The ECU uses info from the MAF to determine if the EGR valve is working.

When the EGR valve opens it allowes exhaust gas into the inlet. There will then be less air flowing past the MAF. If the ECU sees no reduction in MAF when it opens the EGR it assumes the EGR is faulty so flags up a fault. So your problem could be the MAF on the edge of failing or just a poor electrical conexion to it.

Where would the EGR vacuums connect to the inlet manifold. I thought they were fed via a one way valve from the brake servo vacuum pipe? Must admit there is a hissing noise missing that used to be present after you turned the engine off.

When you turn off the engine you can see the valve on top of the EGR move (anti shudder?)

By the way the car generally drives so well the variable vane part of the turbo must be working - it has never shown under/over boost

The MAF sensor being on the edge looks as though its worth investigating. I'm guessing this could also cause the occasional lack of low down grunt(below 2000) as if MAF is under reading presumably it will also under fuel.

Why would this vary with different fuel? woud better diesel draw more air?. The fuel issue may be misleading though - The light has come back on today with Shell Diesel, the difference is that it has taken 200 mile to do so whereas with the supermarket diesel light comes on (repeatadley) within 20 miles.

Thanks, J...

As Dieselnut quite correctly says, the ECU uses MAF information to monitor the effect of the EGR system by detecting a reduction in air flow when the EGR valve is commanded open. The sole purpose of the EGR valve is to allow a controlled amount of exhaust gas to re-enter the intake manifold when then reduces combustion temperature, which in turn prevents nitrogen and oxygen forming Oxides of Nitrogen (NOx). In other words the EGR valve is for emission control, and has nothing whatsoever to do with performance enhancement. The EGR does not operate at idle or the engine would run rough. It is only commanded to open during mid-range operation.

On your car the EGR control can be altered, but to do it you need to find someone with VCDS. The EGR has a default setting which equates to a certain volume of air measured by the MAF. Increasing the EGR setting will adapt the ECU to expect a greater flow of air before it operates the EGR valve. This has the effect of lower EGR. This will mean the intake manifold is much less likely to clog (which they do quite commonly) and performance and fuel economy are improved. The engine idle will also be smoother. The downside is that levels of NOx will increase, but since this is not an MOT requirement you have nothing to lose by doing it. You can even adapt the ECU so the the EGR does not operate at all....

On the 2003 model PD motors it was possible to disable the EGR valve without throwing a fault code, by pulling the vac pipe off the EGR actuator, inserting something to block the pipe (I used a piece of snug-fitting plastic on my 2003 PD130) and reconnecting the pipe. My own car did some 5,000 miles like this before I sold it.

This way, the EGR valve stays shut, so you get less soot and claggy stuff building up in the inlet.