Idle speed way too high
 

I had a problem where the idle speed keeps getting higher until it's unbearably high (2k rpm +).

Turning the idle speed adjustment screw made this ok for a while, but with it turned all the way down the idle kept getting higher. This happened over a period of weeks.

I cured this problem by replacing the throttle body with a 2nd hand part. This lasted for a good 6 months or so then the exact same problem happened again. I've just replaced the throttle body again and i'm back to a 900rpm idle as set in my MSPNP. I'm not sure if this will fix the problem for good or if it's just a matter of time before this happens again.

What is going on? Has anyone got any ideas why this is happening?


Car Details:

1994 Eunos Roadster NA8C
MSPNP

check for vacuum leaks first off. use brake cleaner and spray it all around the flanges of the TB.

Well the problem is now resolved, but I'm not sure how long it will last. If it was a vacum leak I'd expect the problem to still be there even after I replaced the TB? Unless the nuts holding the TB on have come loose over time?

ah, i read it wrong. i didn't see you replaced it again.

its possible the shaft seal went bad. I had the same issue once where i just couldnt get my TB to seal between the IAC and TB....I ended up just swapping it out

Some more info:

Ok I'm running a MSPNP and I idle around 13.3:1 I'll check the vac pressure at idle, but replacing the TB has fixed this twice so I'd be inclined to think its a problem with the TB and not a vacuum leak elsewhere. Surely if it was a vac leak the problem would still exist even after I replaced the TB. I also don't run a MAF.

The idle raised over a course or weeks / months. At its worst the idle was around 1800 when i first switched the engine on, and upto 2200rpm after a drive.

There was also a deep sucking noise present which has now disapeared after replacing the TB. It's a different noise from the noise it would make with me holding the throttle at 2k.

So you think its the gasket between the TB and the idle speed control valve?

I replaced my TB and caused a vacuum leak. Some of the TB to IM gasket broke off allowing air to leak in. You might want to check there. Of course do a check like Brain suggested to narrow down the problem.

The metal gasket betwen the TB and Intake Manifold is all in one piece and like I explained, it's now idling perfectly after replacing the TB. The reason I started the thread is because this has happened twice now.