Mechanic replaced the water pump on my 93 300E (3.2 liter M104), and now my idle fluctuates up and down at idle, when the transmission is in any gear, and only when the engine is completely warmed up to operating temperature.

My independent MB mechanic (own shop for 19 years) has gone over everything he knows to go over.

He has a Baum CS2000 scanner, and it gave him codes for the coolant temperature sensor and the mass air sensor.

I ordered both parts from Fastlane, and neither solved the problem.

He hooked up the scanner again and then got a fault code 31, which means: fuel adaptation (lambda) control, mixture too lean (rich stop), (intake air leak, fuel injectors, fuel diaphragm pressure regulator).

He checked and there is no intake manifold air leak. He replaced the fuel diaphragm pressure regulator. Didn't help.

The weird thing is the check engine light never comes on, even though it should.

He's going to sleep on it, and try one more time tomorrow. If not, I guess I'll take it to the dealer.

Any help - Benzmac, M.B.DOC, Stevebfl?

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".