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Old 02-14-2006, 04:19 PM
Stevels's Avatar
Running Used Cooking Oil
Join Date: Nov 2005
Location: Providence, RI
Posts: 37
Lightbulb A Strange Glow

Hi Forum folks,

This may be a question already answered on a thousand posts, but once again, I could not find my answer within a half hour, and decided to make a new post.

I have a '91 350SDL in great overal condition. I converted it to a Diesel/Waste Vegetable Oil hybrid and it has been running great for months.

About two weeks ago, my glow plug light no longer comes on when turning on the ignition on a cold engine. Interestingly, the engine still seems to start on these cold New England mornings, but it seems rough and a lot of white smoke comes out for the first few minutes when cold.

Here is the strange part… occasionally, the glow plug light will come on WHILE THE ENGINE IS RUNNING, stay on few 10-20 seconds, and then go off. This usually happens when the engine is not quite up to temperature, but certainly not ice cold.

The glow plug light never comes on anymore when starting the engine – even when ice cold, and occasionally comes on when the engine is farily warm and running. WTF?

I have not a clue what to do, where to look or what this means.

I assume it is not a simple "short", since it behaves consistently – though incorrect.

Besides wanting to fix this, I truely wonder why the car still starts up without the glow plugs preheating the cylinders? In fact, the engine actually seems to start up faster now that I do not have to wait for the glow plug light to go out before I crank the engine… but how… why??

Please advise me, oh people smarter than I!


Providence, RI
Current Fleet:
1991 MB 350 SDL running 2 tank Frybrid Waste Vegetable Oil
2005 Toyota Prius
1997 40' Bluebird Wanderlodge motorhome running running world's first 3 tank Frybrid Waste Vegetable Oil
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Old 02-14-2006, 04:32 PM
69 mercedes 220d
Join Date: Oct 2005
Location: Bozeman, Montana
Posts: 417
glow plugs

I'm not familiar with the glow plug system on your '91, but it does sound like a short in the ignition switch or in the glow plug relay. All the white smoke says your glow plugs are not functioning when they should. As to the glow plugs coming on when warm running, this also sounds like an inconsistent short.
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Old 02-14-2006, 05:35 PM
babymog's Avatar
Loose Cannon - No Balls
Join Date: Jan 2006
Location: Northeast Indiana
Posts: 10,767
In my '87 a no glow-light means at least one burned-out glow-plug. Either #1 is burned out, or more than one of the remaining plugs.

- Jeff Miller

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