Questions and Answers

Mercedes Benz Glow Plugs Q. I am the proud owner of a 1985 Mercedes Benz 300D with 135,000 miles. Recently, the old girl has been giving me some problems, and since I plan on keeping her till she goes out of style (never), I want to fix her right. Living in CT with the car for the last winter has definitely challenged the glow plugs on the car.

However, I found that even on nights with -0° temperatures, if I repeated the glow plug cycle warm-up at least four times, she would start with a little motivational throttling. Yet recently, with temperatures even reaching a balmy 30°, she won't start no matter how long I let the glow plugs do their thing.

I had to get a tow-truck to spray ether into the air intake and jump the battery (I had killed it after engaging the starting motor for up to 30 seconds at a time) to start it up. Since the battery supplied plenty of power to the starting motor to turn her over again and again, I assume that the problem did not originate there.

This leads me to the glow plugs. I read in an earlier column that when the glow plug dash lamp fails to light up, at least two of the plugs are bad. This has been the case for the last few starts. Can you please go into how to change out the glow plugs on my car?

I picked up five of them at the auto parts store, and think I can handle it on my own. I'd love to really get into maintaining her, but I need to get this fixed A.S.A.P.

Thanks a lot,
Ian

A. Ian, Ian, Ian... never ever use starting ether to start a diesel. Not unless you enjoy the sight of bent and twisted connecting rods and pistons. Another 300D owner once told me he uses WD-40 as a starting fluid and has no knocking with it. But that is his opinion, not mine.

Changing glow plugs is easier than changing spark plugs except for one thing. Each glow plug has a wire attached to it. Take the wire off of each one and remove the glow plug. I have a tool that I designed specifically for these glow plugs. It is basically a ¼" drive flex socket on a long extension. But what I do, since they are tight in there, is break them loose with an offset box wrench. Then I use the flex socket to completely remove them.

Now here is "the one thing". The pre-glow chamber should be reamed out before you install the new glow plug. There is a special reamer that screws into the glow plug hole that will clear the carbon from the chamber and keep the new glow plugs from shorting out. The only place I know of to get this reamer is from a Mercedes Benz dealer.

You can probably get away without using it. It is more as a precaution then a necessity. When you put the new glow plugs in, tighten them with the offset box wrench. Don't over tighten them. 15 foot pounds is plenty. Basically an easy, Saturday afternoon job. No headaches or surprises.