pete53

Hello,
I get a rough idle when stopped at a red light.I have 120,000 miles on it. It will try to run smooth but stumbles every 2 to 3 seconds (at the same time my voltage meter gauge will drop down)Does it worse when AC is on. Otherwise it runs OK.I have changed the TPS.I looked at a spark plug (could not get it loose) but am assuming they are fairly new by ceramic part condition but they could be original due to it being stuck or maybe anti-seiz was not used on threads. My spark plug wires look original due to they have cylinder number printer on them ??? I have also ran seafoam through gas tank. It also does not have a check engine light on.Any suggestions?

Thanks, Pete

Injuneer

Manual or auto trans - that determines the programmed idle speed?

What RPM does the tach show when idling, coolant fully warmed up? When you turn the A/C on, does the idle drop? If so, how low? If so, does it recover?

With the car in the dark, pop the hood and look for arcing on the plug wires and coil. Mist lightly with water, if necessary.

pete53

It has auto transmission. It idles at around 1000 RPM in park. When in drive and stopped it is around 750RPM. The RPM does drop (maybe 100 RPM) with AC on but recovers quick. I'll try the arc test when I get home tonight. I looked some at the spark plugs and plug wires and they are VERY hard to get too.Maybe I can get some pointers (without taking out half the engine) to get them swapped out if necessary....Oh and forgot to mention I have NO check engine light.

Injuneer

Stock programming for the auto trans, coolant fully warmed up, is 650 RPM in park/neutral, and 550 RPM in gear. The stock tach typically reads 100-200 RPM higher than actual. You need to put a scanner on the PCM to get the true RPM.

When you turn on the A/C the PCM should adjust the idle air control (IAC) valve to hold the RPM constant.

If your coolant temp sensor (CLT) is reading lower than actual temp, the PCM might elevate the idle speed if the temp is low enough. A CLT that is "off" doesn't set a code, unless its an open circuit or a shorted circuit. Or could be a failed IAC valve, and there is no code for that. Or the IAC pintle or the passages can be dirty.

And yes, you did mention no SES in yourfirst post.

pete53

OK thanks...I'll do the arc test then look at the IAC.

shoebox

At 120k, you can pretty much count on needing a tune-up (plugs, wires, maybe distributor cap/rotor if the rest of the opti is ok).

95_Z28/SS

Check fuel pressure. My car did the same thing but I have a 6 speed. The idle would just go up and down. And one day she just quit. Running wide open at 28 lbs lol. Fuel pumps. Gotta hate em