A couple of questions.

'85/auto. The car idles rough. Newer injectors, TPS set to spec, newer IAC motor, New EST module, Knock Sensor, New throttle body, New coil, wires, plugs, new battery, alternator. Newer ECM & PROM (stock.)

It's not just that the vehicle idles rough - at times it will lope rythmically between 500-800RPM - for minutes at a time, rythmically, as if someone was tapping their foot on the gas pedal. This happens both in gear and out of gear, but never off idle, just @ idle.

I have tried resetting the IAC per the FSM instructions (setting minimum air, etc.) And it does no good. The plenum, intake runners and throttle body blades/iac passages are all very clean, as is the IAC itself. The vehicle has absolutely no vacuum leaks, this was verified by feeding propane along the TB, intake & runners and watching for a change in integrator or block learn.

I have never gotten a code for a faulty MAF, but it's also the only part I've never replaced on the vehicle. I can't really monitor datastream while the engine is loping, as (on an 85) it enters "diagnostic mode" which raises the RPM to 1k and fixes spark. Exhaust backpressure is in spec, between 0-1PSI @ all RPMs. Newer O2 sensor, with plenty of crosscounts. New fuel filter & pump. Plenty of pressure @ all RPMs, even when the vehicle lopes.

Has anybody had any experience with a rythmically loping idle? Any suggestions of where to look? Is a bad IAC motor a likely cause? I also have intermittently sluggish performance - very sluggish, lacking power. That's why I'm thinking MAF. But I don't want to throw a $200 part at the vehicle.

Thanks,
Pablo

 

The IAC remains in a fixed position intil the ECM command it to move. Since the IAC is newer and all the IAC passages are clean, the only suspect *I* can come up with, in light of your exrensive checks, is the EGR valve. The EGR is suppised to be closed at idle. If the EGR is receiving vacuum (it's not supposed to) or has a weak spring it MAY be opening. Or it may be dirty and leaking. THAT would cause an otherwise undectable "vacuum" leak.

RACE ON!!!

 

Should've mentioned - the EGR is 2 months old, and as of right now (doesn't matter either way though) I have the vacuum to it blocked off.

 

Any modifications? What are the vacuum readings at both normal and the problem idle?

RACE ON!!!

 

mods: ported plenum, 52mm throttle body, SLP cold air, NOS, accel 24 lb/hr injectors, accel HEI coil/wires, delco rapidfire plugs. AFPR.

vac readings: if I can remember right, I had around 19 in/hg idling w/out the loping problem, and maybe 16-19 in/hg while it was loping rythmically.

 

one thing i failed to mention-

i plugged my mt-2500 in to monitor data stream while driving (even though it's not adviseable on an '85....because of the diagnostic mode, etc.)

i noticed that with the scanner plugged in it never lopes, and i have full power. i also remember something about diagnostic mode advancing spark. I honestly think i have an ESC problem - but the module is new, the knock sensor also new, and the wiring for the system intact & clean. I just don't get it.

could a faulty ecm be a possibility?

 

Its possible I suppose. Anyway to get your knock counts? Perhaps something is setting off your sensor.

Do you have a different set of injectors to try?

 

sounds like a vac leak. I had a problem like it. Mine turned out to be a leak around the injectors. I sprayed some carb cleaner around the injectors, the idle smoothed out and ran fine for a second or two. Could also be a timing issue.

Hope it helped