Hi,

I seem to be making a lot of posts to the forum just now - the way it goes I guess..

I had my XM serviced last week - remember the leaking gasket..

I seem to have a new phenomenon, when the engine runs at high revs it surges and then loses power and then surges again. It first happened yesterday going up a steep hill. I wondered if it was a blockage in the injectors so kept it in 3rd gear (4 gear automatic) and took it up to about 60 mph. The surging didn't go away. Anything over about 65 mph in 4th gear produces the effect.

I'd be grateful for any suggestions

Regards

Andrew

Sounds like you have an air leak/blockage in the fuel system, have you changed the fuel filter lately? if so could be a leak around the cannister/top seal, if you havn't, could be a blocked filter.

Peter.N.

Hi Peter,

it was just serviced last week, hasn't been out much so maybe it's something dodgy from when it was serviced. I'll take it around tomorrow and see what they say.

Thanks for the tip, gives them something to look into..

Regards

Andrew

I 'm quite sure (signature) that it's a petrol car, in which case I don't think an air leak would be possible??

Cheers,
Niek

Hi Niek,

Not wishing to put words into Peter's mouth but when we talk about an air leak in a diesel car we mean an air leak in the fuel line. In a diesel the injector pump is always on the engine block which means the pipe between the pump and the tank is sucking all the time. If you have an air leak into the fuel line prior to the injector pump it causes the pump a lot of trouble.

On a petrol car, especially an injected one, the pump is in the tank and the pipe between the tank and the injectors is pressurised therefore it is nearly impossible for air to be sucked into the line. However, a petrol injection car usually (not always) has an air flow meter. The amount of fuel injected is proportional to the air flow measured. If you have a crack in the inlet manifold air can be sucked in after the air flow meter. This means the amount of fuel injected is wrong because it hadn't accounted for the air passing through the crack. Alternative sources for the leak is where the brake servo is attached to the manifold for it source of vacuum. The pipe between the manifold and servo could be leaking.

Confusingly, the phrase 'air leak' can mean two totally different things depending on whether you are talking about diesel or petrol.

Hope this helps.

cheers

noz

Thanks for the advice guys. I took it out yesterday and it was absolutely fine until it warmed up. Another clue maybe..

Regards

Andrew

Hi,

just got the car back from the garage. They put it on the engine diagnostics and got an error with the "Vehicle Speed Sensor" part of the gearbox. Does anyone have any insights on this? Is it prone to errors?

Thanks

Andrew

Hi,

i've got a series 1 (90 model I think) V6 24 valve 5 speed and have the exact same problem. I have had my mechanic look at it and he can't find anything. I did read some time ago on a forum that this was caused by a "potentiometer" but I get blank stares when I mention this. The problem arises at around 60 - 65 MPH and I get a sudden loss of power for a short instance so the car feels like its dropped out of gear (it hasn't - it's just that the drop off is that dramatic) then it comes back again and drives normally. This happens intermittently so I can have periods of normal highway driving and others when it drives me crazy.

Anyone heard of a "potentiometer"?

Yes a potentiometer is just a posh word for a variable resistor. They can get a bit dirty and the signal can become non linear or noisy thus affecting whatever system they are giving a signal to. You can experience this effect when you adjust the volume on an old radio and you get a terrible crackle whilst it is adjusted.

If a potentiometer in one of the sensors has degraded then this crackle will confuse the ecu, thus the neecd for a new one.

best of luck

bigjohnh

Hi Anj,

as I mention in my postings, the problem just went away of its own accord. My suspicion falls on one of 3 things:

1) Fuel - somehow something unwelcome got into the fuel tank down at the garage.

2) That the problem was the speed sensor but intermittent and it fixed itself somehow

3) The head honcho at my indy garage thought it might be a catalyser problem. Apparently, bits tend to fall off the ceramic active component in the catalyser and block free flow of exhaust causing this problem.

I had the problem for about 3 days and then it resolved.

Hope this helps (and it goes away!)

Andrew

I'll take a look at the resistors. My problem has been around for a few years and I have learned to live with it. Any ideas where I can source a circuit diagram that might point out resistors that feed the ECU?

Any advice would be greatly appreciated.

Tend to drive my Safari a lot at the moment!