compression numbers
 

What compression numbers can one expect for a '76 300D engine (617.910) in good shape? Thanks!

numbers
 

according to my MOTOR repair manual from 1980 it is 313 to 331 with a minimum of 213. presumably tested cold.

tom w

Heh only a few people know it seems... thanks a lot! :pleased:

it's me again. A seller just sent me the numbers of his 300D and they are:

Cylinder 1= 209
Cylinder 2= 211
Cylinder 3= 210
Cylinder 4= 211
Cylinder 5= 207

He added a new engine has about 220, and everything above 200 is considered "good".

Tom, could it be that your numbers are for a different engine? Otherwise the seller would lie to me flatout....

Compression values
 

Although the variation between your figures is minimal which is good, those #'s look quite low overall and good compression is critical to a diesel. A recent test of a family members 240D was 275 PSI on one cylinder and 350 PSI on the other three.

Are the numbers you are providing PSI values?

In theory and not taking into account wear, leakage, and gauge inaccuracy:
one atmosphere = 14.7 PSI X the compression ratio (approx 21:1) = the approx. compression value in PSI. This also concurs with t walgamuth's MOTOR manual #'s.

Try posting your question in the Diesel Forum, some of the experts may offer you some valuable info.

well
 

unless he is using another measure, the statement is simply wrong. i cant say if he is just misinformed or deliberately lying. if the numbers are psi, they are very low for a diesel. just above the minimum. so if i decide a person is lying i usually would simply move on and find a car that is honest and an owner who is too.

i would assume that the minimum from the book represents a figure below which it becomes difficult to start the engine reliably. so if you buy that car there is not a lot of life left in it, but it might start and drive pretty well. if you live in a warm climate it might be ok for you. on a diesel, when they get worn they become hard to start but often run fine once started. it is less critical if in a warm climate.

good luck.

tom w

I'm afraid it's too late. I already bought the car. The seller had the comp checked by a mechanic who told him that the orig. MB book says 220 equals NEW condition. Anyway, the car will go to southern AZ (right now it's >110 degrees here which I'd say is pretty hot). I'll maintain it regularly and it will never see snow, ice let alone salt. I'll see how it goes....

best of luck
 

with your new purchase. you will probably get a lot of good service out of it even if it is a little low on compression. i assume it starts and drives fine or you wouldnt have bought it.

i bet you were having a little buyers remorse... most of do from time to time.

tom w

Told the seller that something's wrong with these numbers.... he'll double check with his mechanic. The story is not over yet :)

FYI- just another point of reference. I just tested my 300SD W116 w/125k miles (617.950) and it was 390 to 410 across all 5. I would think yours should be in that ballpark as well. I can't seem to find your engine nominal compression numbers (Braingears has your manual) but I think you have a squawk against your seller.

Thanks for the info. I guess these numbers are quite good, your engine is probably on the zenit ot its life. It seems Braingears have removed the manuals for W115s, at least I can't access the .pdfs. Maybe someone else could hook me up with these manuals.... Not gonna buy the car if the numbers are below 330.

personally
 

330 is extermely good. my 350sdl is 300 290 280 and it runs perfectly, has great power and good fuel mileage. the higher figures may be from testing a warm engine. mine was tested cold. if testing cold i would listen to the MOTOR specs listed above. insisting on 330 will get ;you out of buying what may be a very nice car. does it start and run good? idle well?

tom w

Sorry- yes the ~400 PSI values were in a hot engine. That's the way the factory manual says to test them. IIRC the acceptable values (hot) were something like 300-450 (but in bars). And I was able to find your W115 300D on braingears this morning, just couldn't put my finger on the compression specs

gotcha! The pdfs would not open under Firefox - weird, but whatever they worked fine in IE. And that's what I found:

22-24 bar (approx. 330 psi) at 80C coolant temperature
15 bar (215 psi) compression minimum
3 bar (44 psi) maximum compression difference between any 2 cylinders

I bet he had the compression tested on the cold engine.

it seems
 

all comes together with that set of facts. and youre just above minimum. so the car is not really great on compression as the seller had stated. so maybe you are in for a refund or a partial refund if you still want the car.

tom w