That's pretty low, I don't think I would install and engine with compression that low.

According to the FSM, OM617 non-turbo normal readings are 22 - 24 bars (approx 320-350 psi) - with a minimum compression of 15 bar (220 psi). The OM617 turbo compression norms are 24-30 bar (350-435 psi), with minimums of 220 psi as well. Both have a maximum of 3 bars acceptable difference between cylinders.

My '78 non-turbo readings were all between 300-325, and those readings were on a cold engine (FSM says to do this on a hot engine (80 C), which should result in higher psi's). I'm not sure how you do this on a hot engine...

ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

I wouldn't either but the question was what was the minimum. If looking to buy a used engine I would look for a lot better than that too. 220 is the minimum cold reading for reliable starting cold.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I was just surprised that 220 was considered acceptable for cold starting. I'm not sure I would want to have to start a 220 psi engine in 0F conditions. Personally, I would be looking for closer to 320.

What a strange thread.

The pressure produced in a cylinder will not be specified as an exact value because there are too many variables such as how well the piston rings are bedded into the cylinder walls and how gas tight the valves seal in the head; the temperature of the engine; to a lesser extent the ambient air temperature; viscosity of the engine oil...

...blah blah blah...

...that's why there's a range given in the FSM - that's why there's a value that compares the measured values for all of the cylinders in the engine.


######

Strange to see the names of two prominent members who are no longer with us though.

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

A fact some people find difficult or impossible to understand, that has and will continue to generate discussion forever.

For those wanting the range = the data in this thread will answer their basic question.

.

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ASE Master Mechanic
https://whunter.carrd.co/

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
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Dynamometer.
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1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/

Im amazed that a recent valve adjustment hasnt been mentioned... BEFORE .. making such a test.

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

how good the starter is, how fast the motor spins, motor oil type (yeah I went there), are the valve rotators working? valves adjusted, motor temp? outdoor temp, altitude of engine at testing, type of compression tester, location of the compression tester connection... etc etc etc...

I've seen 535psi on some well worn motors that were well maintained, and serviced recently, and I've seen 200 reliably start with zero blowby and good glow plugs...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

actually, I think a case could be made for a motor with low compression having a better condition that one with high...
ring seating and oil dispersal could show up as low compression test numbers, with perfect combustion... I'd have to think on it.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Im curious how my cars engine would do ,a 400k mile beater car but service is always done when needed.

Hot vs cold matters too. If you want really impressive numbers, do the test while the engine is hot & running.

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

Your technique can matter. To get an accurate reading, the compression gage must displace the same volume as whatever component you removed (injectors or glow plugs). My Harbor Freight tester has a ring you must use on the "injector simulator" to fill the volume. It also has a tubular hole that seems different than the real injectors. I have had mixed results from leaving the injector heat shields - last time the shield increased the reading a bit. Seems smarter to use them. The volume that matters is up to the check valve in your tester, and that Shraeder valve must be a special type with very weak spring (usually a white collar), not one for a bike tire, auto, or AC port. You cannot calculate expected psi from the compression ratio, because the camshaft makes a big difference. Different cams can cause a x2 difference in gasoline engines, especially "racing cams" with high intake-exhaust overlap. Fortunately, we have little choice in camshafts. Cylinder pressure also varies w/ speed (organ pipe dynamics in the intake), so the starter spinning speed could make a difference though that is more an effect at much higher rpm's. Perhaps a greater starter effect is how well the oil seals on the rings - faster stroking leaves less time for leakage. I expect that the thicker oil in a cold engine would increase piston sealing, and also give less thermal loss to the cylinder walls (as does faster speed). In my tests, it takes ~12 compression cycles to reach max pressure on the gage. In gas engines, people say it should come up in ~3 cycles (usually does for me), but I suspect that has more to do with the design of the tester, i.e. how much volume between the check valve and gage.

I recall measuring ~250 psig in most cylinders when my 85 300D turbo was getting hard to start on 35 F mornings. It failed a year or so later. The 82 replacement engine (craigslist) measured ~410 psig in all cylinders (cam looks new) and my 84 engine (130K miles) measures ~405 psig in all except one 350 psig one (will check valve gaps). These are all with a warm engine (hot when I started removing injectors).

BTW, while your injectors are out is a great time to adjust your valves. It is so easy with no metal tubes in the way. I once tried using the glow plug holes for compression, but seemed harder. Since I made my pop tester, it is a good time to check the injectors anyway.