Irregular RPM Drop When in Park

[lazabby]

I have an automatic. Recently I started having an irregular RPM drop at low idle when I'm in drive. I can be at a light and the car will suddenly drop 200-300 RPM before going back to normal. Otherwise the car runs fine while driving. The RPM drop doesn't seem to do it when in park. The only work I've done recently was to replace the fuel hoses by the fuel pump (not the one in the tank) but I can't think that would have anything to do with it. Could there be a vacuum hose leak at the transmission causing the transmission to put a load on the engine? Or alternator slowly going out?

[DMCMW Dave]

I have an automatic. Recently I started having an irregular RPM drop at low idle when I'm in drive. I can be at a light and the car will suddenly drop 200-300 RPM before going back to normal. Otherwise the car runs fine while driving. The RPM drop doesn't seem to do it when in park. The only work I've done recently was to replace the fuel hoses by the fuel pump (not the one in the tank) but I can't think that would have anything to do with it. Could there be a vacuum hose leak at the transmission causing the transmission to put a load on the engine? Or alternator slowly going out?

1 - no
2 - not likely
Check to see if you have any engine compartment wiring running near the ignition distributor. Sometimes random electrical pulses, for example the fans coming on, changing the blower speed, hitting the brake pedal (turns on the brake lights) etc., will glitch the inductive pickup in the distributor and cause the coil to fire at the wrong time. Also look for loose igntion connections, plug wires etc. This sort of thing can cause a drop under load at idle (i.e. an automatic in gear) but be unnoticable at other speeds/no load.

[Bitsyncmaster]

Also could be a sticky idle motor. You can pull the idle motor, clean the idle motor valve with solvent then oil it. There are no brushes so it won't hurt to flood the idle motor valve with oil, just let most of it drip out.

[lazabby]

Thanks Dave and Dave. I'll check it out in the next few days.

[lazabby]

1 - no
2 - not likely
Check to see if you have any engine compartment wiring running near the ignition distributor. Sometimes random electrical pulses, for example the fans coming on, changing the blower speed, hitting the brake pedal (turns on the brake lights) etc., will glitch the inductive pickup in the distributor and cause the coil to fire at the wrong time. Also look for loose igntion connections, plug wires etc. This sort of thing can cause a drop under load at idle (i.e. an automatic in gear) but be unnoticable at other speeds/no load.

I did sit in the driveway for a while with the car in gear and it would so the drops even though the fans weren't on or any other activity. But there could be a loose wire somewhere.

[Spittybug]

Shannon.........you don't have an idle motor. You have an IAC valve, remember?

Capture that log file we talked about. I want to see if the voltage drops in conjunction with (or slightly ahead of) this event. Dave S.'s comment on stray electrical and the pickup coil has me thinking about the sync issues that you also continue to have. I'm wondering if your alternator is dying.....that could explain several things.

[David T]

I had one car where the idle had an odd bump to it. It turned out to be the reluctor in the distributor was not evenly gapped all around on each of it's points. Some were closer and some were farther from the impulse coil causing some "missed beats". Probably caused when someone replaced a bad impulse coil and didn't take the time to check the gaps. Any imbalance between the 6 cylinders can cause this, from a valve adjustment to a dirty injector to a spark plug not gapped right. A good place to start is a compression test.
David Teitelbaum