All was good, now it is not. The symptoms include no glow plug light prior to ignition (can hear the click after about 30 seconds with ignition in on position), then the light comes on for a minute or so. I have tested the plugs and found one with an open circuit and needing to be replaced. The car starts ok, but misfires and smokes for 10-15 seconds. The odd thing is that the normal startup nailing doesn't start until just before the engine starts to smooth out, say the same 10-15 seconds after startup. I never noticed the delay in nailing before the glow plug system stopped working properly.

Can just one bad glow plug cause the symptoms? How can i be sure the relay is working at all and that the glow plugs (albeit 4 out of 5) are getting juice? would the car start easily if the glow plug relay was shot (it has been in the lower 30's in the mornings)? is it just one bad plug or a bad relay?

The fact that you hear the relay click off after 30 seconds is a pretty good sign that it is working.

No.

If one of your plugs is bad, then that would be the thing that needs to be addressed first. I had one plug out, and at 30 degrees, the engine would start, but would have a slight nailing or miss, which it did not have prior to the glow plug going out. It would sound that way until it ran for a while and warmed up. Plug replaced, nailing no longer an issue. Good luck

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96 E300d

SG,

How many miles on the 602.962?

'Got the small alligator clips on the Meter leads?
'Turn on meter,set to Volts DC.
'open the hood clip one lead (positive) to the tip of the #1 glow plug,
clip the other lead (Negative) to a ground (Block/Head).
'set the meter up so you can read from the pilot's seat.(through the
windshield)
'Turn key to GLOW position.Observe the meter.If glow relay works you'll
see the proper voltage at the plugs from the meter.

(If it's starting [as you said it is] the relay is working.At 30 degrees without
the glow system it would not start.You may in fact have a bad plug,which
would account for your problem.)

I had a glow plug go out last month and it did the exhact thing your's is doing.
The glow light would not come on but the relay was working. The engine would start, run rough for a little bit, then it would smooth out and the glow light would come on for a few minunits. Replaced the bad plug and it was back to normal. By the way, my car won't start at all if the glow plug relay or fuse is bad. My diesel is a 91 but I am sure they are almost the same.

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84 500 SEL (307,xxx miles)

if you have your owners manual you will find that the glow light not coming on until after the engine starts is intended to alert you to the fact that you have one or more failed glow plugs.There is plenty of information on this site for testing methods,but the easiest is to connect your test light alligator clip to the battery positive post then unplug the glow plug wire harness from the relay and touch each socket with the test light,when you find the one that does not light the light look closely and you will see the sockets are numbered coresponding to the cylinder numbers.It looks more difficult than it is to replace the plugs,if you have a decent set of 1/4 drive sockets and a universal joint they can all be replaced with the manifold ect in place. My $.02 Don

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Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles

Thanks to all for your advice. It was indeed one bad glow plug (cylinder #3). Changed it today and all is good again.