gday all i have a 91 TT Z and it's just come into this problem when 1st start the car up hot or cold it will idle fine but after driving it round the block or wherever the revs stay around 2k i clutch in and brake revs go down a little then when stopped jump straight back to 2k i already cleaned TPS, IAAC, AFM connector, checked coolant levels, and now i got no idea, any help would be great. Cheers
my zed does this, once it warms up it stops. ive tried to find the prob but had no luck. i find it does it more in winter then in summer.
The unit that controls cold idle speed is called the air regulator unit which is just behind / below the balance tube in the middle. Its job is to regulate the amount of direct air reaching the plenum via an internal butterfly which in turn adjusts idle speed until the engine reaches normal operating temps when the idle control unit (AAC) takes over control. Signs of this unit failing are usually that the engine does its cold-start fast idle but idle doesn't come down smoothly when the engine is hot. Either it stays at ~1500-2000RPM or will come down Ok only to bounce back up unexpectedly. The best way to check this unit is with a multimeter set to resistance across the male terminals. (They are facing the drivers side) Anything around 70Ω (ohm) is fine. The other way (easier but more likely to damage an older brittle air hose) is when the engine is at normal temps and the problem high-idle is occurring, use a flat head pair of pliers to carefully squeeze and hold the 20mm 'dog-leg' hose between the air reg and the balance tube together. If the idle drops immediately, then the air reg is faulty/failing. If idle remains high, then the air reg is unlikely to be the culprit. As others have suggested, the TPS might be out of adjustment or the idle control unit (AAC valve) is gummed up. Easy to remove and overhaul... But I have a hunch that your prob is the air regulator so try that first.
76 is borderline but within tolerances. Try clamping that hose also when it is misbehaving (high idle) after the engine reaches normal temps.
The Air Reg was faulty on my Yellow Z. I never measured the voltage, but the pliers trick did work for me. replaced the Air Regulator and overhauled the AAC at the same time.
If your TPS value is correct, your CTS is clean and working, and the IACV and AAC are working correctly, then try the below. I have this issue occasionally as well and mine is caused by my TPS wiring. I have a throttle position readout on the dash and occasionally the throttle % will jump around while at idle and cause the same symptoms of a high idle. Play with my TPS wiring and it comes back down. Now I've replaced the plug, and re soldered up some new leads off the loom, but 3 months later it undoubtably comes back. When the idle is high, try jiggle the wiring from the horizontal plug and see if the idle changes.
It was the Air reg connector plug wasn't doing it's job so put another plug in now it works perfectly, thanks to everyone and there input greatly appreciated. Cheers Tyson
