1999 E300 Glow Plug Confussion
 

For background I replaced all of the GP's in the summer of 2005. At that time I did not check to see which or how many were bad. I just replaced all of them. When the GP light came on the Check Engine also came on. Once the GP's were replaced the GP light went out and I had the Check Engine light cleared out.

Last November a GP went out. This time there was no GP light or Check Engine light, but a cold started resulted in a rough running engine for a few seconds. This time I checked the resistance at the GP relay connector and found which one was bad. Only one was bad...replace and no problems.

Now today I start (very quick start) and I have a rough running for a few seconds at a cold start and both the GP light and Check Engine light are on. I checked the resistance at the GP relay connector and only two show any resistance; the other four are 0.0. I find that difficult to believe because I would think that the engine would have been much more difficult to start if four GP's were out. Also, why and I getting a GP light and Check Engine light? Is the GP light controlled by the resistance of only one GP? What is the parameters for the GP's that triggers the Check Engine light?

By the way the engine starts I would think that only one GP is bad (maybe two) and that is what I want to replace. Any wisdom on this??

Maybe your voltage regulator is putting out too much voltage and burned out several GPs at once? I don't see how, if you are measuring resistance correctly, you could have 4 out one day and not have noticed any issues before that unless something caused all 4 to go at once (in other words, if you had lost a GP at a time you should have seen some hard start conditions).

I know when I changed the battery in one of my diesels, the very next start I lost a GP which tells me that they are like light bulbs and a surge can burn out a weak one. So a good strong battery, in my case, pushed a weak GP to death.

Today you said you did a "very quick start" which i surmise means little or no glow? That would set PO380 GP Circuit Malfunction when start-up occurred on cold GP's. If that were the case the CEL would go out after the 3rd cold/hot run cycle if the fault did not re-occur within that time.

It has been in the high 50's today. After letting the car cool down as much as it could I started it. The glow plug cycle seemed normal and the engine fired right away. After running for 3-5 seconds the GP light comes back on. I can not imagine that more than one is out, but that surely does not line up with my multi meter readings.:confused:

Note: the battery was changed about 18 months ago.

I guess that begs to question your method of testing then...did you go from ground to each pin on the plug and if so are you sure you had a good ground?

I ground on the head and then to each connector. I am puzzled.

Very close to the relay (6" toward door) is a neg ground. Holds down 3 or 4 wires. Use it. It isnt aluminum like the head.

Bingo! Thanks, that worked. Five read 0.8-0.9 and one read 1.6. I thought that the last time the bad one read 0.0.

I have glow plugs and a cash of intake manifold gaskets. It has been removed so many times on this car. I need to find a away to attach the intake with Velcro.

The head should be a good ground, after all that's how the GPs get grounded, but you might not have had a good solid connection at it when you first tested them. User error should always be high on the list of suspected reasons when test results defy logic...no shame in it, we have all been there and done that. Good to hear you got it right this time.

FWIW the GP at 1.6 ohm is probably still working, just on its way out so it might not be worth rushing in to change it...if you wait until you have another reason to pull the IP off you can kill 2 birds with one stone. Of course if it were mine I would go ahead and fix it now...I hate to have anything I know is wrong go uncorrected, but no harm would come from waiting, especially now that the weather is warmer.