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Greetings! I have been trying to get my head wrapped around this issue, so I can smog and sell this car of mine. I've tested the EGR with a hand pump and it stumbles. I've cleaned the tube on both ends, clean as whistle. I'm replaced the EGR Switchover Valve new from Mercedes. The car drives like a dream no rough idle no stumbling. I reset the code and about 6 or 7 starts later the check engine light is immediately back on. Any thoughts would be greatly appreciated.

This car has no symptoms other than it sounds like my fuel tank is gurgling sometimes. Like the sound of a faint faucet and a lot more pronounced when the car is running and the trunk is open. Maybe these things are related? It's the only clue I have on this current issue, thanks in advance!
 

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2003 G500, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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First and foremost, has the wiring harness been updated? A degraded ecco junk harness can cause all sorts of strange issues and so must be updated before the car can be reliably diagnosed.

Assuming that has been done, it sounds like you have covered all the bases on the EGR, so I'm going to go way out on a limb here with a couple of additional suggestions.

I'm not sure how the W124 "tests" the EGR system, but from other FI systems that I am familiar with, I know of two methods. One, is that the ECU looks for a change in the A/F ratio upon activation of the EGR flow and the other is that the ECU looks for a change in manifold pressure upon activation of the EGR.

To that end, how old is your oxygen sensor? These do degrade and if the ECU is using an A/F method, the sensor may not be properly picking up the change in A/F ratio.

If you have a California car, it is equipped with a manifold pressure sensor that feeds into the CA required OBDI diagnostic module. If the ECU is using that signal to test the EGR, perhaps the sensor is malfunctioning.

Again, just a SWAG. Please post if you find the problem.
 
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