I have owned my car for a year and did a compression check a few weeks ago here is my story/dilemma/question.

-Car: 1982 300D
-Engine: 1985 D or 1984,85 SD (I think this, because my engine has a wire for California emissions and I have been told that it was first introduced in 84 on the SD model and in 85 for the D CD TD. But I can’t confirm it because I can’t find my block number (maybe someone could let me know where that should be))
-Mileage on odometer: 130k
-Miles I’ve put on it: ~10k
-Time I’ve had car: ~1 year

Compression test procedure:

Results:

Note: Placed two tablespoons of oil into cylinder #1 and re-tested. Gave me a reading of 250. (I have been told this last test indicates the rings in cylinder 1 are bad and causing the low compression in that cylinder) I was too depressed to perform this last test on the other 4 cylinders.

General performance and condition of car as applies to this troubleshooting:
-Cold mornings (~25F) I have been glowing 6 times w/o thinking about it all winter so I assume that in the early winter I was having starting problems w/ only 1 glow.
-Very cold mornings (>20 and about to snow) have had to glow 40+ times (glow 10 times try to start repeat until car starts) Note: I park on street and cannot use my block heater. Note.2 I have verified glow plug operation with an amp probe ~10amps at each plug
-Warm mornings: Glow once, starts right up. (some times will knock a little until I give it a little gas then is fine)
-Runs great (in my opinion) my father-in-law has an 85cd with a rebuilt engine (30k miles on engine) my car does not run as well as his but I assume this is the 100k mileage difference and that I have not adjusted my turbo or the vacuum to my transmission.
-DOES NOT SMOKE!!!! DOES NOT BURN OIL!!! DOES NOT LOOSE OIL!!! My assumption is that if my rings were bad it would smoke like a mess. Am I wrong?
-Battery is putting out proper voltage when car is off ~12.5V (when its on I am reading the alternator(13.9v), right?)

My questions:
-Aren’t these compression readings INSANELY low/bad? (I’ve read 235 is like 1 foot in the grave)
-Should a 300D be able to start with compression this low?
-Shouldn’t a 300D smoke and burn a lot of oil if the rings are bad?
-Could an under performing starter be giving me these readings
-I assume a bad test could too…?
-Where should the engine number be on my block (a picture would be more helpful)

PLEASE!! PLEASE!: I do NOT need to hear comments like “you should have gotten a compression check before you bought the car” Lesson learned! Rubbing it in it just cruel and I didn’t spend all this time writing this post just to made fun of. I am a beginner, this is the first Mercedes, or diesel, I have owned and I started the whole backyard mechanic thing only a year ago when I bought this car.

Thank you for reading this and thank you for any help you can give.

Welcome to the forum.

Those readings are not correct. I'm confident that the engine would never start with such numbers. So, as far as that test is concerned, it's not worthwhile to base any decisions on it.

If the rings were bad, you'd have significant blowby and and the oil consumption would be easily noticed. My SD has very little blowby but it consumes a quart every 2K or so.......probably in the turbo.

If the starter is spinning the engine exceptionally slow, you'd definitely have slightly lower readings, but, probably not that significant.

Now I've got some questions:

1) When was the last time you checked the camshaft timing?

2) When was the last time you checked the injection pump timing?

3) What figures did you use for the valve lash?


A compression test comes after the basic maintenance chores are performed and the engine still won't start or run properly. In your case, 10 cycles of the glow plugs is not acceptable performance.

camsaft timing= never checked
ip timing= never checked
figures for valve lash= i am assuming you are asking me what size feeler guages i used? (sorry like i said im a begginer) intake .004" exhaust .012"

As I thought.

These two items need to be done. You may have a perfectly good engine with camshaft and/or IP timing that is way late due to an elongated chain.

BTW, not critical, but the exhaust should be .014". .012" is too tight. Are you positive that you identified which are exhaust and which are intake? Usually you need to be out there with a diagram and check off each valve when done.

when i did valves we had a chart set up and checked them off one by one as we went around. we also ordered the feeler guages from a pretty reputable place so i think they are the right size, but i will definately check that!

actually as long as we are discussing valve adjustment i did one when i first bought the car a year ago. and as i already mentioned i did one a couple weeks ago. well only one of the valves were tight and need repositioning. #4 if i remember right.
is it ok for your valves not to be way outta whack after 10k miles ad a year of daily driving? or is that a sign of something?

also i am curious (because i am trying to learn) how would IP timing affect a compression test? i thought it just delivered fuel, but when i am doing the compression test no fuel is being delivered...?(i really dont know much about the IP please dont laugh)

thankyou again for your help it gives me some hope!!

10K on valves is less than the recommended 15K, so, you would not expect all that much change. Many of us don't check them every 15K........and 25K is probably about the mean around here........

IP timing would not affect the test. As you noted, it simply delivers fuel at a specified point in the cycle.

Don't worry about not knowing much about the diesel engine. Almost all of us knew nothing when we got here. Take a look at the DIY section that WHunter recently put together. It has quite a few procedures that are relevant to maintaining the entire vehicle.

ok thanks

so i did a search but cant seem to find a quick answear
what is camshaft timing?
how is it adjusted?
if ip timing doesnt affect compression tests, does camshaft timing??

Camshaft timing is the relative position of the camshaft with respect to the crankshaft. It's adjusted by using a Woodruff key on the camshaft sprocket or replacing the timing chain.

Camshaft timing won't affect the test results either provided the valves are closing on the compression stroke.

Check this out:

http://www.peachparts.com/Wikka/OM617TimingChainStretch

ok cool

so if i understand what you are saying...

its that either my tester is bad or i did the compression test wrong?

since my valves are adjusted correctly (im pretty certin)

you are suggesting ip and cam shaft timing to help correct my starting issue?

btw thanks for all the help this is alot of cool stuff!

Random, you don't need a friend to hold the throttle open either, at least normally you don't have to on a diesel. They don't have a throttle plate to restrict airflow as you do on a gasoline engine - though I think I read that some of the early 240D's do have a type of throttle plate and maybe an open throttle is needed there. But normally diesel are wide open to air and only use the throttle for fuel.

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1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

ouch that checking for timing chain strech looks complex..
is it easier to just replace the timing chain.... I dont have to do all that
measuring to replace the timing chain do i...

One of the two possibilities is highly likely.

I'm suggesting that the IP and camshaft timing need to be verified. They will both affect capability to start........among many other items.

What's the condition of the battery and the starter? A slow turning engine is always difficult to start. Cranking speed is the king.

Use the short version and align the camshaft marks. Get a reading........and.......if it's less than 4 degrees.........don't worry about the procedure with the dial indicator.

ok,
I have rebuilt a m103 but i have not
really touched a diesel so i am diesel virgin..

So are you saying align the cran to zero... then look at
the cam and if it is near alignment.. say within 4 deg your
ok...

is that what your saying

Align the marks for the cam. Check the DIY........it explains this. The marks must be dead on.

Then read the crankshaft. If you are at 4 degrees or less.......just leave it alone. More than 4 degrees requires some better measurements with the dial indicator before you put a Woodruff key in it.