Did A Compression Test Yesterday... - PeachParts Mercedes-Benz Forum

Jim300D · #1 04-14-2011, 11:21 AM

I picked up a '78 300D from a friend of mine for cheap exactly a year ago. Since then I've put some TLC into her but haven't invested too much yet because I wanted to do a physical on the motor first. Well I finally got around to doing it last night after I gathered all the tools to do so. I did a valve adjustment a week ago and have driven the car maybe 100 miles prior to this compression test.

The compression test was performed about an hour after driving home from work (engine reached full operating temp).

These are the maximum numbers reached:

Cyl.
#1 - 280psi
#2 - 340psi
#3 - 325psi
#4 - 359psi
#5 - 275psi

Not the results I was hoping to see exactly.

The car has 170k on the clock. 3 out of the 5 cylinders have fairly good compression. I plan on doing a leakdown next to determine the cause of low comp. on cylinders 1 & 5.

My thoughts are this: because of the relatively low mileage and the healthy #'s on 2-4, it's my hopeful guess that the valves have carbon on them and are not sealing well on 1 & 5. Especially since I have no idea the last time the valves were ever adjusted.
If that proves to be the case, how much effort is it to pull the head and do a valve job? What does a valve job typically run for these heads? Maybe it only needs a good cleaning? Just trying to figure out if it's worth continuing on with this project car. I was going to rebuild my injectors, replace timing chain, and set IP timing if the compression test checked out, now I'm not so sure...

Thanks guys.

vstech · #2 04-14-2011, 11:31 AM

in my opinion, none of those numbers are healthy. a warm engine should have over 400 on the pots.
it's possible that the valves need further adjustment. if they were tight, they could have damage that caused them to seat further after you drove it after the first adjustment. readjust the valves and try again...
good luck!

kerry · #3 04-14-2011, 12:13 PM

Does the engine run ok? If it were me, I'd just drive it.

funola · #4 04-14-2011, 12:35 PM

Per the Mercedes tech spec data manual, compression on a healthy engine is 320 to 350 psi. It is more important that the #'s are close to each other than how high they are as long as they are within spec.

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Quote:

Originally Posted by vstech

in my opinion, none of those numbers are healthy. a warm engine should have over 400 on the pots.
it's possible that the valves need further adjustment. if they were tight, they could have damage that caused them to seat further after you drove it after the first adjustment. readjust the valves and try again...
good luck!

vstech · #5 04-14-2011, 12:39 PM

absolutely true, but on a warm recently adjusted valve compression test a HEALTHY engine will have excess of 400psi. cold numbers in the 3and change range are fine.
nothing wrong with 350 warm numbers, but I don't like to state HEALTHY that low.

the nearly 25% difference high to low is certainly the biggest problem!

funola · #6 04-14-2011, 12:46 PM

I'm going by Mercedes spec. If you measured 400 psi on one engine, the possibilities are:
1. your gauge is wrong
2. the engine has carbon buildup in the combustion chamber, thus raising the compression ratio.

Quote:

Originally Posted by vstech

in my opinion, none of those numbers are healthy. a warm engine should have over 400 on the pots.
it's possible that the valves need further adjustment. if they were tight, they could have damage that caused them to seat further after you drove it after the first adjustment. readjust the valves and try again...
good luck!

Jim300D · #7 04-14-2011, 12:52 PM

Quote:

Originally Posted by vstech

absolutely true, but on a warm recently adjusted valve compression test a HEALTHY engine will have excess of 400psi. cold numbers in the 3and change range are fine.
nothing wrong with 350 warm numbers, but I don't like to state HEALTHY that low.

the nearly 25% difference high to low is certainly the biggest problem!

Agreed the biggest problem is the variation, and granted 3-and change may not be healthy, but if it's acceptable I'll take it. The engine lacks power, when floored going up a steep incline it actually decelerates...
The car starts up no problem however, especially after converting to newer style glow plug system. It idles nice and ran a bit smoother after the valve adjustment. However it still runs too noisy IMO and can rattle/knock at certain engine speeds/loads. Exhaust is a bit smokey at startup. The blowby doesn't seem all too bad when I take the oil cap off.

Next step is definitely to do a leakdown test and maybe do a wet test to determine ring wear. I'll update with results.

kerry · #8 04-14-2011, 02:31 PM

Quote:

Originally Posted by Jim300D

, when floored going up a steep incline it actually decelerates...

The question is, did it ever do otherwise even when brand new? 617na automatics are no speedsters.

Jim300D · #9 04-14-2011, 02:43 PM

Haha...I would'nt know! I do know it makes my dakota r/t feel like it's a supercar tho! lol.

I'm just kinda stuck at this point, I was gonna order nozzles this weekend but now I don't know... I gotta say it drives well and I've taken a liking to this car. Maybe the compression will improve with good maintenance and diesel purging...

Stretch · #10 04-14-2011, 04:40 PM

Quote:

Originally Posted by Jim300D

Haha...I would'nt know! I do know it makes my dakota r/t feel like it's a supercar tho! lol.

I'm just kinda stuck at this point, I was gonna order nozzles this weekend but now I don't know... I gotta say it drives well and I've taken a liking to this car. Maybe the compression will improve with good maintenance and diesel purging...

If you are worried do a leak test - otherwise I'd just drive it and use it for a bit. This is based on the assumption from your first post that you've driven 100 miles in a year - is that right?

GregMN · #11 04-14-2011, 05:05 PM

Quote:

Originally Posted by vstech

in my opinion, none of those numbers are healthy. a warm engine should have over 400 on the pots.
it's possible that the valves need further adjustment. if they were tight, they could have damage that caused them to seat further after you drove it after the first adjustment. readjust the valves and try again...
good luck!

X2

Adjust your valves again.

Do a compression test.

Then do a "wet" compression test. If you see improvement over the "dry" test, you have leakage by the rings. If no improvement you have leakage past the valves.

If it still starts and runs, drive it.

leathermang · #12 04-14-2011, 05:18 PM

My Inspirational story ....

I bought my car for $500 and drove it home 200 miles. It had been looked at by many MB people at the Austin Get together.... had been trucked down from Ft. Worth area for them to look at it because it shows no compression in the number one cylinder.
It was losing oil... not burning it at that time... although it could use valve stem seals now... it had a beat up copper oil drain washer, valve cover gasket leaking, oil too high in the crankcase... so I fixed those things ....
But it has always run great... it has always gotten 24 mpg ....and will run 75 all day long... with number one cylinder measuring ' NONE" for compression.... so I say drive it... making sure the brakes are in great shape..

pawoSD · #13 04-14-2011, 06:39 PM

Quote:

Originally Posted by leathermang

My Inspirational story ....

I bought my car for $500 and drove it home 200 miles. It had been looked at by many MB people at the Austin Get together.... had been trucked down from Ft. Worth area for them to look at it because it shows no compression in the number one cylinder.
It was losing oil... not burning it at that time... although it could use valve stem seals now... it had a beat up copper oil drain washer, valve cover gasket leaking, oil too high in the crankcase... so I fixed those things ....
But it has always run great... it has always gotten 24 mpg ....and will run 75 all day long... with number one cylinder measuring ' NONE" for compression.... so I say drive it... making sure the brakes are in great shape..

Does the #1 cylinder actually fire? If it measured none you'd be blasting fuel out the exhaust and mixing it into the oil.....not to mention you'd have about 50hp on a good day with 3 cylinders working!

Ol'Smokey · #14 04-14-2011, 07:47 PM

Generally speaking how much does it cost to rebuild an older mercedes diesel engine assuming you do most of the work yourself, minus the machine work obviously? And minus a new injection pump

leathermang · #15 04-14-2011, 11:41 PM

Quote:

Originally Posted by pawoSD

Does the #1 cylinder actually fire? If it measured none you'd be blasting fuel out the exhaust and mixing it into the oil.....not to mention you'd have about 50hp on a good day with 3 cylinders working!

I have no idea. I have just driven it... pretty peppy with the manual trans....
I guess I could crack one fuel line at a time and check to see if the RPM drop was the same... but I just have not worried about it..
My secret theory... is that they were taking its compression through the glow plug holes and number one was completely clogged...
I only put about 20 miles per year on it...6 of them to get an inspection sticker...The rest are here on the property going to the mailbox and back... I start it up about once every two weeks .... and often keep the battery charged with trickle charger in between.. the AC does not work.... and I have three other vehicles ( all R134a) which keep me comfortable here in Texas....