Does anyone else with an XM V6 24v have a misfire under about half-throttle acceleration around 2750 - 3250rpm?
Mine's done this since I got it. I've changed the spark plugs (including the 3 under the inlet manifold), checked the air filter, fuel filter and all hose connections, tried disconnecting the battery overnight so the ECU returns to factory standard settings and nothing's made any difference. It feels like it's running lean in the mid-range, if I go to full throttle or force the auto box to change down a gear by going into sport mode then everything's fine - ie it goes like the proverbial off a chrome shovel!
One theory I have is that it had stood for some while before I got it and I wonder whether a couple of the injectors are partially gummed up. I've tried a 'throw in the tank' proprietary injector cleaning fluid to no effect. Has anyone any other suggestions before I yank the injectors out for a professional clean, or is this a characteristic of the breed? I'm averaging 26mpg in commuter mode and a shade over 30mpg at (legal) motorway cruising speeds.
Also: does anyone know what Citroen have tweaked to get an extra 20bhp or so out of the same engine in the C5 v6? Or have they just played with the ECU programming?
Thanks all,
Martin
One for XM 24v owners
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mwj666
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David Goddard
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I assume we are talking about the post 1997 engine.
The most favorite is the throtle position variable resistor. That's on the end of the butterfly spindle. An ohmeter (ideally a old meter type) can be used to check that the resitance changes smoothly as the butterfy opens. Any sudden changes will have a really bad effect on how the engine performs - flat spots etc. as this is the only way the ECU knows how much the throttle is open and so how long to keep the injectors open - result well weak mixture (as you describe). It is a sealed unit - costs about £50.
Next I would check the MAP sensor. It's located on the intake manifold. Is the 3 core cable connected? One wire has a constant 5 volt feed. One is earth, the other gives an output with respect to earth of:
Ignition on, engine not running = 4v
Engine ticking over = 0.7v
If it doesn't drop, the sensor is faulty. It can be removed from the manifold (one screw I think) and the aperture checked to see if it blocked. Low pressure air across (not into) the aperture may clean it. A suitable gentle solvent may work. By the way, the only simple way to measure the voltages, is to stick pins through each core!
Another reason maybe the flywheel sensor. The ECU may compensate at low engine speed, but not when turning over at higher speeds. One sign is if the rev. counter shows an irratic reading at certain speeds. Otherwise need a scope to check it - actually, I don't know where the sensor is - hidden deep down I assume!
My 3.0 Xantia does 31/32 mpg on motorways, with or without the a/c on - no complaints there. It's done 110k - only problem (with the engine) has been one of the temperature sensors - resulted in stalling until the engine warmed up.
As it's a pretty high compression engine you get more "go" running on super unleaded. On regular petrol, the knock detector turns the power down a bit under heavy acceleration.
The most favorite is the throtle position variable resistor. That's on the end of the butterfly spindle. An ohmeter (ideally a old meter type) can be used to check that the resitance changes smoothly as the butterfy opens. Any sudden changes will have a really bad effect on how the engine performs - flat spots etc. as this is the only way the ECU knows how much the throttle is open and so how long to keep the injectors open - result well weak mixture (as you describe). It is a sealed unit - costs about £50.
Next I would check the MAP sensor. It's located on the intake manifold. Is the 3 core cable connected? One wire has a constant 5 volt feed. One is earth, the other gives an output with respect to earth of:
Ignition on, engine not running = 4v
Engine ticking over = 0.7v
If it doesn't drop, the sensor is faulty. It can be removed from the manifold (one screw I think) and the aperture checked to see if it blocked. Low pressure air across (not into) the aperture may clean it. A suitable gentle solvent may work. By the way, the only simple way to measure the voltages, is to stick pins through each core!
Another reason maybe the flywheel sensor. The ECU may compensate at low engine speed, but not when turning over at higher speeds. One sign is if the rev. counter shows an irratic reading at certain speeds. Otherwise need a scope to check it - actually, I don't know where the sensor is - hidden deep down I assume!
My 3.0 Xantia does 31/32 mpg on motorways, with or without the a/c on - no complaints there. It's done 110k - only problem (with the engine) has been one of the temperature sensors - resulted in stalling until the engine warmed up.
As it's a pretty high compression engine you get more "go" running on super unleaded. On regular petrol, the knock detector turns the power down a bit under heavy acceleration.