PeachParts Mercedes-Benz Forum - View Single Post

jay_bob · #6 06-05-2019, 07:00 PM

In a nutshell the EGR prove cycle works like this. Doing this from memory so if I have something wrong let me know.

When the engine has achieved operation and stabilized, the EGR prove cycle is started. The EGR valve is opened a defined amount, and the mass air flow is monitored, to make sure the air volume passing the MAF dropped by a predicted amount. This predicted amount is based on how much exhaust gas (coming through the EGR) the computer thinks the engine should be producing for the given operating point.

If your EGR vacuum transducer is not working then the EGR valve cannot open. The computer senses the lack of EGR valve opening and throws a P0400.

The diode/resistor hack works to fool the MAF sensed voltage input to the computer, when the EGR transducer is actuated. The resistor and diode couple some of the voltage used to pull the transducer open into the line for the MAF, thus fooling the computer.

I ended up removing my EGR hack that the prior owner installed on my W210 because it was causing issues with P0400 codes being thrown if the car was started if you forgot to wait the whole glow cycle when it was cold. Which happened every time I let a stranger (repair shop, valet park, my kids until they figured it out) drive the car. So I ended up removing the diode/resistor, and the obstruction in the EGR vacuum hose, and putting the system back to stock.

If changing the EGR vacuum transducer does not fix the problem you will probably need to scan with the HHT emulator found in Xentry. This goes through the 38 pin port in the fuse box under the hood and provides system level access to the computer. The OBDII port was a first gen implementation and it only passes the federally minimum required emission test codes to meet the law as it was in 1996. Of course on modern cars you can get complete access through the OBDII, but that does not work on a W210 from the late 90s. An iCarSoft or other third party OBDII connected high level scanner will not get any more data from a W210 than a generic code reader.

I thought of one other thing. You said that you had to pump the EGR valve numerous times to get it to cycle. I would pull the intake manifold/EGR and give everything a good cleaning and get the EGR valve operating properly.