Series 1 24v XM (PRV V6) won't idle...

[DHallworth]

Have already put this on Club-XM but will put it here incase anyone can offer any other suggestions...

When starting the car it idles at 2000rpm, then after it's been running for 5 or 6 minutes the revs will drop to 1200rpm and will stay there no matter how warm the car is.

Anybody got any ideas what could be causing this?

I've changed the following so far with known good ones:

MAF sensor
MAF sensor pipe
Idle Control Valve

All of which individually or together have made no difference to it.

Any help would be appreciated. Biggest concern at the moment if it's holding to high an idle is that it's generating to much heat and risks overheating.

David.

[addo]

What's your compression and fannymould vacuum like? I'd put my fingers on an air leak. Like the blue electrovalve on my car, that had come in two.

FWIW most engines don't overheat at a mildly elevated idle as there is no load - just better water circulation.

[xantia_v6]

A high idle is almost always due to too much air getting into the inlet manifold. Try blocking off the air supply to the idle valve and see if that drops the speed or stalls the engine.

Check also that the throttle is closing fully.

The only other (non-air related) cause I have seen (on a Jag V12 HE) was an ignition fault which caused the timing to be over-advanced by a few degrees at idle, which meant that the correct amount of air was consumed too efficiently, spinning the engine too fast.

[citroenxm]

Just to add to Xantia V6s last post, the 24v PRV has NO adjustible dizzy timing, theres no vacuum pipes on it either.... Its a fixed dizzy head.. all idle is controled by ecu useing a vaccum somewhere or how...

I cant pin point this for david without really being WITH the car...

Paul

[DHallworth]

Thanks for all of the tips guys.

Have been checking over it with a Lexia tonight and have found something quite interesting.

The idle speed according to Lexia is around 850 - 890rpm however when checking on the rev counter it's showing as 1100rpm. I've also got the engine management light on with the stored fault code showing a permanent fault with the throttle potentiometer, (full message is: Short circuit to -ve or open circuit). In the parameter measurements on Lexia the throttle position is constantly "mid" no matter where the throttle is.

I've pinned out the loom to the plug on the ECU and checked it with another car without any issues and the readings are the same, no resistance on any of the wires and only one of them goes to earth. I've tried two throttle potentiometers and am still getting the same issue. I've even tried another ECU in case it was that giving me problems but nothing has made any difference.

Am totally at a loss now!

David.

[addo]

In terms of the dash display, could there be a fault in the tacho conversion box?

What's your manifold vacuum?

[DHallworth]

Addo, manifold vacuum is around 1400 with the engine off and that drops to circa 750 at idle. One thing I did notice was that at 4000rpm the ignition timing is being advanced by 45 degrees according to the Lexia. Luckily we've got another 24v to compare it to so I'll do that tomorrow too.

Well I'm just back in from the workshop and can confirm that it's definitely not the throttle potentiometer which is triggering the engine management light.

There's a 5v feed to the sensor as stated in the technical bulletin for the 24v engine and the voltage varies in 0.5v increments when the potentiometer is moved.

There's a list in the manual of what does/doesn't trigger the engine management light so tomorrow evenings job is to check the following:

Air Temp Sensor
Water Temp Sensor
MAP Sensor
Front Bank Knock Sensor
Rear Bank Knock Sensor

Luckily in the technical bulletin it tells me what the values should be and how to test them with the multimeter.

I bought a 24v XM that somebody had cut in half with a stihl saw a few months ago so have got access to all the sensor I need so hopefully with an hour or two tomorrow evening I should be able to get her sorted.

Really hope I can as MOT is on Friday at 8.30am!

David.

[addo]

750mBar is about 22 inches of mercury - a really good figure for a high compression motor like a healthy PRV. You can rule out manifold vacuum leaks.

An erroneous knock sensor should not advance timing by 45°! I'm surprised it ran if that figure is correct.

Substitute the ignition driver module (next to the coil and battery) with a known good. The other absolute bâtard to access is the CAS - at least it's the same as a Xantia one for swapsies.

[DHallworth]

An erroneous knock sensor should not advance timing by 45°! I'm surprised it ran if that figure is correct.

Substitute the ignition driver module (next to the coil and battery) with a known good. The other absolute bâtard to access is the CAS - at least it's the same as a Xantia one for swapsies.

That's what I was thinking Addo but it's running beautifully, it's as smooth as the other 3 that I've driven so I ws wondering if there's a sensor that's reporting a false values somewhere.

I bought the half XM for the engine and wiring loom and luckily it's got all of the sensors on its well so I'm going to try swapping them over one at a time and see if the management light will go out.

The crank angle sensor actually isn't to bad to change on the XM, if you take the pipe going into the inlet manifold off and get a use a 1/4" ratchet you can change it in a couple of minutes so I'm going to try that tonight as well.

Will keep you posted!

Regards

David.

[DHallworth]

All my sensors are giving correct readings, all the plugs seem to be intact too, there's no breaks that I can find anywhere on the loom using the multimeter either.

I've changed the crank angle sensor, cam sensor, both water temp sensors to no avail. I was going to change the MAP sensor but the one I've bought from Citroen Services arrived in the post damaged. Unfortunately the plastic spigot that the rubber pipe goes onto is snapped off. It's not in the box or the packaging so it must of been snapped before they sent it

Hey ho, maybe the light will clear when it's getting a run to it's MOT first thing tomorrow morning.

David.