XM 2.1 TD preheat system

Doc · Post by **Doc** » 09 Dec 2004, 15:36

Just wondering if anyone out there knows how long current is supplied to the glowplugs at an ambient temperature of 10 deg C.
Over the last few weeks my 1992 XM 2.1 SED has struggled to start when cold. Where as it used to take about 5-8 seconds before catching, it now takes at least 20 seconds. Once started however, the car idles and runs smoothly and also starts okay when fully warmed.
Using a test meter, I timed the glow plugs at 15 seconds before the preheat relays cut power. Is this correct when outside temp is 10 degrees or under?
Note: Car is in good mechanical order with 20,000 miles/2 years use on Citroen glowplugs and Bosch Filter/primer pump housing. (Original filter housing was allowing air to be drawn in)
Thanks in advance [:)]
Doc

Robin · Post by **Robin** » 09 Dec 2004, 18:09

Doc, did you test the preheat time with the engine stopped? The preheat switches off early if the starter motor is not engaged. I understand the circuit may then remain energised for up to two minutes in extreme cold after the engine fires up.
I would however begin to question whether you have a couple of inefficient rather than failed plugs. Robin

NiSk · Post by **NiSk** » 09 Dec 2004, 18:21

1) Does the starting improve if you perform repeated glow passes (i.e. turn on ignition, wait 15 secs, turn off ignition, turn on, wait 15 secs, repeat a number of times)?
2) Does the starting improve if you pump up the fuel with the hand pump (on the fuel filter) before starting?
3) Have you done your time measuring on the actual supply to the glowplugs and not the supply to the indicator lamp?
//NiSk

Doc · Post by **Doc** » 09 Dec 2004, 18:40

Hi NiSK
There is a slight improvement when I cycle the ignition a couple of times but I don't want to rely on this method due to increase in wear and tear on ignition switch and preheat control. I tested the supply to the glowplugs themselves via a test meter clipped to the plug feed and then rested test-meter on windscreen with bonnet up so I could see results from drivers seat. There was supply for about 16 seconds then it cut. I continued to crank engine without re trying and it eventually started. It idled about 750 rpm for a second then stabilised to 950-1000 (cold engine, 10 deg ambient)
Car runs great once started. Averaging 42 mpg mostly motorway driving.
Doc

NiSk · Post by **NiSk** » 09 Dec 2004, 18:44

What about pumping up the fuel by hand?
Don't worry about the ignition switch - I've never yet come across an XM with that sort of problem and as far as the preheater control is concerned, if thats whats giving you the trouble, you're going to have to replace it anyway!
//NiSk

Doc · Post by **Doc** » 09 Dec 2004, 19:33

Thanks NiSK
just going to try pumping by hand.
I'm hoping it isn't this as it's only two tears old, although the filter was changed less than a year ago.
I will let you know what happens...
Doc

Doc · Post by **Doc** » 09 Dec 2004, 21:45

Just pumped fuel by hand. Didn't make any difference to time taking to start, however I did try glowplugs twice and it did start much quicker - I think you're correct in the worn glowplug prognosis.
Where does the glowplug control unit sense temperature - engine coolant temp or is there a built in sensor in the unit itself?
Doc

NiSk · Post by **NiSk** » 10 Dec 2004, 00:19

From the wiring diagram I've got there doesn't seem to be an external temp. sensor, so I suppose it just senses the ambient air temp. (if anything!)
//NiSk