help with compression test - PeachParts Mercedes-Benz Forum

nibiru · #1 02-09-2003, 01:43 PM

Hello board: Damn I love this board

I have been searching for days. are there instructions on doing a compression test on a diesel on this board

help please and thanks

Peace;

nib
new to 300TD'82

leathermang · #2 02-09-2003, 04:27 PM

Find a compression tester for sale at a reasonable price... a couple of people have found them at a good price... keep looking on the threads to find that..

Generally you are going to need to be sure that you have a compression tester adapter which will fit into the hole you are going to use for testing... this could be the fuel injector hole or the glow plug hole...

Smart money is using the glow plug hole ... 57 times less work.

Then you take out ALL the glow plugs and disable the fuel delivery to the engine...I am not sure which method is best for this...

Then, with the battery in good shape you turn over the engine with the starter for a certain (same) number of rotations with the gauge in each glow plug hole in order... keeping records of the psi reading each cylinder gets to. Usually 8 to 10 revolutions per cylinder is enough to " top out" the guage to whatever high that cylinder is capable of reaching...

Some people think you need to perform this test on a cold engine, then a hot engine... I think a correctly done cold test will give you enough information to evaluate the engine and the cylinders compared to each other...

nibiru · #3 02-09-2003, 07:59 PM

Thank you very much

I shall do it and I am sure report back

peace

Nib

Fisherman · #4 02-09-2003, 10:43 PM

check your injector pump type also as some require that you wire the throttle open full and some do not.
On mine the IP has no butterfly inthe intake so no need to wire the throttle open.
this could affect your readings.......

McRoth · #5 02-10-2003, 11:49 AM

Anyone in the Cleveland area have a compression tester I could borrow or come over & use? I'm not having any real problems yet, besides cold weather starting, I'm just curious.

md21722 · #6 02-10-2003, 12:26 PM

From what I have ever heard you must do a compression test on a HOT engine. Doing it cold is meaningless. Also, for MB diesels, its done thru the injectors not the glow plugs. This ain't no VW TDI.

leathermang · #7 02-10-2003, 01:45 PM

Brian, I love Bold Clear Statements.... I will be looking forward to your interpretation of the following photos of the 617.9 shop manual...

leathermang · #8 02-10-2003, 01:46 PM

second..

nibiru · #9 02-10-2003, 02:07 PM

McRoth

I am very interested in making Biodiesel

care to share. This is why I wanted to go diesel

No matter what the trouble with my engine I intend to fix

please and thank you

Peace:

nib

md21722 · #10 02-10-2003, 02:12 PM

If you have such an adaptor such that it will fit at the glow plug location that is great and use it with the possiblity the glow plug may break. The manual mentions the glow plug bore or the prechamber, my interpetation is the prechamber is the injector threads. For those without an adaptor or a 603 engine then the injectors are the way to go. What does the shop manual say about the engine being hot or cold???

McRoth · #11 02-10-2003, 02:25 PM

Nib, Sure, I'll share any experience I have. What do you want to know? I bought my 240D to run Bio in it & eventually convert to SVO. I built a big, 45 or so gallon processor for making it in. I've run about 100 gallons so far with no trouble except for the bio cleaning the sludge out of my fuel tank and clogging my filters. I had to pull the tank and clean it out, not a big deal in this car. I moved this fall & haven't gotten my processor running again yet, although these diesel prices are going to push me to start again, probably this weekend!! I get methanol for $1.22/gal from work, a paint factory, you only use 1 gallon of MeOH for each 5 gallons of oil. I get the oil for free from restaurants.
You can see pictures of my processor on my site at www.emeraldisleforge.netfirms.com

TimFreeh · #12 02-10-2003, 10:26 PM

md21722

You should try to make your interpretations as clear and bold as your original assertion. Maybe you just should have said "I stand corrected"

Tim

md21722 · #13 02-11-2003, 09:01 AM

I am not perfect, nobody is. I don't know why it is that when somebody posts many times and one or two of them don't come across perfectly right that some get defensive.

I do believe that the compression test thru the injectors is a better way because it removes the risk of breaking a glow plug. There are threads about broken glow plugs, but I am not familiar with any regarding broken injectors. The shop manual describes two ways and pictures one. Since then they may have updated their internal procedures thru programmed repairs or modifications or TSBs. The shop manuals are great but they are neither perfect nor completely current. I blend the advise of the manual, mechanics, and foreman whenever possible. Also, I have never heard of a cold compression test that had any meaning. I don't claim to know everything & am open to correction.

nibiru · #14 02-11-2003, 10:18 AM

fellow diesel friends

This brings a good point. As I have been scanned countless pages of collective wisdom on this site, I still ask- which is better

cold compression test or warm. ???

I ran my 300TD for 30 minutes before I pulled her into the garage to set 2 space heaters to get to know her better.

Alas I discovered I did not have a 27mil deepwell so ran to Sears before they closed. I knew the engine would not remain warm forever so I took my chances

I understand ( and stand ready for correction ) that cold is the way we start up in the morning- so would it make more sense that a cold compression test is what really matters.

I did read on one of these pages ( and may I say right now I am addicted to this - I love learning , crave it ) ( Thank you all for anything you have added - this new guy is most appreciative ) that the only compression test you need is a cold start in the morning - a leak down test is what really matter.

Understood this is one mans opinion or maybe more

I ask because why I love turning wrenches is the logic and engineering that blend in alloys and metals that lay before me- almost inviting me to get dirty. It may not be for everyone but I would rather be under a hood than.....................

please and thanks in advance

Peace:

Nib
'82 300TD

#15 02-11-2003, 12:01 PM

Unless I could read where MB specifically recommended taking a compression test hot, I would question doing it that way. Reason being on most engine pulling a screwed in plug out of a hot head can be a real problem. The head will cool and shrink back without the injector in there and the hole can then reshape itself. Could have a problem getting the injector to go back in there.