Audi AEL 2.5 fitted to Volvo v70 EGR question.

Hi guys,
I have a 2000 model (old shape) volvo v70 with the AEL engine (called D5252t) running a MSA15.8 ecu.
I removed the vacuum pipe to the egr and blocked it but this has threw a code.
This engine has no sensor on the EGR itself being a conventional vacuum operated type, and I can see no flow sensors or otherwise in the inlet manifold, not can I see any reference to this in the wiring diagram.
The EGR controller is a 2 wire solenoid type with 2 vacuum hoses connected to it, 1 to vacuum source (vacuum pump) and one to the EGR.

So, my question is how did the ECU detect the EGR fail"? First thoughts are either the MAF sees some change in flow versus the lambda readings but this does seem unlikely to me, or the vacuum circuit somehow is monitored.

I also wonder if I can leave the EGR connected to the vacuum side of the circuit but block the either feed or outlet side (exhaust or inlet) without a code.

Any ideas? Volvo forums have thrown a blank so far., but I don't like being beaten by things.

Your guess is right, the ECU indirectly measures EGR throughput by observing differences in the MAF signal. The Volvo's MSA system is superficially different from the EDC used in Volkswagens but I suspect it is identical in this respect, given the same complement of actuators and sensors used. The engine is fitted with a mass airflow sensor, in addition to air intake and manifold absolute pressure sensors, therefore it has two different ways of calculating intake air mass. The ECU is able to use the "speed-density" method (i.e. calculating using engine speed * intake air temp and pressure) to estimate the amount of flow through the MAF it should expect to see, and then it can dial up EGR to a desired amount, tailoring EGR quantity by tracking the difference between actual flow through the MAF vs expected flow based on the speed-density calculation.

Blocking the EGR passages triggers a CEL because when the ECU orders some EGR and opens the EGR valve, but doesn't see a corresponding reduction in intake air flow through the MAF, it concludes that something has obstructed EGR function -- either failure of the valve or a control side problem, or, in your case, an intentional blockage.

The way this is usually dealt with on a TDI is through chip tuning (or can be done through adaptation to a lesser extent) -- altering ECU logic so that it never calls for the EGR valve to open and therefore doesn't turn on the light when it figures out that something is not as it should be. No doubt this is readily available for the MSA system, and could give you a nice bump in power too....