89StangGTConvertible

My '89 GT idles at around 1,000 rpms, but it drops to about 850 rpms, then back to 1,000... it does this constantly while idling... should it be at a consistent 1,000, or is this slight up and down "normal"?

Bowtietraitor

What you have is called a "Rolling Idle" More likely than not it'll be emissions related...

Check for vac leaks, bad AIRB/AIRD Valves, bad check valves, vac test the EGR & check the EGR switch. Might be the MAP sensor, aould even be a bad AIR pump- though not likely. Sometimes a flatspot in a TPS sensor will cause this too. Get a good multimeter and vacuum set for testing. It might take a minute to find the problem, but you'll know as soon as you do.

Oh yeah- Idle specs:

Auto Idle: 575-725 (in drive)
Man Idle:625-675 (in neutral)

89StangGTConvertible

Thanks for the info, I appreciate it. It's a 5 speed, so I see now that in addition to the rolling idle, it's also idling too high... I recently purchased the car, and it only has 55k original miles, so I am hoping not much will go wrong...

Bowtietraitor

No problem. One more thing I forgot to mention- until you test it's accuracy ignore the tach. Factory tach's in '80's sports cars sucked. Check it with a scanner or external tach before you trust the one in the dash. Once you get that idle roll stablized if the idle is still high check the computer to see if it's the right one. You never know what the previous owner may have done (aftermarket, junk yard, etc...). Mine idles smoothly at about 610 (below spec) so the book specs aren't gospel to have a nice running car, but yeah- yours is a bit high. That makes me think... Check your timing while you're at it. That can screw things up too. Shouldn't cause the rolling, but might be contributing to the high idle.

I just bought my '90 GT vert last month & have been working on it since. Mine ran perfectly, but the interior electrical, instrumentation, airbag, and qtr windows were all dead. Almost everything is fixed now- I'm down to the A/C, the cruise, fix the seat lean, replace the back window & then detail it.

Mabey I'll give it a bath today & put some pics up...

89StangGTConvertible

Thanks again for all the info. Seems like you're making a lot of progress with your GT. 1990 must have been the first year they introduced air bags into these models, as mine is not equipped with one. I need to find a mechanic in Maryland who has experience working on these cars... I will need to do a lot of research! If anyone has a recommendation, feel free to chime in!

GT350son

I had similar problems with my 87 motor. Checked everything I could also replaced a lot of the stuff. Finally bought a timing gun and timed my motor. It was reading 30º ATDC. Yea... Did everything as above too. So if nothing above works check your timing.

AdderMk2

contact MJR46 here on the forum... he can steer you in the right direction.