the ECM to a higher idle RPM by opening the minimum air position. That only results in the ECM pulling ALL the IAC steps trying to get the idle down to its calculated target, and makes the IAC very slow to respond to any other changes in variables. The result of the slowed reaction can be hunting, surging, stumbling at low RPM... Sound familiar?
It also doesn't help to "fool" the ECM to a higher idle RPM by opening the minimum air position. That only results in the ECM pulling ALL the IAC steps trying to get the idle down to its calculated target, and makes the IAC very slow to respond to any other changes in variables. The result of the slowed reaction can be hunting, surging, stumbling at low RPM... Sound familiar?
Vader, thanks for replying, I must have read hundreds of your posts over the last 4 years.
I agree, the minimum air position trick I'm using is just that, a trick to try and rule things out as well as be able to drive the car. I will say, with the minimum air screw letting in more air, the car doesn't surge or stumble at lower rpm. But it does have a harder time coming to a stop. It's not a fix, that's for sure.
This is why I'm trying to figure out exactly what the issue is.
09-08-2024, 07:51 AM
