My 1987 300TDT has a problem in that it smokes and idles rough during cold starts. Occasionally the engine will not start on the initial start. It seems to emit a white smoke that smells of unburnt fuel. When the engine is warm, it starts without a problem.

After reading a bunch of posts in the Shop Forum, I thought the problem might be a bad glow plug or bad glow plugs. I measured the resistance of all of the glow plugs at the glow plug relay connector and they all measured between 0.6 and 0.7 ohms which indicates they are all good (no shorts or opens). I did find one of the previous owners had soldered across a crack in the 80A fuse in the relay to fix it, so I replaced the fuse. The cold starts did not improve.

I then measured the current to each of the glow plugs in the start position. The current would start out around 18-20 amps initially, and just before the relay shut off it would be down to 13 amps. I think the intial current is supposed to be closer to 30A according to the MB shop manual, but my meter was not fast enough to catch it. I also timed how long the glow plug lamp stayed on and it was about 1 second. The ambient temp outside was about 51 degrees F, which is about 10.5 degrees C. According to the MB shop manual the glow plug light should stay lit for 3 to 5 seconds at 10 degrees C. I also measured the time the glow plug relay stayed closed after switching on the ignition and it was 6.4 seconds. I'm not sure how long the G.P. relay is supposed to stay closed after the ignition switch is turned on, but I thought it was 30 seconds or so. I then tried to see if a longer wait after the the G.P. lamp went out would improve a cold start. The engine started easier with the longer glow time and smoke was reduce substantially.

Based on the data I gathered above, I think the G.P. relay is going bad. Does anyone have any other ideas on what else might be causing the problems? I want to make sure the G.P. relay is actually bad before replacing it since it costs $132. The previous owner also mentioned the glow plugs had been replaced in 2003 so I don't think they are bad.

Sorry for the long post, but I wanted to make sure all of the info. was present.

Any ideas or thoughts are welcome. Thanks in advance.

-Steve

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1987 300TDT smoke silver w/ burgundy leather interior
2000 VW Passat wagon indigo blue w/ beige leather interior
1985 Mustang SVO
1970 Chevrolet K10 fleetside, shortbed