I had the engine management warning light come on a few times recently in the Astra during urban driving. I guessed from the pattern of it's coming and going that there was a problem with the EGR. I removed the EGR valve and found it was covered in a combination of hard carbon and some softer sludge. I cleaned it thoroughly, so it operates freely again, but was very disturbed by the thick buildup of sludgey cack in the intake manifold, as well as on the valve itself.
I also removed the manifold pressure sensor and found it too was full of sludge. I'm amazed it could even detect the boost pressure properly!
This mess is unboubtedly the result of oily diesel fumes entering the intake manifold via the EGR valve. I also hate EGR on because it increases turbo lag. [1].
On the escort I disabled EGR by disconnecting the electrical connector from teh EGR vacuum control valve. This causes the valve to remain shut. Unfortunatly on the Astra this casues the EMS light to come on and it goes very gutless. Similarly, if I disconnect and plug the vacuum pipe to the EGR valve, the same thing happens! How on earth can the dammed ECU tell that I've dissed and plugged the vauccum pipe!?
Does this mean that I've got to leave the crappy EGR system working and filling my manifold with crud, or is there some clever way to out-wit the ECU?
[1] I noticed reduced lag on the escort after disabling the EGR. My theory regarding this is as follows; The EGR recirculates exhaust gas before it reaches the turbocharger, under part load conditions. This means less gas to spin the turbine, so at part load the turbines are spinning at a much lower speed. This in turn means that when more throttle is applied, it takes longer for the turbines to reach the optimum velocity.