2.5TDi engine compression

marius_b

Member
Joined
Apr 15, 2003
TDI
Audi A6 2.5TDI 140HP, 1995, Z4V
Hi hi,

I would like to know something about engine compression...
Engine: 2.5TDi 103KW(140HP), 5 cylinders in line

So here I go:
1) In the manual they write that the normal compression for this engine is 34 bar and it shall not get under 26 bar. What happens if it gets under 26 bar?
2) They also write that when you measure the compression, the engine has to be warm so that the coolant temperature is 80 C. What if you measure the compression when the engine is not warm, actually it is cold, like after driving 5Km in a cold winter morning(-10 C)? Is the compression test result close to reallity(like a difference of max 1 bar) or far from it?
3) Is it possible that the engine "smokes" a lot because the compression is bad? And how bad it has to be, in order to smoke a lot? An engine with bad compression smokes even when the engine is hot?(if it smokes at all because of bad compression)
4) Is it a must that a low compression engine "eats" oil? Or eating oil is independent of the compression?
5) If the engine starts up without much of a problem on cold mornings(like max 2 secs when the morning is cold, but not cold enough so the plugs have to turn on) and if the engine starts up in less than one sec on cold mornings when plugs are used... - is the compression that bad?
6) When is a compression too bad for the car itself, not just that you see that the numbers look bad?

Hope someone will take some time and help me understand this.
Thanks a lot
 

marius_b

Member
Joined
Apr 15, 2003
TDI
Audi A6 2.5TDI 140HP, 1995, Z4V
And here are some more information...
Just tested my car like this:
started in 1'st gear without any acceleration and drive without acceleration for some 10-20 meters, then
change to second gear without any acceleration and drive like this some 40 meters, then switch to 3'th gear w/o acceleration and drive like that some 100m. I could have drove more but useless, the engine wouldn't stop and there was no sign that the engine was very unhappy about this(some more vibrations than usual but that's normal I guess).

Is it possible to do such a thing with an engine that has a bad compression?
 

TDIMeister

Phd of TDIClub Enthusiast, Moderator at Large
Joined
May 1, 1999
Location
Canada
TDI
TDI
1) In the manual they write that the normal compression for this engine is 34 bar and it shall not get under 26 bar. What happens if it gets under 26 bar?
The engine will either not start very easily, especially when cold, or if it does will run rough and smokey until it warms up.

2) They also write that when you measure the compression, the engine has to be warm so that the coolant temperature is 80 C. What if you measure the compression when the engine is not warm, actually it is cold, like after driving 5Km in a cold winter morning(-10 C)? Is the compression test result close to reallity(like a difference of max 1 bar) or far from it?
Testing the compression on a cold engine will result in compression readings that are lower than in a warm engine. The reason is that a cold engine has higher clearances between the piston and cylinder walls for blow-by leakage to occur. Also, a cold engine will result in increased heat transfer from the compressing air to the cold engine surfaces, further reducing the measured pressure.

3) Is it possible that the engine "smokes" a lot because the compression is bad? And how bad it has to be, in order to smoke a lot? An engine with bad compression smokes even when the engine is hot?(if it smokes at all because of bad compression)
Yes, see answer to #1 above. Also you could very well have worn and dirty injectors.

4) Is it a must that a low compression engine "eats" oil? Or eating oil is independent of the compression?
Possibily, but not necessarily. Low compression is indicative of a worn engine, and implies leakeage primarily through the piston rings; secondarily though the valves. If the rings are worn, compression will be low, and oil consumption will increase.

Some oil consumption is not cause for worry. If the compression check is OK but there is high oil consumption and exhaust smoke then there is a possibility that the valve stem seals are worn.

5) If the engine starts up without much of a problem on cold mornings(like max 2 secs when the morning is cold, but not cold enough so the plugs have to turn on) and if the engine starts up in less than one sec on cold mornings when plugs are used... - is the compression that bad?
My TDI starts almost instantaneously regardless of the temperature (except real extremes like <-20*C). 2 seconds is unacceptable!

6) When is a compression too bad for the car itself, not just that you see that the numbers look bad?
Don't really know what you mean by that, but if I understand correctly, if a compression test shows it to be below spec in the manual, and you are experiencing longer cranking times and smokey starts, don't you think it's about time to do something about it?
 

marius_b

Member
Joined
Apr 15, 2003
TDI
Audi A6 2.5TDI 140HP, 1995, Z4V
Yes, see answer to #1 above. Also you could very well have worn and dirty injectors.
I checked the injectors, they spray very well.
How do I know when the injectors are worn?

4) Is it a must that a low compression engine "eats" oil? Or eating oil is independent of the compression?
Possibily, but not necessarily. Low compression is indicative of a worn engine, and implies leakeage primarily through the piston rings; secondarily though the valves. If the rings are worn, compression will be low, and oil consumption will increase.
The engine doesn't eat oil.
I use only 0-40 full syntetic, and I change the oil filter each time I change the oil(which happens every 14-15000 KM).
The difference of oil level after I change it and before I change it is like 1mm(without refilling it), so peanuts.


My TDI starts almost instantaneously regardless of the temperature (except real extremes like <-20*C). 2 seconds is unacceptable!
Could it be that it doesn't start easy not because of bad compression, but other things?


Don't really know what you mean by that, but if I understand correctly, if a compression test shows it to be below spec in the manual, and you are experiencing longer cranking times and smokey starts, don't you think it's about time to do something about it?
Well, I do want to make something, but not until I am 100% sure what is the problem(remember the dude that sold his engine on ebay because he had bad timing for injectors, or some like this?). I will check the compression one more time, this time respecting what is written in manual(coolant temp = 80C).

And if I get a bad compression, what could cause it? Shall I change the entire engine(which is not cheap after all) or the engine could be rebuilt?

Thanks for your answers
 
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