Uneven cold starting after glow plug replacement? - PeachParts Mercedes-Benz Forum

bhatt · #1 01-12-2003, 09:12 AM

Hi guys..

My '86 300SDL (160k mi) is doing something weird since I replaced all 6 glow plugs. I replaced them strictly as a maintenance item (purchased the car 2 years ago and have no idea when they were last changed..)

When I turn the key to start, the glow plug light now takes about 1-2 seconds to light up, where it used to come on immediately. Once the light goes out, I start the car and it starts all bumpy, as if if one cylinder is "missing".

If I wait a little longer after the light goes off then start, it starts MUCH better. I know this is the best way to start a diesel in the cold, however I never had to do this before! Once the car is warm, it runs PERFECT.

My guess is one bad glow plug and/or I screwed something up while hooking everything back up.. Any thoughts on what I can check before I tear everything apart and look for the problem?

I'm thinking of testing glow plug resistance.. this should hopefully tell me if there is a bad new plug and/or if I screwed up in connecting one of the glow plug wires or something. Do I just pull the connector off the relay and test from each plug wire to ground? Is there a ground terminal in that connector or do I just find a good ground area in the engine compartment?

Thanks for all your help!

Neal

Ken300D · #2 01-12-2003, 10:09 AM

I think the glow plug resistance check is your first diagnostic procedure to take.

Somewhere I read that if you over-tighten the electrical connection to a glow plug it will damage it and cause a failure. So there is a torque specification for the nut that holds the wire.

Ken300D

bhatt · #3 01-12-2003, 03:37 PM

I tested the glow plug resistance and each of them is identical at 0.8 ohms. I was REALLY hoping one of the plugs or connections was the problem.. not sure where to look next.. HELP!

Neal

Ken300D · #4 01-12-2003, 07:28 PM

What brand of glow plug did you put in?

Ken300D

bhatt · #5 01-12-2003, 10:05 PM

I put in 6 brand new Beru plugs.. are you thinking I have a bad plug? Wouldn't the good resistance test prove that the plugs are all good?

Neal

Ken300D · #6 01-13-2003, 11:36 AM

I don't know. Maybe. I would have to recommend the Bosch glow plugs - sold right here on FastLane.

The next test you could do would be a current test. This requires a multimeter capable of measuring about 20 Amps of DC current. Most multimeters cannot do that, but I can point you toward one that is relatively inexpensive if you want to try it.

The idea is that all the glow plugs should draw similar current when tested through the multimeter with the car battery being the source. Typically this starts out around 15 Amps and settles down to maybe 8 amps when the glow plugs get hot.

Sometimes a bad one will not draw much current and will not heat properly. Something like 3 or 4 amps.

Ken300D

joe p · #7 01-13-2003, 11:37 AM

I've seen more than 1 set of plugs destroyed due to over torquing them into the head. The only accurate test for glow plugs is a amperage draw test which requires a amp meter capable of handling 30amps.

The plugs should start cold around 21 amps and fall to 15-12 in 15 to 20 seconds.

Joe