Does sound rather like overboost despite the things you mention - what boost level does it peak/hold? How about removing the restrictor to show it runs 8 PSI? The smaller bore restrictor on the MY99 would only make overboost worse.

The AE800/801 ECU runs a boost target system with 13.7 PSI as the target. Using a lookup table it finds the present boost error from target and then ramps the wastegate solenoid duty cycle up or down at four different rates depending on how large the error is. The highest two ramp rates for 1.7 and 3.0 PSI error are really very aggressive, and will tend to setup a midrange boost oscillation in high gears rather than a full blown overboost fuel cut which has a trigger point of 17.6 PSI held for a few seconds typically.

A Select Monitor log of Primary Wastegate duty cycle or whatever the Selmon calls it along with manifold pressure would give this away.

[Edited by john banks - 11/6/2003 8:09:14 PM]

right then it's a t reg turbo with 120,000 mls on the clock and when your in 4th (and sometimes 5th) at 3000rpm which is about 60 mph and floor it so WOT at 4000rpm it misses like the waste gate control valve is sticking but it's just had a new one so it cant be that as it was there before the change and then it does it again at 4500rpm then at 5000rpm then it's ok. the waste gate can be moved so it's not stuck i tried a smaller restrictor but still does it i even blocked the pipe off to the waste gate but it still does it. if you accelerate gently it wont do it it hasnt got coil packs just the big single one on the manifold any one got any ideas before i go swapping parts?

cheers for that john, i must say that thinking back im sure putting the smaller restrictor in did make it slightly worse. so if it is overboosting how do i stop it i thought putting the smaller restricter in would cure it.

Larger bore restrictor, standard is 1.2mm, drill it out to 1.3mm and remeasure the boost. 0.1mm goes a long way, not a linear relationship.

The boost goes down with more flow through the larger restrictor because more pressure gets through to the actuator to open the wastegate to lower the boost.

On older models where the restrictor was on the vent part of the control loop, a larger restrictor would raise the boost as it allows more pressure to be bled off from the actuator, so the wastegate stays more closed and the boost is higher.

The other thing that is handy is that the actuator is adjustable on the 99/00, loosen the 10mm locknut lenghten the rod by 1/2 turn at a time, but obviously there needs to be enough to hold the rod on. Shortening the rod will raise the peak boost. This can be a nice fine adjustment once the restrictor bore gets you in the right ballpark, and can put it back up a bit if you've dropped it too low.

You need to work on the boost measurements though to sort it back to spec rather than just adjust it by feel. Otherwise, come a cold wet day when the car is loaded up, it could fluctuate again if the boost error gets too large and the ECU s***s itself.

john your a star thanks for that :):):)