W124 eratic tickover

[MW]

1992, 2.3 automatic, 142,000 miles

Normally runs very well but lately the idle tickover is a bit eratic and rough. It normally runs at about 750-800 revs and is very smooth. Now, it sometimes runs at 550 revs and seems to be nearly going to stall. Starts fine, hot or cold, and the economy is normal at 25-6 mpg in town, and 31 on a run.
I have serviced it with some petrol tank injector cleaning fluid, new airfilter, and plugs. There was a little improvement, but it is has now returned. It is only ever present for about 30% of the time.
Any ideas?

 

[Richard Moakes]

Sounds like the idle air valve or it's control circuit. It behaves like an electronic throttle and allows a certain amount of air past the throttle butterfly to keep the idle revs stable.

It might be old and stuck, and need some WD40 inside it to loosen things up, or if the OVP relay fuse has failed and the ABS light is on, then it isn't getting any power from the ECU.

 

[MW]

Thanks for the help, The ABS light is off (after start up) and normal. Is it likely that this valve is gunged up. It has probably never been touched, and can it easily be taken out and cleaned up with carb cleaner?

 

[Richard Moakes]

Carb cleaner, WD40 are all good for this, see if the rotor will move with a screwdriver when you have removed the valve from the car.

Looks like you are the 2nd person with a faulty one of these at the moment.

 

[MW]

UPDATE. Some developments, that might help others.
I had the rotary idle control valve out today. In and out in about 50 minutes. It is held on the 4 cylinder engine in a rubber sleave that simply is a wedge fit in front of the most backward pipes in the inlet manifold. Two air pipes are push fit in the top and bottom of the unit, one from the rocker box, and one goes into the bottom of the throttle unit. Very tricky to get out because of lack of space and the fear of snapping some fuel injection gear if ham-fisted.
Cleaned the unit out with carb cleaner and then WD 40. It was pretty clean, but the rotor was solid, open about 3 mm, and would not turn. Not sure if it has to, until the electric motor is energised.
Anyway on start up all normal, except the revs now drop to zero.
Checked one of the air pipes had not come off, but all OK.
It is now at my local independant garage, who are pretty good, and will gas test it etc, and hopefully fix it.
Will update.

UPDATE
The garage relaced the rotor control valve, and she now runs like a dream again, very smooth, with a rock solid tick-over at 750+ revs. It cost about £165 but my local independent gave a bit of discount at £115 + £60 labour and it also fixed a rattle on the exhaust heat plate, + vat = £205. It is a very straight local garage, yet stuffed with new analysers etc. A world apart from the cold expense of Mercedes who seem to need weeks before they can order anything. Our local Eurocar parts supplied and delivered the Bosch unit in 30 mins to the garage.
Thanks for all the advice.