Citroen XM Forum

I was having a chat today with a Prelude owner who was having some issues with his car, the trouble was it would not rev beyond 3k rpm, idle was ok but power was down, massive trouble with this car for weeks trying everything that would usually stop a car reving like bad ignition coil, rotor, distributer body, leads and plugs, fuel pump, filter etc etc.

The probelm turned out to be a badly fitted crank sensor which was gapped too far from the reluctor ring, setting the crank sensor into the right place solved the issue.

Just a thought, i know you checked the crank sensor but with the tach dancing about all over the place we know youhave an issue there, might be worth looking for problems on the crank sensor and wiring loom for that sensor as well.

D

I wonder how the gap between sensor and target would be checked? Is there any adjustment. I recall on the used abs sensor that I fitted, I needed to use a piece of sacrifical cardboard stuck to the sensor end with a grease smear.

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well this is the catch, you cant set the gap and you cant really fit it wrong either, its the same sensor as your Prestige and every other Citroen Pug engine out there uses, you cant really fit them wrong but if that cable was damaged or getting interferance from something......................

Im just thinking, the issue with the tacho and the not reving and this guy had a 'not reving' problem and it was a poor crank sensor.

To be honest i always thought they either worked, or didnt but i guess its like ignition, a bad signal breaks down at higher frequence / revs.

D

Many thanks for that Dean, although I have changed the crank sensor and there is no adjustment as you say. I also replaced the plug on engine loom since the contacts did look a bit dodgy once again with no effect, but I will revisit this area. With regards, Matthew T.

Is the tacho still misbehaving?
Do the engine ECU and tacho closely share the crank sensor signals, or do they have their own independent electronics? I don't have a V6 diagram.

Even though the crank sensor (and a substituted one) can only be fitted in one position, it could be that the rotating element below it (is that the flexplate's or the t/c's periphery, or the starter ring?) is not located correctly in relation to the sensor tip.

This type of sensor will usually work ok at low frequencies (revs), but as the frequency climbs "misreading" occurs as the waveform gets more difficult for the electronics to interpret. The gap between sensor tip and the "passing metal" is critical, and should be as small as possible.

I think the misbehaviour of the tacho might just be a result of the rev limiting effect Chris and purely mechanical down to the gauge and I have had occasional oddities on other cars. The ring from which the sensor reads is attached to the starter cog and securely bolted to the crankshaft and can only be in one position really, nor can it change alignment. The gradual worsening of the problem is also clearly significant and I think I must keep focussed on the fuelling. If the sensor or wiring were faulty presumably the engine would not rev up when petrol was added? With regards, Matthew T.

Have you checked the fuel vapor canister in the wheel arch.
Cars were built with a one way valve in the piping.
This can fail causing starvation problems according to the Citroen report.
There was a recall in 1993 and repeated in 1995 when over one third of the affected vehicles were still outstanding.
As this was a performance fix and not a safety fix DVLA search for current owners was not permitted to be used.
So any sold on XM's relied on a dealer seeing them for maintenance and checking the fix had been done.
Citroen fix is to remove the valve and replace with a short length of steel pipe.

John

Not sure. Wondered about that too.
I'm just assuming that correct fuelling depends on correct reading of engine revs/crank position. Presumably correct spark timing gets driven from the crank sensor too.
Whacking in that fuel did make it go, somewhat!

Thanks John and I have run the engine without the intake pipe and no change so I think the canister is good. I did wonder if the crank sensor had any influence on fuelling Chris but there is certainly no issue with spark or timing! Regards. Matthew T.

Hi Matthew

The instruction suggest the system needs to be closed and free flowing.
Failure of one way valves is unpredictable so they are best all checked for and if present replaced with straight pipe.

John