When I first bought the XM, even on it's test drive, it would stagger slightly when cruising. I bought the car nevertheless and when I got it home I started to investigate. Couldn't see anything obvious so set about trying to assess the conditions when the event happened. When cruising along a flat road at 70-80 the engine would falter every 8 seconds - only momentarily and would recover again. The stagger would last 1-2 seconds.

I put a teepiece in the pipe to the MAP sensor and attached a pressure guage so that I could see it whilst driving. The manifold pressure remained at 2 bar(g) but fell slightly in time with the stagger. I did think that 2 bar(g) was a bit high for a turbo and coudn't understand why it didn't vary very much under the control of the ECU.

I found out why! The turbo isn't controlled by the ECU at all. The wastegate default position (spring return) is closed. In other words, full boost. The wastegate position is controlled by a purely mechanical pneumatic acuator (no electrics at all) which gets it's pressure signal from a tapping into the fresh air pipework downstream of the turbo. ie the higher the pressure produced by the turbo, the higher the pressure entering the actuator, the more it opens the wastegate, the less exhaust gas works on the turbo drive vanes and the less pressure it produces on the fresh air side. In other word it is self-regulating. The manifold pressure rises proprtionally to the pressure/volume of exhaust gas until it begins to compress the spring in the actuator. At that point, if the turbo tries to produce more pressure the actuator opens the waste gate and prevents it from going any higher. On my car that pressure is approximately 1.3 bar(g). This is subject to the accuracy of the central heating pressure gauge I was using at the time.

Anyway to get back to the problem. The arm of the actuator which connects to the crank on the wastegate was seized. The turbo was full on all the time. The rubber pipe connecting the aftercooler to the inlet manifold blew up like a balloon. The stagger was a self protection mechanism by the ECU. The map sensor saw the high pressure and momentarily retarded the injection pump or reduced the fuel flow, I don't know which. The self protection circuit is obviously programmed to check the pressure every 8 seconds.

I freed off the seized linkage and the problem has disappeared. I have to say that the engine performance with the turbo stuck on was amazing but I don't recommend it long term. Although it does make you wonder about altering the actuator spring rate for more turbo boost !!!

cheers

noz 8)

So what are the chances of tuning the Turbo boost/ wastegate control do you reckon?

I recall seeing a chip advertised claiming an extra 15 bhp, but it could only have been re-mapping the pump ECU from what you're said.

Any mechanical improvements to be had for free?

Regards,

Roy

Roy,

You are correct. There is no way the chip can affect the boost pressure. I did consider chiiping my 2.5 and thought that the boost pressure was part of the deal. It is not possible.

To 'tune' the boost you would have to alter the spring rate inside the actuator. Although not impossible, you would have to cut the outer casing open since it is spun closed. If you were determined enough I'm sure you could manage it. You'd need to calculate the new spring rate or have a selection, trying them out one by one and recording the results.

Happy motoring.

noz 8)

ps

why not join the forum as a member instead of a guest and swell the numbers:-)

oops, just notice you have, sorry