Citroen XM Forum

Summary of work & observations.

Throttle: Swapped out the old throttle sensor for a different one. Adjusted the position to get 0.5v at closed throttle. 0.5v measured between signal and return at the tps connector. Initially, the lexia showed 'idle' with a closed pedal and showed the 0.5v. Thereafter, it continued to show 0.5v but called it 'mid'. Made no difference to the mis-fire.

Chasing Earths: Measured the resistance between the battery -ve and the earthing point behind osf headlamp. It measures about 5ohms - compared to 0.5 ohms on the good gold car. Cleaned up the earth terminals behind the airbox. In summary, when we disconnect the batter +ve, the resistance between -ve battery post to any of the earth points was about 0.5ohms. Connect up the positive, open and close a door, and it jumps to around 30 then falls back to about 5 ohms. Couldn't figure out the reason for this. Certainly linked to the interior/puddle lamps and the electrovavle activating. So this is a difference between the two cars that we can't explain.

CO% at idle : Using an old 'vintage' Gunson gastester, the Co reading went off the scale in a negative direction - confirming the lexia readout that the car is running very lean at idle.

MAP Sensor: Put a vacuum pump on the map sensor and used the lexia to see what the engine controller is reading. a 50% vacuum halved the lexia reading - so that seems okay.

Tachometer. : Some strangeness was observed from the tacho - at one point it was stuck in the red rev limiter range whilst the engine was at idle!

Rail pressure: At idle the cars are running as follows {gold - warming up, 64'C}@768rpm/11deg spark/41%idle valve/477mb vs {grey, hot 83'C} 570RPM/17deg/36%idle valve/538Mb. So the grey car is idling 200RPM lower. Inj timing seems to move in .5mS increments on both cars and was of similar values. A test of the actual rail pressure value would be good. Matthew says he has tried substituting with known good replacement parts. The vacuum connection to the regulator was dry and held a vacuum.

Odd behaviour. Disconnecting the o2 sensor stalled the engine. Not so on the gold car. So that's another difference. Matthew substituted a new sensor -loosely mounted in the engine bay. Engine ran - some evidence of the engine racing as if receiving extra fuel. Whether the sensor was earthed to the engine, or not, seemed to make a difference to the engine running. Can't recall the details of what happened next but I think engine stalled and we could hear a hissing - opened the throttle and very strong and visible vapours appeared. Engine would not start after that - battery flat. It seemed like the fuel injectors were open putting fuel into the manifold.

So my summary of events: Throttle sensor is now working but makes no difference. Some oddity over the grounding resistance. Tacho shows odd behaviour. Very odd behaviour with the o2 sensor. Engine is running lean all the time. Steve

Thank you for the suggestion to check the regulator Dean and I note your comments about the intake manifold and possible collection of fuel. Unfortunately I do not have the equipment to perform a cylinder pressure test, but I cannot see why this should have changed when the engine was running fine before. I also do not think there is any problem with the ignition including the spark. Any physical fueling problem has I think been ruled out by the pump on the engine exercise and I have also dropped the catalyster off the car with no change. I was also thinking that my next task might have to be changing the engine loom and especially given the odd behaviour of the sensors as pointed out, while luckily I have a fairly decent spare. With regards, Matthew T.

Many thanks for that really excellent summary Steve and for your kind assistance earlier today. It will be very interesting to see what other comments are received! With regards, Matthew T.

Looks like you made good headway, connecting an O2 sensor in the oxygen rich atmosphere will signal a lean mixture to the Ecuador and will inject extra fuel, how this would walk backwards into the intake is beyond me and it would need to be a shed load of fuel injected to do that.
I think the answer is in your work today, that Tapachula dancing around must be a clue too, does the tach get a speed signal Tom the coil? I think it uses the crank sensor myself but I don't know for sure and I'm sure this is a clue too, mixture readings swing between a wider range on the grey car but that (I think) is a result of the problem not a cause.

Matthew I suspect everything at this stage, we know the engine runs better with added fuel, there is an intermittent problem with the engine speed signal as seen by the tacho, the engine speed is limited as well as power due to a (severe) lean burn.
The engine seems to be stuck in closed loop operation too, even when fed unbelievable data.
D

Agh! iPhone predictive txt!!

MTXM wrote:Mixture readings

Grey car/ Gold car

600/800 rpm -18% and +7%
1,500/1,600 rpm -15% and +9%
2,600/2,700 rpm -22% and +19%

I think these differences are really significant!

The readings confirm the engine is being starved, which is fundamentally the problem, as graphically illustrated by hand pouring in the petrol. I just need to establish what is holding back the fuel.

Regards,

Matthew T.

I am not sure that you have correctly understood the meaning of those figures. The mixture percentages are showing the amount that the ECU has adjusted the fuelling map after correcting the lambda readings from the oxygen sensor, so a negative reading means that the ECU has shifted the map towards the lean direction because the oxygen sensor has been detecting a rich mixture.

It would be a good idea, as suggested previously, to measure the mixture with an external gas analyser. based on the current data, I expect that i will show that the open loop mixture is a little rich, but the closed loop mixture will be correct (as the mixture correction has not hit its limits).

Many thanks for that Mike and I take on board your points, however, the 02 sensor readings were similar on both cars while the mixture readings were wildey different! The mixture was measured by Steve at the exhaust with a gas device and confirmed to be lean. Hopefully the external O2 sensor was the reason for the excessive dumping fuel in the manifold Dean as you suggest! I also think the tacho works off the crank sensor via the ecu but I am not sure how significant the odd behaviour is. With regards, Matthew T.

Matthew I suspect everything at this stage

We have been 'on the case' for a little while now Dean and here is my attempted summary! Is there anything I have missed? Regards, Matthew T.

At this point any odd behaviour is significant, if it didn't do it before the poor running it must be thought of as part of the problem.
Compression guages are cheap and useful for checking engine condition, borrow or buy one and check the compression, it will show up other problems as well like bad timing, valve damage etc.
This overfueling thing is nagging me, yes pulling an O2 sensor out into atmosphere will riches the mixture but how on earth does it get into the intake? I think it's all down to fuel tipped in but if it isn't you have to ask why is fuel being blown backwards through the intake system? That would be bad valve seats or timing.

D

I really don't believe that this engine is suffering from fuel starvation, the fuelling is close enough for the closed loop operation to keep the lambda (as measured by the car's sensor) correct. The indications are all that the engine is running a little rich, caused by the high manifold pressure that is fooling the ECU into overestimating the air flow.

I have no faith in the videoed test of pouring fuel into the intake. The rate at which fuel was added would make the mixture totally incombustible in the engine, I suspect that it was combusted in the catalytic converter, perhaps causing further melt-down of the core, resulting in the subsequent worse performance of the engine.

Another cause of the high inlet manifold pressure could be some mechanical load on the engine, perhaps due to a fault in the transmission. I know of a case with somewhat similar symptoms where the cause was eventually (after 2 engine rebuilds) traced to a jammed pressure relief valve in the oil pump.