i have a 91 wagon with a 4 banger in it. Its a manual.

the problem: when at idle the engine revs to 3k rpm and then goes back down and repeats. This is at a steady rate.

what i tried: I tried to eliminate sensors, although no CEL. When i unpluged the big sensor on the front of the throttle body the engine went to the idle it was supposed to go to and jumped up and town to 1k. When given more gas it didn't run right so obviously the sensor is important. I changed it out with a junkyard part and the same thing happens.

What could be the problem and what should i do to fix it?

 

no takers

 

Possible timing problem.

 

ignition??

 

replace your pcv valve

 

I'll have to check my Helm manual. I can't recall exactly what it might be, but maybe your air idle control valve. That sounds like what you may have disconnected. That usually gives a rough idle though. I'll take a look in the manual tonight if I remember

 

vaccume hose leak?

 

well polakatl- The manual points to the idle air control valve for all rough idle conditions. Beyond that, a fuel injector or vacuum leak. What was it that you replaced already??

 

i wish i knew what it was. here is a description:

its a big sensor at the front of the intake manifold. It has two hoses hooked up to it and when i took it off (3 or so bolts) there were two ports on it. One had a mesh screen.

i'll check the pcv. you just shake it and see if it rattles right?

I thought it might be a vacuum leak so i checked the hoses to my best ability.

 

roger.