High idle problem
 

I've had a slightly high idle on my 2003 mk2.5 since I got it (around 1000-1400rpm when warm) and when jumping GND and TEN and adjusting the throttle screw, it did nothing so I knew something wasn't working correctly.

I've since swapped that throttle body and it's attached TPS with one off a 1998 mk2 (a 1999 to you lot in the USA) and the idle came instantly good (around 850rpm when warm)

I then started up the 1998 mk2 and the high idle problem had transferred between the two cars so I know it must be a faulty TPS or the actual throttle body itself. Out of interest I then tried to adjust the base idle on the 1998 mk2 with GND and TEN jumpered and again there was no reaction when twisting the idle screw - just like when it was fitted to the 2003 mk2.5.

Can anyone diagnose for certain if it's the TPS or the actual throttle body that's at fault?

Cheers,
Craig

Adjusting closure position correctly on the TPS itself might fix it. I'm not sure of the pin layout on the particular unit you have, but typically the bottom two pins are used to indicate throttle open or closed. It should be set so that it's closed (resistance < 50 ohms) when the gap between the throttle stop and the throttle stop adjusting screw (not the idle bleed screw) falls below 0.7mm. With throttle opened byond 0.7mm then it should go to open across those two pins.

If that fails then you might check the other two switches that help the ecu determine idle speed - transmission neutral switch and clutch pedal switch.

I'll have a look at the TPS settings once I get hold of some feeler gauges.

I've been playing some more and by pulling off the small clip connector directly under the TPS connector on the throttle body the idle comes down by about 1000rpm, if I then reconnect it straight away, the idle remains unchanged.

The best example being this morning on a cold start. I started her up, the idle was 2500rpm and climbing! I pulled the plug (bottom right of the throttle body, under the TPS plug) and the idle dropped to 1200rpm. I then replaced the plug and the idle still remained at 1200rpm. As the car warms up on the way to work the idle just hangs at 2000rpm ALL the time - it's fooking well embarrassing as everyone is thinking I'm revving up to screw it away from every set of lights!!!

Please help :(

you has leak. plain and simple, more air is being ingested than should be, put a plug on it.

I dont think it's a leak as the throttle body with attached TPS and IAC valve have been swapped from car to car and the problem transferred with it. This is why I'm thinking the TPS, IAC valve or actual throttle body is to blame.

3 x very expensive parts to just "replace and hope"

So even if you don't have a multimeter and feeler gauges then just try loosening the throttle body mount screws and rotating it a few degrees clockwise to see what it does. 5 minutes work at no cost might give you an answer. If you go too far then the car becomes a bit jerky to drive at low revs in heavy traffic, that's all.