Low Compression Expansion Tank

[derekcarden]

Hello Everyone.

Basically.. I have a cracked expansion tank.. the new one is on order. This was caused by overheating or another faulty part i'm sure. So here i go to do a compression test. first cylinder is ok. 2nd and 3rd read 90 psi.. wayyyy too low!!!!

ive read about bleeding coolant to release possible air pockets.. so i will do so asap.

also.. it seems like that fan used to turn on as soon as the car turned on and is not right now.. but it does have tension both ways and i believe it works..

so i will get a new thermostat becuase i think thats what the cause is..


My Question is.. what would cause the low compression...
i know it could be head problems.. gasket.. etc.. but i am wondering..

a) would air pockets lower my compression .. as there is no oil/water(coolant) mixture that i see...

b)do i have to have ALL spark plugs out when i compression test because i did one at a time.. took one out tested it.. put it back in.. moved to 2nd cylinder etc. .. would it change results.. does having throttle all the way open change results?? and would low coolant change results??

I know its alot but please help if you can. thanks.

[E36graduate]

firstly, if your expansion tank is cracked, it is a common failure part as I have replaced many in the 400,000 miles on my bmw's.

i'm not sure how why you are relating low compression and your expansion tank, unless you overheated your engine very bad?

secondly, if you have an electric fan as I do, my expansion tank blew and shortly after that I figured out my electric fan was toast (again). I got sick of paying $400 for this fan and jury rigged it so whenever I am driving slow or in stop and go with a/c on, I put the fan on manually with a switch...works well, just dont trust my wife with my car.

since your first 3 cylinders were the same at 90 pounds(?), I would think the tool you used was not calibrated or reading another unit of measure?

if you are concerned you blew your head gasket or something, you would have the tell tale signs that are common.

overhaul that cooling system and see what happens....you should be all set.

[Nico3k]

Results of a compression test have absolutely nothing to do with your coolant. If the results are correct, then your engine needs to be taken apart ASAP.

[derekcarden]

The thing is that i compression tested before i realized the cracked expansion tank.. I always jump to compression test first thing after i see overheating.
results of compression test.
1st- 155
2nd-90
3rd- 90
4th- 155
5th- 155
6th- 155

so i'm thinking overheating caused by faulty tstat or simply from the expansion tank alone.. causing blown head gasket..