After a couple runs at an autocross test and tune a few weeks ago, my stock FD started a high, fluctuating idle. With no load or AC, it will surge between about 1600 and 1800 rpm. When I put a load on, it will rise to 2000rpm and stay there, but smooth. When I put the AC on, it will drop to about 1500rpm and hold smoothly. When I hold the throttle at a constant position, it will hunt up and down about 300rpm, unless I turn on the AC, then it will hold smoothly. When I connect the GRD and TEN pins on the data link connector, nothing happens.
When starting from cold, the rpm will fluctuate wildly,from 1800-2500rpm, whether in gear or not, until it settles down to the pattern above.

I've checked for vacuum leaks, found a loose hose going to the Air Bypass Valve, but fixed it with no change. The TPS was adjusted last year and hasn't been touched, it's still tight. The car otherwise runs and boosts as normal.

Any suggestions out there?

 

what do you mean nothing happens when you ground the diag box? does the check engine light come on when you put the key in to the on position?

 

do you have a boost gauge? how much vacuum are you reading at idle?

have you check to see if your throttle cable is to tight or caught? also check the cruise control cable.

-bill

 

chances are that you have a vacuum problem that idle is way too high you should be pulling 5 inches below absolute vacuum which is 21 hg (around 16) stock vacuum lines are bad and should be replaced as soon as you buy a car with silicon lines if that doest work then your next two considerations are either false air( a leak into your intake side causing a bypass of your air flow sensor resulting in a too lean mixture, or the most dreaded and expensive a computer air fuel mixture problem. oxygen sensing could also be a factor

 

When I jump the pins in the diagnostic box, the idle does not drop, as the shop manual says it should. Also I'm not seeing any check engine light. I've checked the throttle cables, they are just a bit loose. I actually completely removed them and the idle was still the same. Vacuum readings are normal, around 16in.

I'm beginning to second guess the TPS. Today I noticed it was bucking and surging while driving slowly in 1st gear as well as with the cruise control on at highway speed. It feels a lot like it did when I mis-adjusted the TPS last year, so I'll have another look at that.