2001 Mercury Grand Marquis. 125K miles.
There's two issues here. The one I need to solve is the low idle in gear when warm. If I can keep it idling above 570ish hot, in gear, It won't set a p0305. I'll take care of the possibly low compression in the spring.
When warmed up (after a highway drive, even sitting in traffic), it wants to idle at 500 RPM. This has been causing a cylinder 5 misfire. I do believe that cylinder is a bit weak and won't fire well at that low of an RPM. If I do a clear flood and crank over, every 8th cylinder sounds odd so it's not a flat spot on the flexplate's ring gear. I would assume it's a weak cylinder? But that only manifests when it's around 500 RPM. The car likes to idle at 900 or so in neutral when hot.
I've squirted around with starting fluid, some propane and have not found any vacuum leaks. At idle (900-1000 RPM) in neutral, it's pulling 18.5 in/hg of vacuum. That's well within spec. Idling in gear, it's 13.5 in/hg at 500 RPM which seems normal. I'll try to smoke it this weekend in case I missed something.
I cleaned the IACV (and did the relearn) and that seems to have made the issue worse. Now if I stop hard (instead of coasting where it will figure itself out), it will drop down to 400 RPM for a second. Same happens when I put it in gear warm.
Tested the voltage on the TPS and it appears to be working 100% , the readout on my basic scanner also agrees with it working right.
There is no pinging / hesitation during moderate throttle acceleration with torque converter locked at low RPM. If I get on it too much it kicks down. But no pinging / hesitation during low RPM acceleration , and no pinging / hesitation at part throttle cruise leads me to believe EGR and DFPE are fine.
I believe the IACV to be bad. If I'm in traffic or a drive thru where it wants to idle low and rough, if I two foot it and lightly blip the gas pedal a few times, it will eventually compensate and adjust and idles at 570-600 smoothly and won't set the misfire code. And it will work until I take off from the stoplight / drive thru and come to a stop again.
Someone else tried a very basic way to figure this out before. It still had all 8 original coils but the plug in cylinder 5 was an autolite, the other 7 were original motorcraft. I also cleaned the MAF when I got the car as maintenance. It's something I do with CRC cleaner on my 2001 F350 with the 5.4 regularly.
- 8 new motorcraft plugs and coils - wasn't troubleshooting anything, just maintenance.
- Idles at 500 RPM, drops to 400 sometimes
- Cylinder 5 misfire
- Can't find any major vacuum leaks with starting fluid or propane
- If I tap the gas a few times idling in gear it'll figure itself out and idle normally
- Eliminated TPS as problem
- No pinging / hesitation ; don't believe EGR or DFPE
- Cleaning and relearning the IACV made it worse
- Adjusting idle screw and relearning didn't do anything
- Cleaned MAF with CRC MAFS cleaner
Obviously if I replace the IACV it's only going to be with a Motorcraft one. Not interested in aftermarket junk electronics.
If NY didn't require no CEL to pass inspection I wouldn't care.