A friend has asked for my assistance with a problem he is having with his 1990 300TE.

It starts fine from cold and runs fine for several minutes from cold. It then develops an irregular intermittent misfire that is so bad at idle that it is very difficult to drive in traffic and may stall. Some time later after the engine has been at normal operating temperature for a while it reverts to running perfectly. He has also found that it can be very difficult to start after it stalls whilst the problem exists during the warm up phase.

I have only had a quick look for him to rule out a few items. I checked the duty cycle at the diagnostic socket and it was cycling around 45 to 50% as expected, even when the problem was occurring. I unplugged the idle control valve whilst the problem was present and the idle speed increased slightly as expected but the rough running continued. I unplugged the EHA and the rough running continued. With the problem remaining with both the ICV and EHA unplugged suggests to me that it is not related to the ECU or any electrical/electronic item associated with the fuel injection. I also forced the mixture richer by slightly depressing the flap in the air flow meter and the rough running did not change suggesting it is not air/fuel ratio related.

I am now thinking it may be ignition related. He has had ignition HT waveforms checked on an oscilloscope and they looked fine (although this was with the engine hot and running fine). Spark plugs are fairly new and distributor cap and rotor look fine. Given that the problem only shows during the warm up phase (ie. at a certain temperature), I am wondering if it may be an issue either with the crankshaft sensor (flywheel sensor) or the EZL ignition module.

Anyone have any suggestions? My next idea is to try and get it connected to an oscilloscope whilst the problem shows itself.

Greg

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.