Engine reving up and down at idle.
 

It's happening in a friends car. Began when he was at a stop light and the engine started to rev up and then down.. He says it felt like the car was gonna blow up.. He made it home but we can't think of what the problem could be.. I thought maybe the lcuthc was sliping but he has a fairly new clutch and the car is bone stock. I haven't had a chance to drive it but we started up before work and while parked in neutral it was reving up and p then down and then up again. throttle body was also just replaced.. and ideas?

thanks in advance.

...the not so obvious - clutch switch on the clutch pedal? I've heard people diag P FC idle issues say this can cause erratic idle if it's bad?

could it be the map sensor; vaccum leak somewhere?

i'lll look at that as well then .. thanks

do a test for error codes

Were black clouds spewing out of the exhaust. Sounds like the map sensor. Its what happened to me.

That was my first guess too

If the throttle body was just replaced, maybe you forgot to hook up the TPS sensor?

It could be your idle air bleed screw in the throttle body, I bet the new one isn't set at the same point the old one was and the ECU is trying to figure it out. Take off the intake elbow, screw the idle screw all the way in and then back it off 1/2-3/4 of a turn, clear the ECU and restart the car.. let it idle for 10 minutes and the idle hunting may disappear.

Could be a blown lower intake manifold gasket. Made my car hunt at any idle RPM below 1800rpm!!! :eek:

K

Yeha the new throttle bottle has been in place for a couple months and the car ran fine before yesteday.

TPS is pluged in.

Idle bleed screw is set properly.

checke map and lines are ok.

damn .. gonna have to pull out alot to check out the lower intake manifold gaseket. but will do since i'mm acheck all the vaccum lines as well.

thanks all for great suggestions.. I will check it out.