I have been asked to look at a friends 1993 400E with 107K, always had good care. Normally it runs perfectly, but it has a maddening and frustrating problem.

Occasionally but recently more frequently it will begin to run rough and sometimes die and not restart after 5-10 minutes of running. This can occur at idle or while driving. This started out happening if the car had not been driven for at least 3 days but now it can occur after one day's sitting. It seems to be worse during rainy or damp weather.

The car has been cared for by an excellent local Mercedes-only indie, but they have never been able to reproduce the problem. There are no codes stored in any of the modules.

Caps and rotors were replaced, and a new engine wiring harness was installed in September 2007 with no change.

Here is my experience with the car over the last two weeks.

June 9

Replaced MAF sensor

June 11

Pulled the car into the garage. Hard start, died in gear, ran rough.

Removed one spark plug from each side – plugs were dry

Tested coolant sensor – had correct value for approx coolant temp

Went to move car out of garage, no start.

Checked fuel pressure at rail – normal

Checked spark with timing light – had spark

Swapped out Base Module for unit known to be good except for AC function – still no start. Put original Base Module back in.

Successful at re-starting by pressing down on accelerator while cranking. Engine would go rough when pedal returned to idle, went rough when moving car slowly up slope into parking space. Light throttle cured roughness.

No CHECK ENGINE LIGHT

June 13

Started the car, running perfectly. Then I noticed that the coolant light was on, so I shut the engine off and topped it up.

When I re-started the car to move it, the idle went rough, and the CHECK ENGINE LIGHT came on for the first time. Light throttle cured roughness.

Pulled the following codes:

9 – Intake air temp open or short circuit
17 – Data exchange malfunction
25 – Knock sensor or ignition control module defective

June 14

Cleared codes, started car. Ran perfectly

NO CHECK ENGINE LIGHT

So from my experience with the no-start situation, it appears that the engine has spark but no fuel. Since there is pressure at the rail, the ECU is not opening the injectors. I am thinking this is either a bad ECU or a bad throttle actuator. I have no idea why the CEL would come on suddenly with those codes.

Anybody have any thoughts?

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe