Cold compression numbers anyone? - PeachParts Mercedes-Benz Forum

#1 06-01-2002, 09:54 PM

Well my risk on a 1976 240D which I bought for 850.00 Canadian in mid winter is at this point is for not. Cylinders 1 and 2 are pretty much no compression with lost of blow by when it runs. numbers 3 and 4 are 320 pounds. I have access to a parts car missing the tranny for about 300 Canadian. If I can't get it started can I do a cold compression on it and see how that motor measures up cold? What type of numbers should I get cold before I pull the motor from that for mine?

Thank you so much

David

engatwork · #2 06-02-2002, 08:44 AM

There is a machine shop here in town that has a block, crank and head assy for a 1979 240D that he is asking $150 for with buyer paying shipping. It is torn down ready to rebuild. If you want the name/number let me know.

tcane · #3 06-03-2002, 12:46 AM

M-B only provides compression specs for an engine that is hot with 80 degree C coolant temp. (or very warm anyway since it takes time to remove glow plugs or injector nozzles and install the compression gauge fitting). Normal compression is 319-348 psi and I've seen readings as high as 400-420 psi. Minimum hot compression is 218 psi.

I found that you can expect a 20-40 psi lower reading on a cold engine if the compression is toward the lower end for a hot engine spec of 319 or so and 30-50 lower psi if the hot compression is toward the higher end of 375 +. If the used engine is in minimal condition, I would be very leary of cold compression of 190-200 psi. If the used engine has been sitting then it could have low cold compression that MAY improve when the engine has been run for a while. My numbers are for outside temps of 60-70 + degrees F. If it is colder than that in your area of Canada, then the compression readings will be lower.

The type and condition of the compression gauge used can also make a difference in the reading you get. It would be helpful if you are familiar with the compression gauge you're going to use or the person you borrow/rent it from.

Will the seller of the used engine give you a refund in case it turns out to be in worse condition than known?

Good Luck!
Tom

#4 06-03-2002, 02:24 PM

Thank you for your replies. I will go and see what I can find out about the other motor. The motor is no cost to me. But it has to check out to some credible degree. I am wondering if it isn't the head gasket on my car. There is some oily substance bubbling up up from around where a large bolt is recessed on engine block. The bolt is facing upwards as you look down just past the large fuel filter canister. What do you think?

Thanks again.

tcane · #5 06-03-2002, 05:06 PM

The large bolt in the recess on the head just behind the timing chain area and close to the fuel filter mount is part of the drive mechanism for the oil pump. An oil leak there has nothing to do with the head gasket.

Forgot to mention, compression readings will also be dependent on the speed the starter motor turns the engine. Suggest you use as fresh a battery as you can, hook the battery to a high output battery charger/starter, let the starter cool about 2 minutes after checking each cylinder, open the throttle all the way, crank the engine at least 8 revolutions per cylinder to get a maximum reading that is consistent for comparison, remove either all the glow plugs OR the injector nozzles depending on which compression gauge fitting you're using, shut off the fuel flow to the injection pump (either clamp the inlet fuel line or use the injection pump's manual shut off that requires disconnecting the throttle linkage).

Compression can be low if the valve clearance is too small. Usually, M-B diesel engine valves lose clearance between valve adjustments. If compression seems low, you may want to check the valve clearances on the cylinders with low compression that otherwise may be a good used engine even though the compression is low due to valve clearance being wrong.

Good Luck!
Tom

#6 06-03-2002, 07:52 PM

Just a note. Holding the throttle down all the way on a diesel makes no difference. There is no throttle plate.

TimFreeh · #7 06-03-2002, 08:40 PM

MB Diesels that have venturi governors have throttle plates.

In fact as I recall your 75 240D does have a throttle plate. I'm sure 190/200/220D engines all have a throttle plates since they use injection pumps with venturi governors.

The mechanical governor MB engines do not have throttle plates.

Tim

tcane · #8 06-03-2002, 08:58 PM

Diesel Power has a point (my post was from memory and Diesel Power's comment made me check my official M-B engine manual).

Engines using the pneumatic governor on the injection pump require opening the throttle all the way to check compression (see page 00.4 - 115/2). The pneumatic governor is found on some 615/616/240 engines and have throttle valves. You will have to look to see if there is a throttle valve in the intake manifold/tract.

Engines with the mechanical governor on the injection pump require that the throttle remain closed during the compression check (see page 00.4 - 115/2). Generally most 617/300D engines and some 615/616/240D engines do not have throttle valves.

Good Luck!
Tom

#9 06-03-2002, 10:23 PM

Quote:

Originally posted by TimFreeh
MB Diesels that have venturi governors have throttle plates.

In fact as I recall your 75 240D does have a throttle plate. I'm sure 190/200/220D engines all have a throttle plates since they use injection pumps with venturi governors.

The mechanical governor MB engines do not have throttle plates.

Tim

Yes it does, I had a lapse of memory on that one. I haven't opened that part up yet, so I don't know how "tight" the fit is. The Datsun diesel pickup that I sold last winter also had one. I had noticed on that one that the throttle plate was very "poor" fitting in relation to what I've seen on gassers. I wouldn't have thought anythnig of not holding it open on a compression test, as it looked like the gap around the plate was too large to result in a restriction during cranking speeds.

Now for a correction - "MOST" diesels do not have throttle plates.