High idle 940
 

Tomgang has managed to go high on my -98 lt, approx. 12-1500 rpm.
Has followed a bit of extra with the last time and it's after the engine has gotten hot and I've driven a few miles, and then it seems like it goes down to normal rpm when the car is standing still/wheels not turn. Otherwise it starts okay and normal, no cheating electricity The damper housing, lmm, idle engine is cleaned and tps "clicks".
Changed.
Idling valve
Air mass sesnor
Temp sensor for ecu
Throttle pos. switch
Vacum hoses on intake manifold
Fuel ecu
Crank sensor
Abs/speed sensor on rear axel

When I shut og the engine and start it up again the problem is not there until the car has been shut of for aprox. 6-7 hours, depend of the outside temp. The problem seems to increase when outsidw temp is low from aprox. +5 C and lower. The problem comes after driving to the engine temp gauge shows about 1/4 warmed up.
In the winter time when I use the electric engine heater (230 V ac) and the engine i a little bit warmed up the idle is normal.
It is a very old thread on this forum were the car is acting exactly like mine.
Hope there is someone who can help with this irritating issue.

Not much help but I have a high idle problem too, It only occurs after I have driven the car fast or on WOT, If I switch it off and start again it immediately goes away. Idle is same as yours at around 1500rpm when fast. It doesn't bother me too much but I wouldn't mind knowing how to sort it out, or is it just ' one of those glitches?'

High idle
 

The engine management can only adjust the idle speed when the engine is warm and speed is (almost) zero. If you start a cold engine, the idle speed setting will be set higher because of additional friction of cold engine. During driving the engine warms up, but if you do not stop, the engine management system has no opportunity to re-adjust the idle speed for warm engine. The idle speed will then be higher, especially when the outside temperature is low. You need to fully stop and wait for two or three seconds before the idle speed is lowered back to 800 rpm. Can be annoying if you get into stop-and-go traffic without a chance to stand still long enough.

Sometimes it happens that I stop after having driven continuously from cold start for some time, the idle speed will be lowered but stick somewhere around 1000 rpm. If I then slightly let go of the clutch to let the engine speed drop to 6 or 700 rpm for a moment. Then the engine managament sets the speed at a steady 800 rpm.

In my 940, the idle air valve was only operated when the revs drop low enough to need it, so unless it is sticking, it should be closed during normal driving.
If it is slow to react then it needs cleaning (and lubricating using clean engine oil - safe on CAT etc), but as the OP says they have replaced it (not cheap I guess!) then that shouldn't be an issue.

Is there an air-leak causing more air to go into the engine, so it behaves as if the idle valve is open?

Thank you for all answers.
It makes sence that the engine managemanet can only adjust idle speed when it is warm, on a other side I just can`t understand it, because the engine did not behave in this way for until 3-4 years ago. I also had a 1996 940 with B230FK before this car who did not behave like this.
All the parts I have replaced is original parts.

High 98945lt,

There is an old thread on here that might help (see below). Check out my response which is the last reply on the thread as to how I got a cheap fix to my high tick over.

https://www.volvoforums.org.uk/showthread.php?t=248168

Hope this helps.

Cheers,

LeeP

Have read your last reply now and will check it out late next week because I am away on work.
Have checkes all the vacum lines/tubes with quic start gas without responce, but can pay it att. again.
What I really wonder in is that the idle becomes normal after shuting of the engine and starting it up again straight after. Is there a connection between ecu and the speedometer??

The idle on my 940 stays high when it's been started from cold. I find if you let it warm up to the point where the idle first drops without touching the throttle it works as intended.
I've not found out why, it is all set up properly and the throttle body isn't gummed up. In fact the only part I haven't changed is the lambda sensor. I did wonder if being old it took a bit longer to warm up and take the correct reading but as others have said it might not be doing anything until the engine is warm enough.
If the idle is high then switching off and re-starting fixes mine as well.

I had had two ancient 940s with loads of miles on and neither of them had problems with the idle, except when the idle valves themselves got stuck.

If it is not the idle valve, something else has failed. If the coolant sensor was incorrect or not getting its signal to the ECU, the ECU will think the engine is still cold, but I would have thought your MPG will suffer as a result.

I have a spare LH2.4 ECU, if you wanted to see if that is the issue.

Have replaced the colant temp sensor with a brand new original one.
Have forgot to tell that I have replaced the ecu, with a new chip (ecu is a second hand) for a week ago, but no different.