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.

Well, I just added a whole bottle of PS Diesel Kleen, a 20oz. bottle of Chevron Techron to a full tank of truck stop diesel (hoping to get a "fresh" supply). I then got on the interstate and ran it up to 75 MPH in third (RPM's around 4500-4800) and kept it there for about 7-10 minutes. When I finally got off at the exit and stopped at the light, the vibration was reduced significantly. It's starting to feel the same whether the car is in Neutral or Drive at a stopped position. Knock on wood...we may have it licked.

The service writer should have told you the truth instead of BSing you!!!

A miss at idle is always caused by a bad muffler bearing.

That's what I was told anyway...

Not necessarily. Could be a worn battery gasket. Or headlight fluid.

Or the Expedelemus could be shorting...

Hmmmmm, ...expedelemus eh?
aw rats, now why didn't I see that one coming...

I've spent a lot of money on headlight fluid at my indie already, so I knew about that!

Thanks Duke! I'll go ask my indie about it! He doesn't charge much for questions like this.

You guys are way off base - the negative bias to the reverse-biased injectors is probably intermittantly disappearing. I would check the computer that applies the negative bias to the injectors.

It's Back
 

Well, looks like all the cleaning in the world won't fix this vibration. It showed up Tuesday after driving it a while. Looks like it's time to start swapping injectors. I'm taking it in next week for my mech. to work on it. We'll see what happens.:(

Sounds like you have misaligned Johnson Rod.

C'mon Gary - you of all people should know that Johnson Rods are only found on Gas engines!!

You talking about the Magical Mystery Oil? :cool:

It's actually called Marvel Mystery Oil. It's blood-red (in fact, there are those who say that it's re-packaged ATF, but I think it is at least slightly different) and comes in metal cans (although I think theyr'e finally starting to use plastic) and can be had at just about any auto parts store. The can will be mainly red, with black and white graphics and whatnot on the side.

Great stuff, but I actually found a couple things that could work better. One's called Seafoam. I haven't tried it in the fuel yet, but I put it in my truck's oil a while back; it seems to run smoother and be a bit more responsive. It's essentially working to clean out the oil galleries and such. It's supposed to be useable in oil and diesel fuel.

The other...I actually don't remember the name of it right now. :o I got it from Kragen, and put it in the fuel of my truck...seemed to have more power and ran a bit quieter. The jury's still out on it, though...the truck broke down last weekend and I haven't torn into the truck to figure out why yet, but it's definitely fuel-related (need to replace the return lines at the injectors, and I also either need a lift or an injector pump :(), and I don't know either way as to whether or not the additive made a difference.

FWIW (I'm just starting to learn the quirks of M-B diesels, but I've been around diesels for a long time) I vote for an injector. The engine's too new to have anything internally wrong with it (i.e. I doubt you've lost compression), and you'd be getting lotsa smoke and be running rough while cold if it was a GP.

Just my $.02 :)

I have only two words to say about the vibration I used to have at idle....

Engine Mounts

Looks like what I thought was a dropped cylinder was just a simple wear and tear item. My Mech still thinks that the original symptoms were those listed earlier, but, after replacing the mounts is inclined to believe that it was just bad fuel at that time. He told me when I first got the car that I needed to replace the mounts. I just didn't know how big a difference it would make. It's like someone dropped an gas engine in there. Oh, well. I'm loving it now and did have a good time with the posts here. If nothing else, I learned to make sure to keep an eye on my reverse-biased injectors and to be sure that my Johnson Rod doesn't get misaligned

Have a nice day.
Munsey

Deuce:

I had a similar vibration in my 1987 300TD wagon after I purchased it.

About two months after persistent use of PS (double dose at every fill up) and some occasional use of other cleaners, the vibration was reduced both in term of amplitude and frequency. At that point, I got the vibration, maybe 1 out of 5 times (compared to 1 out 2 times when I bought it), when stopped at a light at idle.

Now it is four months after the persistent use of PS, the vibration is completely gone. I could not remember feeing it in the last month or so.

In these four months, I have used some additional cleaners but mainly PS. I have used one bottle of Techron and some Gunk diesel fuel conditioner. Also lately I have been using a Premium Diesel fuel from Crystal Flash.

My experience tells me that you might want to be patient and keep cleaning it up. In the mean time, also try to use some
oil that has lots of detergent in it (CH-4 rated), if you are not already using it.

I was thinking motor mounts all the way but cleaners did the job. It was not over night, but it did it. Originally I was preparing to change the mounts in the spring after the Michigan winter but now I am not going to tough them.

I totally agree with you on the use of additives. I actually am bouncing between PS and Howes right now. Although, I think I ruined all of my O-Rings with the last few tanks because I used too much additives (see previous posts). Last week the think started leaking like a sieve. He's over at the mechanics right now getting a whole new set.

As for the vibration, there isn't one anymore. I had the mounts replaced last Wednesday and it's like a brand new car. It starts smoother, idles smoother and shuts off smoother. If you've got any vibration while idling and still have the original mounts in there, I'd consider changing them out. Based on some of our previous discussions, you probably would be able to do the job yourself. The parts would run you about $100-$120 and take you half a day, tops.

Anyway, thanks for the advice. I'm going to stick with the PS in every tank for maintenance and performance reasons.

Have a good week.