Bad glow plug causing rough idle?
 

Is it possible that a glow plug with a high resistence can cause a rough idle that would make the car seem to be "dropping a cylinder"?

I'm supposed to take mine in Friday to the Mercedes dealership (it's under dealer warranty for a few more days) for a rough idle that my mechanic says is not normal and doesn't appear to be bad diesel related. I talked to the service writer and he said a tech thought that a bad GP may be the culprit. I thought that weird as I was under the impression that GP's only started the car (I am far from a mechanic, as that statement may imply).

Any thoughts?

Yours is probably way different than mine but I would assume that a bad glow plug would only cause rough idle when cold.A bad injector would cause rough idle all the time.Good luck:)

Glow plugs are activated BEFORE you start your car to preheat the cylinders so that the fuel will flash and burn. The glow plugs usually turn off after 20-45 seconds depending on whether your car has fast or slow glow plugs. Not firing on one cyliner properly - definately suggest bad injector.

Thank you!! That's what I thought, but the service writer tried to Barbara Streisand (BS) his way through by telling me that a GP was the diesel equivalent of a spark plug.

Frustrating, very frustrating.

Mercedesshop get-together in NC
 

We'll be there Wednesday to whip his ass!!

Those with cracked leather upholstery will have first crack at his hide.

Give him one for me,Matt!! You would think the MB service writers would be a little more skilled than that!:rolleyes:

Interesting. However, a bad glowplug, if it drops bits off the end, can cause idle problems by partially blocking the vents in the prechamber to the cylinder. Usually causes pretty heavy knock, too.

And I wonder -- my Volvo seems to be much happier now that I have replaced 5 of 6 glowplugs (#6 is a huge pain!). Four of those I replaced had a neat little hole in the end where the filament broke free of the weld, and I assume that they are large enough that there would be a minor change in compression chamber volume if the hollow interior is exposed, rather than sealed off. The little hole could cause strange things to happen with swirl patterns and airflow, too.

I've also heard of glowplugs failing at the other end, too, and leaking compression. This will definitely cause idle problems!

Peter

Thanks for the insight Fred. I've been on vacation for a week but should talk to my mechanic this week and will let you guys know what the cause was.

Oh yeah, I forgot to add this. The rough idle is only when I have the car in gear. If I put it in neutral or park the idle is smooth as silk. Don't know if that matters, but thought it may shed some light on the subject.

I suppose that it could cause rough idle at first as the affected cylinder(s) would not be up to temperature and would need to catch up with the others. My 300SD had 3 bad glow plugs and was hard to start but idled okay although smoky.

In retrospect, we probably shouldn't have killed and skinned that guy.

Live and learn,

Matt

Don't let him off just yet...remember, the idle is smooth at first and only gets rough the closer the car gets to operating temperature. Once it's at operating temp it begins to act like it's "dropping a cylinder".

Again - I repeat - glow plugs are only used to pre-heat the cylinders. If one or two are dead - those cylinders will be cold and not firing properly when first started causing rough idle when COLD. As the engine warms up and the cylinders with the bad glow plugs warm up from compression and misfiring - they will smooth out. If the engine goes into rough idle when WARM - it cannot be the glowplugs.

Update and more questions
 

On Monday I dropped Riggo off at the Doctor's Office (Randy Jensen's Motorworks for those of you in the Greensboro, NC area)so he could ascertain what was causing the weird vibration. As would be expected, the Benz decided not to vibrate that day:rolleyes:. Randy drove the car around for 30-45 min. to pick up parts and even took it home over night to see if he'd act up. Of course, he wouldn't. He and I discussed it and decided that because the abient temp. outside never got over 36 degrees, the car couldn't get hot enough to start the vibration. Since the car ran so well while he had it, he ruled out any major problems. Also, he had seen it 2 weeks earlier when it was vibrating, so he knew I wasn't looney.

Long story short (yes, I realize I missed that opportunity about 40 words ago ;)), he decided put in an oil additive (not a fuel additive) and see if that cleaned it up any. Guess what? It hit 67 degrees today (3 days later) and the vibrations have been reduced significantly. They haven't totally disappeared and if I shift to neutral the vibrations will all but go away as they did before.

So, what components in the car would have been cleaned by the oil additive? Also, could this mean that running Diesel Purge and getting the valves cleaned up could eliminate this all together? I did find out that the 120k was NOT done as the dealership had said it had been. Go figure. So I'll probably get that done this month.

Sorry for the long post. I'm not real good at editing, takes all the fun out of it.

Munsey

What oil additive did you use to stop this idling condition? I gotta get me some of that.

The stuff is called Marvels Miracle Oil. Randy said it's been around since the 20's but is still a good product. I just took it by the shop and he added about a Wendy's medium drink's worth (he couldn't find a measuring cup). I'm going to put a 20 oz. bottle of Techron in it, fill it up and drive the piss out of it at lunch today. All this combined with the 120k service should have it runnin' like a champ.