I've got a customer's car here that mysteriously started to idle around 1200rpm after he took a road trip. He pressure tested everything and found a couple of small leaks and fixed them. Car is at my shop for some new toys and I went over everything today trying to find the issue. No luck. No matter how the BISS is adjusted it idles at 1200rpm. Any thoughts? Car has no MIL on at this time.

This car sprays meth. Could the meth have damaged anything in the TB that could cause the high idle? I've seen Mazda TB's get damaged if brake clean was sprayed into them.

 

The part that gets damaged is the FIAV, since it has parts made of wax, or something similar. The typical failure mode for this part is to not close fully, which would cause a high idle. They don't go very often, and I wouldn't think meth would hurt it unless some liquid got in there. Maybe it condensed near the TB, or the location of the nozzle caused this, who knows. Given the infrequency of these going bad, I have to wonder if this could really be it. But I also can't think of anything else that would bypass enough air past the TB aside from the throttle plate itself to cause the BISS to be of no effect. To rule out the throttle plate, make sure it is colsing fully and the cable has enough slack.

It could be any number of things, I'm just thinking out loud here.

 

I've checked slack and that the throttle is closed and they both check out. It's a freak thing for sure but it's all I can come up with since the leak would have to be behind/getting around the TB to raise the idle.

 

I would swap out the TB to narrow it down to or rule out the ISC nad FIAV.