The Compression Test Blues? - PeachParts Mercedes-Benz Forum

djburt · #1 09-20-2012, 03:45 PM

OK, my title may sound familiar to some of you, cause I recently posted a similar thread on The Valve Adjustment Blues. If you read through that one, you'll see that my valve adjustment blues were mostly cause it was my first valve adjustment ever and I was slow, and then I didn't fully prime it after I reassembled the fuel lines (supply and return). But I still had lots of white smoke, and a rough idle, and it was recommended I stop where I was and do a compression test.

I was finally able to get out there and do that today and thought I'd use a different post since it's a different topic. I started with the HF compression test kit for $25, cause the leak down gauge was like $55 and I don't have air until I can get down to Junkman's place (thanks again for the offer Junkman!).

First, though I drove the car 3-4 miles to the car wash and back so I'd be working with a hot engine, it took me a while to actually get started on the compression. I'd say probably about an hour. Reason was my 27mm deep socket was only deep enough if the inside part of it barely fit over the metal fuel return nipples...I actually had to ever so slightly bend one of those that had been been out (probably by me tugging on the flexible fuel lines at an angle). And also I had to stop and fabricate the wooden stick to hold the throttle to the stop position. So I'm sure that my numbers would have been higher had I started the test within 10-15 minutes.

But I finally got the lines off, injectors out, and everything set up, and had a friend to crank ten revolutions for each cylinder. I tested all five cylinders in order and then went back again and repeated every one again, and here are the results:

Cyl - 1st Test - 2nd Test
1- 250 200
2- 260 220
3- 200 160
4- 75 65
5- 100 80

Ouch, especially cylinders 4 and 5. At first I thought it might be just cheap equipment that was causing the inconsistency, but then remembered how drastic a difference the readings are hot vs cold, and there seemed to be a fairly consistent percentage drop as the temperature continued to lower.

So maybe I would be up in the 300s with a nice hot engine, but still...4 and 5. I guess you guys are gonna tell me to do a leak down next to try and figure out where the leak is coming from in 4 and 5. Is that right? Looks like I need that gauge after all. Could someone please explain (keeping in mind I've just begun to learn the workings of engines) what the possibilities are, and what can be done (if anything) to correct them?

(or feel free to direct me to another thread that details those possibilities)

Thanks everyone!
Guess I'll get to work on cleaning the injectors and replacing the nozzles with my new Monark ones for when I eventually put everything back together.

Simpler=Better · #2 09-20-2012, 03:59 PM

Are you 100% sure that your valves are properly set on 4&5?

Sources of a leak (in no particular order):
rings
head gasket
injector heat shield (improper installation can effect)
glow plug (improper installation can effect)
intake valve seat (poor adjustment can affect)
exhaust valve seat (poor adjustment can affect)

djburt · #3 09-20-2012, 04:27 PM

In short, no. I just did the valve adjustment a couple weeks ago, but I wasn't able to double check all the valves cause my power steering belt started slipping before I finished...and it's also possible that they need to be REadjusted. Some have said if it was a long time since the last valve adjustment, sometimes you should drive it 100 miles and then adjust them again.

Junkman · #4 09-20-2012, 04:57 PM

FWIW, I think I have a HF leakdown gauge. It isn't exactly a precision instrument and I've no idea whether it fits a diesel. Probably not worth much.

Simpler=Better · #5 09-20-2012, 04:58 PM

If it's the first time ever you've adjusted valves ever, you should probably re check them before diving in any deeper. They can be difficult/ sneaky until you've learned the tricks firsthand...then you get a big ol' DOH moment when you see how easy they are.

rscurtis · #6 09-20-2012, 05:55 PM

If you don't want to drop the freight for a leakdown tester, simply lock the engine at TDC on the compression stroke of the cylinders you want to test, and supply compressed air. You will hear the leak from the filler cap (rings), the air intake (intake valve), or the exhaust (exhaust valve).

Diesel911 · #7 09-20-2012, 07:45 PM

If is is an equipment issue when you re-test the compression on #1 it would be lower.

djburt · #8 09-20-2012, 08:52 PM

Diesel911 - Were you saying that the lower second test number could possibly be because of faulty equipment?

rscurtis - Thanks for the tip. Embarrassingly, I had to look up TDC, but now I know.

I think double checking the valves is a good idea, so here's my plan:
-finish cleaning the injectors and replacing the nozzles
-another valve adjustment (probably with Junkman who's offered to be point)
-run the compression test again (hotter this time)
-if still way off on #4 and #5, or still low all around, try the leak down gauge next

That sound like a good plan? Thanks everyone!

djburt · #9 11-13-2012, 01:46 PM

I took a break on the car to work on some other things, and also because it just about needed a jump every time I wanted to start it. At least part of the reason was a bad battery, which I just now replaced. Apparently it wasn't the only culprit, cause even with plenty of juice I'm getting this:

https://docs.google.com/open?id=0B6CVwJWk8h0nWDIyRlFFSmRyT2c

I still need to recheck the valve clearance and repeat the compression test, and I will get to that soon. The difficult starts may be completely related to all that. But in the meantime, should I be checking anything else? Anyone think this could be a weak starter, or does the starter appear to be doing its job?

Also, can anyone tell me what that noise is after I stop trying to start it up? It's a buzzing electrical noise coming from the electrical box at the very end of the video...not sure what that's called. It stops after a few seconds on its own.

Zacharias · #10 11-13-2012, 01:49 PM

Quote:

Originally Posted by djburt

Also, can anyone tell me what that noise is after I stop trying to start it up? It's a buzzing electrical noise coming from the electrical box at the very end of the video...not sure what that's called. It stops after a few seconds on its own.

It's the climate control servo returning to the "park" position when the ignition is shut off. Completely normal.

Also I went back and read your first post and noticed you mention tugging on the soft return lines. Please don't ever do that. To remove the return lines, take a sharp knife and carefully cut an upward slit from the base of the hose, near the actual injector nozzle, maybe 1/4" or a bit more. Then gently work the hose off. If you just yank on the hose you can break off the barbs that hold the hose in place (voice of experience) and that will give you a leak at best or at worst the hose will try to work its way off.

A few other things I will throw out, given the work you've been doing:

1. New injector seals (crush washers) after the compression test (installed with rounded side down)?
2. Injectors torqued to 70-80 ft lbs on reinstall?
3. Certain you carefully lined up and tightened down on the steel injector lines on reinstall?

Assuming all these were covered, I would really check your valve adjustment again.

Listening to it on the video it sounds like it isn't getting any fuel at all. Have someone pump the primer pump while you crank it over. Is your primer pump black or the old white style (which has to be turned to unlock in order to pump)?

djburt · #11 11-13-2012, 02:16 PM

Quote:

1. New injector seals (crush washers) after the compression test (installed with rounded side down)?
2. Injectors torqued to 70-80 ft lbs on reinstall?
3. Certain you carefully lined up and tightened down on the steel injector lines on reinstall?

Assuming all these were covered, I would really check your valve adjustment again.

Listening to it on the video it sounds like it isn't getting any fuel at all. Have someone pump the primer pump while you crank it over. Is your primer pump black or the old white style (which has to be turned to unlock in order to pump)?

1) same as heat shields, right? If so, yes I replaced those when reinstalled the rebuilt injectors.
2) the manuall I had (from the Monark nozzles distributor) says 70-80nM, or 52-59 foot lbs, so I set my torque wrench to 55 ft lbs.
3) yes.

definitely will check the valve adjustment again soon. it does seem like it's just not getting fuel...could the fuel pump be failing, and if so is there a way to test that?

also, for taking the time to read up on this thread and commenting...thank you very much!!!

djburt · #12 11-13-2012, 02:18 PM

Oh, and I replaced the old white primer pump with the Bosch black one last year.

Zacharias · #13 11-13-2012, 02:38 PM

Quote:

Originally Posted by djburt

2) the manuall I had (from the Monark nozzles distributor) says 70-80nM, or 52-59 foot lbs, so I set my torque wrench to 55 ft lbs.

Yes, correct, my bad from digging into memory on that one.

Diesel911 · #14 11-13-2012, 03:21 PM

Quote:

Originally Posted by Diesel911

If is is an equipment issue when you re-test the compression on #1 it would be lower.

Please ignor the above comment. I am not sure what My intention to say was.

djburt · #15 11-13-2012, 04:39 PM

Ok, I tried starting again while a friend pressed the primer pump repeatedly, and voila, I have a running car. So, is the fuel supply / pump looking more guilty? Don't have time to check anything on it now, but what all should I be looking for? I'll at least go back and double check all the supply lines, maybe bleed them again by slightly opening up each line at the injectors one at a time...anything else?

Thanks again everyone. Cheers.