1982 300D Low Compression On #5 Cylinder - PeachParts Mercedes-Benz Forum

hobberslobbers · #1 12-16-2010, 07:55 PM

old "marvin" (see signature) is actually a no-rust, southern car with 106k miles. had a cold compression test done after valve adjustment and i am low 300 psi on cylinders 1 through 4, and #5 (rear cylinder next to firewall) is at 240. can't get him to start hardly at all below 20˚F. stumble starts in the the 20's and finicky starts in the 30's. he runs silky smooth after getting started. i mean, REALLY smooth.

what do you recommend i do? all experts and novices alike, please chime in. i'm just a weekend wrencher and consider the thought of having to tear the motor down as being over my head...as in WAY over my head! my thoughts are:

1. Re-ring only;
2. Re-sleeve and re-ring;
3. Complete lower-end rebuild; or,
4. Do nothing and just drive him 9 months out of the year.

Also, how much would each of the options be in $'s in your area?

layback40 · #2 12-16-2010, 08:17 PM

John,
Has the motor been run on WVO or some thing similar?
Before you go any further, do a hot compression test & report.
It may just need a soak if that mystery oil or better, brake fluid.
Do the hot comp test first !!

Good luck with your car

kerry · #3 12-16-2010, 08:29 PM

Fix your glowplug problem and keep driving it.

t walgamuth · #4 12-16-2010, 10:03 PM

Quote:

Originally Posted by kerry

Fix your glowplug problem and keep driving it.

x2.

Sounds like a duff glow plug. The compression is not low enough to cause any real problems.

hobberslobbers · #5 12-17-2010, 06:17 PM

perhaps some more history is in store. no biodiesel ever used in this motor. did the marvel soak last winter. had a pretty hefty amount of blowby and no-start in cold weather. had the prechambers out and did a 3-day soak and hand turned the crank to get the rings to bust loose if they did happen to be stuck. bore scope confirmed that there is no damage to the cylinder walls with some cross-hatching still visible. helped a little. ran the dickens out of the car this past summer/fall. readjusted again on the valves. they were still perfect and the cam lobes are not worn at all. changed the valve stem seals. i changed out the glow plugs and reamed the gp holes, which were'nt caked up at all. at the same time i went ahead "while i was in there" and replaced the gp relay. i rebuilt the nozzles and balanced them to within 3 bar. all are between 1900 and 1950 psi. car runs awesome and starts great in the warmer months. not even a hiccup. i have check gp's again recently and they are pristine. just curious... could a burned exhaust valve cause low compression. i have an inkling something may be going on top side. any other thoughts?

mach0415 · #6 12-18-2010, 03:21 AM

If you have done all that, then a leak-down test would help answer your question. Have you done a hot compression test, yet? Until you do that, you may be worried about nothing. A stuck ring will probably need more than 3 days soaking in MMO. At least a week if you can do w/o the car is recommended. What was your method of testing glow plugs? A DMM cannot simulate what the GP will do with current flowing, if you are simply ohming them out. Best way is to lay eyes on them while applying 12 volts (if new style - should be) to see if they glow bright orange/white. If so, stick 'em back in and run them.

layback40 · #7 12-18-2010, 05:07 AM

John,
"i changed out the glow plugs and reamed the cylinder walls. "

I think you meant that you reamed the GP holes.

Do you still have bad blow by ?
You need to do comp test with the motor warm.
If #5 is still low, do a leak down test. Listen to hear if you have air escaping from the exhaust.
Many diesels have difficulties starting at 20F
Let us know how you go.

Good Luck.

t walgamuth · #8 12-18-2010, 06:46 AM

I agree to ask how you checked glows....describing them as pristine suggests a visual inspection. I always pull them and use a jumper cable to test them. You gotta be careful though they get white hot!

rscurtis · #9 12-18-2010, 10:26 AM

When I bought my '83, I did so knowing that I had a replacement engine from an '84 wreck if I needed it. The engine smoked heavily (blue) and had some blowby. It only had 100K miles on it, but had suffered from deferred maintenance.

Once I got the car, I did all the neglected PM, changed the oil several times, and drove it. The smoke and blowby were gone, never to return. This was almost 6 years ago, the car starts effortlessly at 10 degree temperatures, and runs great.

hobberslobbers · #10 12-18-2010, 11:28 AM

thanks guys. nope, no history of WVO. i could only dream of that (no blowby and effortless starting). so, i need to do a warm compression, do individual heat test with the gp's, and check for leakage on the valves. regarding the latter, is there a tried-and-true way to do this? i have read different opinions on this. also, would a re-soak of the cylinders be recommended by anyone here? if so, what's recommended on that as well? i really appreciate this forum. i am so itchy to yank the head i just can't hardly stand it....

also, has anyone heard of a no-good turbo shoving oil into the engine to create blowby. there is a little play in the turbine (front to back) but none side to side, but there is a small pool of oil at the bottom of the u-tube. alternate theory if one of the gp's is bad... turbo blowing oil into the intake causing fouling of the gp's prematurely. previous exposure to the oil caused one of the valves (or more) to get burned, resulting in low compression. ??? would that even make sense?

Diesel911 · #11 12-18-2010, 12:00 PM

Dose anyone find it odd that there is a compression issue at 106k miles; assuming that the mileage is correct?

When I did the Marvel Mystery Oil soak on my Volvo Diesel I soaked it for a week; rotated the Engine and soaked for another 3 days or so (I wanted to do it longer but had to use the Car).
It got rid of the hazy gray Smoke I had at all speeds when driving.

Both times I put in 1/4 cup of Oil per cylinder.

hobberslobbers · #12 12-18-2010, 12:04 PM

hi diesel911, yep the miles are accurate and i find the compression issue on #5 troubling as well with no more miles on the car than it has. my '83 with triple the miles has mid-300 psi compressions on all 5 cylinders and starts no prob in the winter. i still plug it in though. at any rate, you think i should give it another "go" with the soak?

Diesel911 · #13 12-18-2010, 12:47 PM

Quote:

Originally Posted by hobberslobbers

hi diesel911, yep the miles are accurate and i find the compression issue on #5 troubling as well with no more miles on the car than it has. my '83 with triple the miles has mid-300 psi compressions on all 5 cylinders and starts no prob in the winter. i still plug it in though. at any rate, you think i should give it another "go" with the soak?

The leak back test would tell you if it was a issue with the Valves. After all if it is not the Pistons/Rings/Cylinders all that is left is the Valves and the Head Gasket (this assmes the Prechambers and Glow Plugs are seated well).
In the attached pic are some Glow Plugs tested on a Car Battery.
All the pictured Plugs were tested for about the same amount of time.

The bad Glow Plug turned out to be the one where the complete tip got hot. But if you notice the very end of the tip is cooler than the part close to the Body of the Glow Plug. Only about 1/4 inch of the tip stickes into the Prechamber so that is the end you want the hottest.

The other 2 plugs the very tip got hottest first and as you see they are yellow hot in the part that sticks into the Prechmber.

In the diagram you get an idea as to how much of the tip is exposed and the Red Arrows show where Carbon buildup or if the Glow Plug is heating more in that area will allow more heat to go into the Cylinder Head instead of heating the air in the Prechamber.

Those are Monark Glow Plugs and after the test I replaced all with Bosch Plugs.

hobberslobbers · #14 12-18-2010, 01:30 PM

i see what you're saying about the cool tip in 1st pic, but am confused about the practical difference between the 2nd and 3rd pics. would they not have identical performance, even if a little hotter towards the middle of the plug? funny you should mention the brand of gp, b/c i do have monarchs in there right now. i will plan to change to Bosch if the test results come back mixed. i noticed there are different levels of Bosch. there is a "standard", a "premium", and a "duraterm"...listed in order of price from $12.95 to $23.95 each. which do you recommend?

so, game-plan is to:
1. test gp's and replace if necessary;
2. do cold and hot compression tests; and,
3. confirm if there is any leakage on valves.

what do you say about the possibility of the turbo blowing oil into the intake to cause what i am seeing as blow-by? and about my "theory" regarding this in my previous post? many thanks....

t walgamuth · #15 12-18-2010, 01:58 PM

Turbo sounds fine too. After fixing the glow plugs you should be fine.