1981 240D: Road to perdition

[ericnguyen]

This is about my damned 1981 240D. It must be on its road to perdition :-(.

I tested my glow plugs by removing them from the engine block
and laying them down on the valve cover so that each plug's body is in contact with the valve cover (as ground) and their center electrodes were still connected to the GP relay. Next, I turned the ignition to the #2 position. I saw that all 4 glow plugs become RED HOT for about 15 seconds. I retested again and again by going through the glow plug cycle for several times. The glow plugs always became RED HOT. If I touched their body (not the heating element), it's extremely hot.

However, after screwing all 4 glow plugs into the engine block and going through several glow plug cycles, I touched the outer body of each glow plug and they were all NOT even warm (same temperature as the engine block). The voltage across the battery did drop, indicating that the glow plugs were sucking energy.

How come? I don't think the glow plugs can dissipate quickly and completely all of its heat into the engine block. So why the glow plugs's outer bodies were not even warm when I touched them after going through several GP cycles while they're extremely hot
when tested outside the engine block?

Could sometimes try to touch the outer body of their car's glow plugs after going through 1 or 2 GP cycles? Hot, Warm or Cool?

Thanks a lot for your information.

Eric

[engatwork]

I don't know Eric. I think you are going about it all wrong. I have seen these cars start with only one heathly cylinder hitting after having the fuel lines off. If it were me I would begin by checking the "start of delivery" and then if the IP is time right I would have to pull the cylinder head and take a look to see what is going on there. If the glow plugs glow on the head I would not worry too much about them not being "hot".

[leathermang]

So Eric, Lets review what has happened....you posted the other thread... and four guys post pointing you towards stuff like reading the compression, starter condition, cranking speed too low,etc....
You don't like their answers (?), so you start a new thread.... concentrating on the glowplugs...
The reason they pointed you in other directions is that a normal diesel engine, in warm weather, getting fuel, and turning over fast enough will start WITHOUT the glowplugs working at all. This is a compression ignition engine... the glowplugs are icing on the cake for less than ideal conditions....
So they are figuring that even if you fix the glowplugs you have a problem which is not yet diagnosed....
and the one guy that has posted on this thread prior to me was pretty blunt " going about it all wrong"......
Of course, if we let you continue down a path we consider unlikely to help... until you get really frustrated, one of us is likely to pick up a fixable 240 and you think we were doing you a favor...

[JohnM]

The body of the glow plug is just dissipating the heat into the engine when it is screwed in and is not fully when the plugs are laying on the valve cover. This is probably leading you down the wrong path. Sounds like something else as the other posts indicate.

Since you asked the question in earnest, I cycled my glow plugs twice and checked if they were hot. They were the same temp as the engine.

[MS Fowler]

You seem surprised that the GPs do not get hot to the touch when installed in their proper location. Think about this for a minute.
When you tested them outside the engine, the heat had nowhere to go. Of course the bodies got hot. They are close to the source of heat and there is no where for all the heat to go, so it stays on the GP body.
However, when they are screwed into the head, the head makes an excellent heat sink. The threads are a good, clean, metal-to-metal contact path for the heat. So instaed of heating up the GP body, it transfers to the head. The head is a large heat sink and can absorb a lot of heat before you could feel any temperature change.
If the volt meter shows that they are drawing energy, it means they MUST be working--otherwise there would be no current flow and no heat.
I would guess that your GP and circut are fine. Look at the other issues addressed by other answers to your posts.

[ericnguyen]

Dear JohnM: Thanks for trying it! At least I know that my 4 glow plugs were actually working OK inside in the engine block.

Dear Leathermang: It's not because I don't like their answers. I'm currently too busy at work and at home (newborn baby and visiting relatives) and haven't had enough time to try what they said. Also I will have to order a diesel compression tester since I don't have one now.

Since it's easy to test glow plugs, I just wanted to make sure that they did glow RED HOT in the diesel pre-combustion chambers so that I will be confident enough to focus on what the other gentlemen suggested when I'll have some time.

Dear engatwork: thanks for the suggestion that I should check the "start of delivery" and IP timing issues. I will try them as soon as I have time.

I've just had a baby girl for several months and only want to spend time for my baby rather than for cars (especially diesel cars with which I do not have any previous experience. I bought this 240D as a project car to learn about a MB diesel car which everyone here is crazy about :-). My wife also complained a lot whenever my head in under the hood :-).

Thanks again.

Eric

[ericnguyen]

Dear MS Fowler:

Thanks a lot for your explanations.

Eric

[leathermang]

Eric, Several people found good compression guages online.... inexpensively... but I can not find the thread they gave the info on... I did run across this thread which might be helpful ... you might even shoot for leakdown instead of compression... all things being equal...

Leak down v. compression test

If you don't think measuring the resistance to ground is adequate then you can test the plugs in the block. Just pull the injectors and sealing rings. Turn on the ignition and look in the prechambers. Watch the glow flies come to life. It's amazing how hot they get. Make sure you have spare seals around and at least a metre of drain line. I usually do this when I inspect injectors for crack/reseat/spray, replace filters and check timing.
All the posts about not needing the GP's in the summer are correct unless the injectors are clogged and not spraying correctly. In this case the GP's help an out of tune engine get started. The same thing could happen if the timing is advanced too far. The air in the cylinder would not be hot enough to cause ignition when the fuel enters the cylinder. Sure sounds like you need to forget the GP's and move on. Good Luck.