BenzWorld Junior Member
Date registered: Mar 2008
Vehicle: 1987 Mercedes 300D, 1983 Mercedes 240D
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Glow plug/relay/starting issue
My 1987 300D (6 cyl.) has developed an unusual problem.
The basic symptoms: I had two glow plugs replaced about 6 months ago. Within a month or so the glow plug indicator stopped lighting. The car still started well (being summer) and continued starting well until about two weeks ago. Just like overnight, it started taking a lot of cranking to get it started, even on 45-50 degree mornings. If it's plugged in overnight, it starts right up. So I took it to my mechanic to change the plugs and when I returned, he said he tested them and they were all with in spec. We decided maybe the relay had gone, so I agreed to buy one online and swap it out. Which we did this evening, and the problem remains.
Here are the weird parts: When I turn the key all the way the first time I try to start it, the glow plug light does not come on. If I leave it in that position for 15 seconds or longer, then turn everything off and repeat the process, the light comes on, for about 5-7 seconds. It's the same symptom with the old and the new relay, and happens the same on subsequent tries. It doesn't seem to help the car start any faster, though.
Another weird thing: when we first put the new relay in and I got the glow plug light to come on after the second try, it stayed on for longer -- even after the engine started running! I "blipped" the engine off and back on and the light went out. For a few times after that, the light would come on and stay on until the engine started, then went out. It stopped behaving like that after a while, though.
Any ideas? Bad plugs regardless of what the mechanic said? I'd appreciate any suggestions.