1992 Jetta 8v, Gasoline, Idles High

[The Horak]

My 1992 Jetta 8v 5-spd idles at about 2000-2500 RPM when it's cold, but within a few minutes it drops down to around 1600 RPM. At times, I'll get off the highway and the RPMs will be back up around 2000. My gas mileage has gone down considerably; from 31 MPG+ to around 25 MPG, and I lost power. I just changed the oxygen sensor, and I think it helped with MPG and power, but it now idles higher!
I don't know enough about cars to know what this could be. All I know is that these problems literally occurred overnight (a cold night, though). Maybe the bad oxygen sensor slowly built crud up somewhere?
If anyone can help me, I'd love to have my old car back. Thanks. By the way, it's at about 86,000 Miles.

[Mr_Diesel]

This sounds like an air metering issue... there are a few things to try.

I had a similar issue with my 1.8L 8v jetta... It ended up being the ISV. They sometimes clog with crap from the crankcase ventilation. The ISV is the device sitting in front of the intake manifold "on top" of the valve cover. They have 2 or 3 wires running to it and a hoses running from the intake hose to the right side of the intake manifold. Take that off and run some throttle body cleaner through it until the fluid that comes out is clear. It may need to be replaced. If it comes down to that, I recommend you get one from a junkyard because they are VERY expensive when purchased new. I'm talking like $250 +. They rarely go bad, but there are exceptions.

The loss of mileage could point to a vacuum leak, which could explain the idle rpm issues. Check all of the hoses and tubing for cracks and leaks. Replace or repair as necessary.

lastly, it could be the VAM (air flow meter)... I doubt this could be the cause but can't rule it out just yet.

[The Horak]

Thanks, I cleaned it, but nothin.