Strange idling 1998 940

[98945lt]

Hello.
My 1998 940 (B230 FK, 135 hp.) has managed to go high, approx. 12-1500 rpm.
Has followed a little extra lately, and it's after the engine has gotten hot and I've driven a few miles, and it seems like it's going down to normal rpm when the car is standing/speedometer of 0.
Otherwise it starts Okay and normal, no cheating electricity The damper housing, lmm, idle engine is cleaned and tps "clicks".

Other parts/things replaced/checked last days:
-Replaced vakum hoses
-Idle motor/valve replaced
-Amm replaced
-Tps replaced
-No error codes
-Coolant temp sensor replaced
-Cleaned up throttle body

It happens almost every time, when the speedo needle is on "zero", the idle goes nice down to normal, if I start trill with the transmission in free the idle increases again, even if the car is going backwards.

Idling is behaving normaly when i disconnected speed/abs sensor on rear axle.
Hope someone can help me solve this irritating and strange problem.

[93Regina]

It happens almost every time, when the speedo needle is on "zero", the idle goes nice down to normal,

I would have to know all inputs/output devices on your setup.

Does it have an EGR valve...have you tested this circuit?

High idling means extra air is entering intake somehow.

If you have a "fly-by-wire" throttle, is there a way to test voltage on its circuit that opens throttle?

In short, one needs to find out how extra air is entering intake manifold.

[98945lt]

No EGR valve, just oil trap, who is replaced with a new one caused bad o-rings.
No "fly by wire". "old style" (steele wire from throttle pedal to butterfly/throttle house.)

[93Regina]

No EGR valve,,No "fly by wire".

Ok, I found a TB thread on your engine: 940 Turbo B230FT/B230FK Differences

Please be specific when this high idling happens...explain again.

So, when vehicle is motion-less, engine idles correctly, yes? Then when does it idle fast, again.

[98945lt]

That's right!
When I start up (engine is cold) and drive 3-4 km, when the engine is normally hot this problem starts.

[98945lt]

......do konw if it id any connection between speed- and/or abs sensor and the idle control on these cars?

[93Regina]

drive 3-4 km, when the engine is normally hot this problem starts.

Vehicle Stopped - Idle works OK

Vehicle Moving (@ normal engine temperature) - Fast Idle, so vehicle does not slow down

Correct?

[98945lt]

Exactly!
And often the idle rise again if the vehichle starts rolling again with transmission (manual) in neutral, both ways (backward/forward).
Specially irritating when you driving and you have to slow down speed before you are coming to a kross,traffic light, turn etc. You have to use the brakes cause it wont retard enough of it self because the engine rpm don't go lower than 1200-1500.

[93Regina]

Exactly!

And this is generally true: "Idling is behaving normaly when i disconnected speed/abs sensor on rear axle."

Bosch LH 2.4 ECU, I think, uses a frequency signal to "drive" the IAC (idle air control).

Using something like a Fluke 78 Automotive Multimeter, one might be able to see if this is a pulsed DC signal.

I don't know which EFI (electronic fuel injection) system you have, but if the IAC has two electrical pins, this test could be performed to see if ECU is controlling fast idle via IAC.

I'm not familiar with your turbo side...any solenoids/etc?

I'll be gone for several days...but if disconnecting abs/speed sensor affects idle speed...some electrical circuit might be "bleeding" into IAC's circuit.

[98945lt]

That's right. LH 2.4 ECU, idle behave normally when disconnectimg sensor in reat axle.
No solenoid on the turbo side, just amm, wastgate.