Compression test on '78 240D (first-timer) - PeachParts Mercedes-Benz Forum

BodhiBenz1987 · #1 05-06-2012, 01:15 AM

I'd like to do a compression test on my 240D sometime in the next few days, which I have never done before (on either car). I've always kind of wondered where the 240's engine stands, since it has loads of blowby and idles rough on and off but starts fairly easily even in the cold. I'm getting discouraged with this car, again, and maybe some compression numbers would help me one way or the other.
I have the FSM and have done a fair bit of forum-searching, but just want to double-check a couple things.
1-I have a remote starting device my dad gave me a while ago. This is going to sound silly, but I'm not really sure how to use it. In the FSM it says to attach to terminals 30 and 50 ... does it matter which leads goes where? At that point do I just press the button and hold it as the engine cranks?
2-Since I only have two hands I can't hold down the stop switch, so was going to try to wire it down. How tightly does it need to be wired? I.e., is it going to try to pop up and start or is slight resistance enough to keep it from doing so?
3-The shutoff line is the one going into the very back of the pump, right? How much vaccuum do I need to put on it to keep it from starting, and will applying vaccuum once be enough?
I could figure some of this out by trial and error but I'd rather not have it start accidentally while I'm trying to take a compression reading.

vstech · #2 05-06-2012, 01:58 AM

if you pull all the gp's or injectors, one, it'll spin faster without draining the battery or straining the starter as much, and two, it'll NEVER start!
you should remove the injector lines from the injectors, but leave them on the IP, and wire up the stop linkage. not the stop lever.

funola · #3 05-06-2012, 02:02 AM

Take all glow plugs out before doing compression. No way will engine start and run on one cyl.

remote goes to starter solenoid and + term of batt

jam a piece of wood cut to the right length between stop lever and IP

vstech · #4 05-06-2012, 02:05 AM

Hmm. I'm not sure you want to just hook to ANY terminal on the starter.
I'd hook to the battery side, and the small wire on the starter. both are on the starter itself. the large wire on the starter and the small wire on the starter.

DeliveryValve · #5 05-06-2012, 03:42 AM

Quote:

Originally Posted by vstech

Hmm. I'm not sure you want to just hook to ANY terminal on the starter.
I'd hook to the battery side, and the small wire on the starter. both are on the starter itself. the large wire on the starter and the small wire on the starter.

I always use the junction box near the battery.

See pic courtesy of diesel911.

Connect the two leads on your remote starter on the outer two screws. Once you press down on that button the engine will turn over.

.

funola · #6 05-06-2012, 11:55 AM

240D does not have a junction box as I recall. There's a 50/50 chance she'll get the correct terminal. If it doesn't spin, go to the other terminal. The big terminal is + 12, if you connect the remote starter across the small and big terminal of the starter solenoid, there's no chance of shorting (if wrong batt terminal connection was made).

As I recall, the 240D has a lot more room than the 300D for glow plug compression testing. Removing the injector lines is not really necessary unless you have huge hands. #4 will be the hardest one with the IP in the way.

You can wire the stop lever shut but use a heavy duty bungee on the wire and hook it on the hood hinge. Wire alone has too much slop for complete fuel shut off.

I'd suggest doing both a cold and hot tests. More work but will provide more data. The cold test will be good practice for getting the glow plugs out fast for the hot test.

Have fun!

BodhiBenz1987 · #7 05-06-2012, 04:05 PM

Thanks guys. I think I've got a pretty clear idea of the process now. Hopefully I'll be able to try it tomorrow or Tuesday.

BodhiBenz1987 · #8 05-13-2012, 11:27 PM

Funola, I should have listened to your advice on doing a cold test first as practice on getting the glow plugs out! It took me so long to get them out (and I broke a vaccuum line the process) that the engine was not at all hot when I was done. Since it's all apart now, might as well wait until tomorrow and do a cold one, plug it all back in, warm it up, and then do a hot one. I don't think it will take me so long the next time.

I did get the remote starter to work, but I'm having trouble wiring the stop lever in the right position. First I discovered it didn't even shut off the engine.

I finally figured out (after a lot of reading) it was just because I had the idle cable adjusted too tight. Now it shuts off w/ the lever, but the bungie doesn't hold it up. I'll have to keep experimenting.

Phillytwotank · #9 05-13-2012, 11:31 PM

Just use a midivac on the shut off valve while you do the test.

BodhiBenz1987 · #10 05-13-2012, 11:43 PM

Quote:

Originally Posted by Phillytwotank

Just use a midivac on the shut off valve while you do the test.

It won't hold the vaccuum by itself, unfortunately, so I'd have to pump with one hand, push the starter trigger with the other, and read the compression gauge at the same time ... I guess that's possible but might require more coordination than I have! I could probably hold the shutoff lever with one hand though, I may try that.

piccolovic · #11 05-14-2012, 03:51 AM

There's always keeping someone in the car to crank it over at your command!!! Anyone think of that???

t walgamuth · #12 05-14-2012, 06:32 AM

I always just put the compression tester in and hop in the car and crank it seven or eight times then go see what it reads.

funola · #13 05-14-2012, 10:46 AM

You can use the key but since you have a remote starter, I'd say use that instead so you can see what the gauge is doing as you crank (be sure to wear safety glasses). The gauge needle should stay put. If it drops after you stop cranking, it's leaking.

The bungee on the stop lever must be strong and tight and pulling at the correct angle. I stopped using that method (because it's dangerous if the bungee let's go) and use a wooden stick to wedge the stop lever and keep the stick in my compression tester kit. I'll see if I can get a pic later.

Not sure what tools you're using for glow plug removal. For fastest results, you should be using 1/4 in drive ratchet, socket, extension on a universal on all except #4.

BodhiBenz1987 · #14 05-19-2012, 12:16 AM

OK guys, I bravely tried a cold test tonight. I think on the first try I didn't have the stop linkage pushed far enough, because I smelled diesel after cranking (but it didn't start of course). After that I removed the idle cable so I could push the stop linkage farther, all the way. I used a socket bit between the pump and arm and it held fine. But I'm wondering if getting fuel in the cylinders in the first place messed up the test, even though I let it sit for a while after that try (and each try actually). My numbers don't sound right: Numbers 1, 2 and 4 were all around the 340 mark and number 3 registed 400.

Does fuel injected during the test cause false highs? I don't mean to be a skeptic, but I don't feel like my 366,000-mile engine would have a COLD (has not run for three days) compression of 400 psi (I'm understanding cold compression test should yield lower numbers, right?). It does start pretty easily, but ...
Other observations: The first two glow plugs had light oil on them when i removed them, and a small bit of oil dribbled out of four glow plug holes during the test ... with a light fuel smell.

During the #3 test, it looked like light smoke or steam was coming out of #2, but I did not notice it again while testing #4. I did notice light oil "spray" on the side of the IP from the open holes.
I want to do a proper full temp compression test tomorrow, but don't want to proceed until I know I've got the fuel shut off correctly. I did try pumping the MityVac on the shutoff while I cranked, on the final cylinder, but not sure if it made any difference. Thoughts? Is a little diesel smell normal during the test?
I guess once I get the glow plugs back in and the car warmed up, I can try starting it again with the same stop linkage setup ... if it doesn't start, would it be OK to assume the fuel is adequately restricted?
An alternative would be to try this with the injector holes instead of glow plugs, I suppose.

funola · #15 05-19-2012, 10:21 AM

Fuel injected can result in false high readings. Make a jamb rod cut to the right maximum length instead of a socket to insure there is no fuel injected. I'll try to get you a pic soon.

One mod before you put the glow plug wires back on for the hot test is to make the ring lugs into spade lugs with a pair of dikes. This will allow taking off the wires without removing the nuts (just loosen them) saving time, and the engine is hotter.