240D Glow Plug removal and inspection

The “Super-Truck” project is something that’s been in the back of my mind for a while. The whole idea is to build a plug-in hybrid pickup, which could run off diesel, battery electricity, and biofuels. Imagine if a Chevy Volt not only ran on bio-fuel, but could also haul firewood and tow a trailer!

Too bad I know next to nothing about diesels.

So, I’ve been slowly learning as I can, reading books and watching online instructional videos, and chipping away at the project. I have a diesel engine out of a Mercedes 240D sitting in my garage. I’ve already test-run it and replaced the water pump. It’s still using the old-style glow plugs. While I’m waiting to eventually get on the same schedule as my diesel-mechanic buddy to perform some REAL work on this project, I thought I would give a shot at upgrading the glow-plugs to the modern style.

I ordered a set of four “pencil” glow plugs from Mercedes Source. My order also included a nice little book showing exactly how to upgrade from the old glow-plugs to the new. So, the other day, I set to replacing the plugs.

To start with, the glow plugs are actually rather difficult to get at with the hard fuel lines between the injection pump and the cylinders getting in the way. The first step was to remove those. I cracked open the connections with a 17mm wrench on both ends of the lines and loosened them the rest of the way by hand. The metal tubes easily pulled away. (I put them in gallon plastic zipper bags and labeled them.)

With that, I could get at the electric connections on the glow plugs. Each has a nut that needs a 8mm wrench to remove. By removing the nut, I could then take off one electric wire, a ceramic spacer, and the other electric wire.

Next, I could remove the glow-plugs themselves. However, I didn’t have the right size wrench. The biggest I had was 19 mm, and the glow plugs seemed to be just a hair larger than that. I used an adjustable wrench to get them started and then used an oversized socket to unthread the glow-plugs the rest of the way.

I had trouble with the plug furthest to the left (closest to the radiator if the engine was still in the car.) Although I completely unthreaded this glow-plug, it still would NOT pull out! I finally figured out that I was spinning only the OUTSIDE portion of the plug, but that the narrow inner part wasn’t spinning at all. It was seized up, stuck in place. I’ve heard of glow-plugs getting gunked-up with carbon, and it looked like this one must have been stuck in it. After breaking the tip off the glow plug with a vise-grips, I started fooling around with several small pry bars. Since the threads were all the way out of the engine block, I figured it was just carbon holding the plug in, and with some prying, I was eventually able to get it out.

Looking through the glow-plug holes, I could see that all of them had some carbon build-up, but that last one was the worst. I’ll have to buy or build some sort of a reamer to clear out the holes before installing the new plugs.

Comparing the old glow plugs to the new, one physical difference is pretty obvious. The old plugs have a loop on the tip. This is so that the current can travel through the plug and on to the next one. They are all connected in series. The big downside is that if one glow-plug fails, it breaks the circuit and they ALL stop working.

On the new style plugs, current goes from the power source, through each glow-plug in parallel, and then grounds through the engine block. These make more heat faster, for easier engine starts, and if there is a problem with one glow plug, it does NOT interfere with the operation of the others. I’ll also be interested in finding out how much current these plugs pull, as I will need to know that so I can get a correctly rated relay and fuse for a glow-plug momentary-on button.

For now, the old plugs are out. Next step is to figure out how to ream the carbon out of the holes. After that, I can get the new plugs in, wire them, and test them out.

Progress on the “Super-Truck” project might be slow, but it always feels good when I get at least a little something done on it.

Til next time, stay CHARGED UP!

-Ben

 

UPDATE: I just tested one of the new glow-plugs, running it to a 12V battery, and it pulled about 8.5 amps. That’s around 100 watts per plug. Imagine the heat of a 100-watt light bulb, concentrated down to the tip of a pencil. Yowza, that’s hot! Looks like I’ll be needing a 40 amp relay to run the glow-plugs in the finished vehicle.

Tagged as: Diesel, glow-plugs