Mercedes E320 Stalls at idle when cold

Maintenance/Repairs
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1

Car: 2000 Mercedes-Benz E320 (W210) 4matic

This car runs beautifully if the air temperature is above 60 degrees or so (maybe 65) Fahrenheit.

If the air temperature, however, is much cooler, and the engine has had enough time to cool to air temperature, it stalls. Here’s the sequence:

  • Start the car. Car runs fine, sounds good. Initial idle speed about 1200-1500 RPM.
  • Wait for car to warm up for a few seconds so it settles into normal idle. Idle speed settles to about 800 RPM after 5-20 seconds, depending upon air temperature.
  • Shift car into gear (D or R - doesn’t matter). Idle speed drops to 500 or so, then the car stalls.
  • Restart the car. Allow it to warm up significantly (how much depends upon air temperature).
  • Shift into gear (D or R). Engine idle speed drops a bit, then recovers to ~800 RPM. No problems after that.

If it’s a little cool (55-60 degrees Fahrenheit), I can drive off once the engine temp gauge needle rises a few degrees (45-50 degrees Centigrade).
If it’s around freezing (20-40 degrees Fahrenheit), I can’t drive off until the engine temp gauge reaches nearly 80 degrees Centigrade.

If I’m on a hill (pointing down), I can start at a slightly lower temperature by allowing the car to start rolling as soon as I shift into gear. In this case, there’s less resistance, so the idle speed doesn’t go low enough to stall the car. If I am on a level surface (or pointing uphill), 80 degrees Centigrade seems to be almost a magic number when the air temperature is below 40 Fahrenheit (near freezing). When the needle is showing about 78 degrees Centigrade, the car will still stall. When it reaches 80 degrees Centigrade, the car is fine, and will not even come close to stalling when put in gear.

ONE CONFUSING VARIABLE: When I took this to my mechanic, it didn’t exhibit the problem for him. When I picked it up (about 30 Fahrenheit), it stalled. He came out and started it, put it into gear, and it idled without stalling. I got in and let him watch while I got in, started it, put it into gear, and it stalled. We have no explanation as to why this would be any different (we tried several times, varying techniques, brake pressure, etc., with the same confusing results).

2

My best guess is that you have a defective temperature sensor, and it is sending spurious signals to the ECM, thus leading to idle speeds that are too low, and/or a fuel-air mixture that is not rich enough.

Aside from the temp sensor, you might also want to have the Idle Air Control (IAC) checked.
That is assuming that Mercedes uses an IAC. Many things are different (and needlessly complicated) on that make, so it is just possible that Mercedes does not use an IAC.

3

This Benz has an electronic throttle . . . no IAC

It might be interesting to hook up the scan tool, and compare what coolant temperature the PCM is seeing, versus the actual temperature at the ECT sensor, measured with a laser thermometer

4

Air Intake Temp sensor - Records the inlet air temperature and sends a signal to the engine control unit. The signal is used for calculating the air mass. The amount injected and the point of ignition are influenced.

5

On this car the air intake sensor is built into the MAF sensor

6

MAF sensor was my first guess, and that seems in line with some of the comments. I have to take it in for service this week, anyway, so I’ll talk to them about changing out the MAF sensor.

I’ve also found evidence (from another owner) that a nearly identical problem may have been solved by replacing the catalytic converter, though I don’t understand that one, at all.

7

@gpseymour

If it’s determined the MAF sensor needs to be replaced . . . use a genuine Mercedes-Benz part, or at least a Bosch part

On your car there are really only 2 reasons to change the cat

P0420 catalyst efficiency code . . . the check engine light would be on, though

The honeycomb has broken apart . . . sadly, this is very common on your car