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Gary, ASE Certified Technician
Category: Car
Satisfied Customers: 1009
Experience:  Senior Technician
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What is good compression value? What will be the telltale

Customer Question

What is good compression value? What will be the telltale sings of a blown heat gasket? And what is the best testing procedure to detect a blown head gasket?
Submitted: 1 year ago.
Category: Car
Expert:  Greg replied 1 year ago.

Hi .

Good compression on your engine will be between 10 and 14 bar and all four should be around the same

pressure with not having more than 5% difference.

All spark plugs should be removed for this test .

A telltale sign is with a hot engine and the rad cap removed you will see bubbles surfacing up the neck of the top .

A better way is to do a Co2 test .

Watch the video attached it will show you what tools are needed and how to do this .

Please click here for demonstration video .

Still need additional information then please get back to me with the reply button, and not the poor or bad service, this way i can get back to you or suggest another expert if needed.

Your deposit does not pay me until the answer is rated in a positive way,
A bad rating does not constitute a refund.

If you are not happy or you have sorted out your problem before the answer then please let me know with the reply button .

Kind regards Greg.

Customer: replied 1 year ago.
Hi Greg,
I did another compression Teas on my 1999 c 70 Volvo and these are the readings.
Cylinder #1 12.7 bar
Cylinder #2 10.34 bar
Cylinder #3 10.2 bar
Cylinder #4 11.3 bar
Cylinder #5 12.76 bar
I do not have any bubbles in the expansion tank.
Your thoughts?
Expert:  Greg replied 1 year ago.

#3 is low .what makes you think you have a head gasket issue are you overheating the engine .

Is the engine oil and coolant system clean .

Now do a wet test on each cylinder .

Use around 4oz of engine oil for the wet test on each cylinder at a time .


Expert:  Greg replied 1 year ago.

Before you do the wet test,refit just the #2 and 4 spark plug and take another compression test on #3.


Expert:  Gary replied 1 year ago.

Hi Dave,

It seems that Greg has given me your 2nd question related to Q.1.

Your compression go from high to low to high.

It looks like a head gasket to me and bent head.

I hope my advice has been of assistance to you.

Kind regards