Inexplicable Problem with XM

[Junkman]

XM 2,0 TCT XU10J2TE 147PS

Car starts in hot or cold condition without any issues. When cold, you can drive about 3km. Then it suddenly starts bucking and doesn't react to the accelerator pedal.
If you declutch it in this condition, the engine stalls, but can be restarted immediately without a problem. You can continue driving some 10 kilometers, then it starts to buck again. This repeats itself several times, then it runs fine all day with no flaws.

When driving on the motorway, during acceleration a very slight jerk is felt. If you drive e.g. constantly 150 kph, it runs without problems. Slowing then to 80 kph on overrun, and stepping on the accelerator again, it takes a moment until it reacts. It stutters a bit and then goes back to normal. The same may also happen when coasting and braking into motorway exits.

Check Engine warning light is off. The error memory is empty. The mileage is not 100% certain, but probably around 300,000 km. Top speed is about 200kph, i.e. there is no real loss in top performance.

Recently replaced:

Intake Air Temperature Sensor
Coolant Temperature Sensor
Oxygen Sensor (Lambda Sond)
Fuel Pump
Fuel Filter
Fuel Pressure Regulator
Coil
Sparkle plugs
Plug leads
Ignition module
TDC sensor
Anti Knock Sensor
Air Filter
TPS
Crankcase breather valve

This has been checked/done:

Idle stepper cleaned
Boost pressure control valve checked - OK
Evaporation valve to the charcoal filter tested - OK
Mass points in the engine compartment cleaned left and right and on the engine block.
All underbonnet connectors cleaned with contact cleaner, including the one underneath the hydraulic reservoir.
New fuse 30A
A different ECU showed no improvement, however, it was from a used vehicle.
A compression test of the engine in cold state yielded 11, 12, 10.5, 11 bar, in the warm state 11, 12, 8.5 11 bar.
So the second cylinder has a bit of a compression loss when warm.
Engine oil and coolant look good. No bubbles in the coolant, cylinder head gasket seems to be fine.

[CitroJim]

Well, you can't say it's not been thoroughly investigated Junk

I once had this happen very occasionally on my Activa (same engine) and that went away when I replaced the double injection relay. Never happened since. I think, in your list, this is the only thing that's not been replaced...

Of course we must bear in mind this is an XM we're speaking of here. XMs are masters of the inexplicable

[Junkman]

Errr...
What's the double injection relay? Where is it located?

[CitroJim]

The relay that supplies power to the fuel pump, injectors and ECU (partially). It's brown, ribbed, about twice the size of a normal relay and is mounted in the 'piggy back' ECU box attached to the front of the main ECU box.

In it you'll also find the cruise relay and ECU.

The operation of the double injection relay is controlled by the immobiliser in part.

[Junkman]

Yeah, I know which one you mean. Big bad brown bugger aka Bitron 240109, right?
Why didn't I think of that damn thing...

[andmcit]

Yep, I was going to say the one missing off the changed / checked list is the brown double relay!
Are the items swapped new or known good second hand as the worst mistake I've made in the past
fault finding was use a duff secondhand item that was 'assumed / believed' to be OK!?

[Junkman]

Apart from the used ECU we tried, all the parts listed are new.

[addo]

Seized cooling fans, or ones with excessive current draw, can cause ugly stumbles with no fault logging.

[citroenxm]

Apart from the used ECU we tried, all the parts listed are new.

If you need anything, Im only down the road in North Wales, Anglesey...

[mick66773]

Hi, I once had a 3.0 V6 XM, Auto, that stuttered rather than bucked, that turned out to be a thing called the potentiomiter, my then local Citroen dealership replaced this and all was Ok, don't know if that helps?

[Mandrake]

The "potentiometer" will be the Throttle Position Sensor or TPS as its often called, which the original poster says has already been replaced.

Looking back through this thread I can't help thinking that the symptoms are very similar to a faulty turbo that I had in my old Daihatsu Charade... the engine was run low on oil once and a few weeks later there was intermittent bucking at higher rpm/load and sudden brief losses of power that would then go away for extended periods of time.

A couple of months later the turbo seized completely and was a write off. Turns out the turbo had been intermittently seizing and un-seizing during driving under higher loads and speeds. When it seizes not only do you instantly loose your boost the stationary vanes constricts the air flow choking both the inlet and exhaust flows.

I notice that of the long list of things that have been replaced or checked in the original posting in an effort to fix the problem the turbo was about the only thing that got overlooked...

[Johnny Cogs]

Alas, we might never know the outcome of great bucking saga!!!