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Gary, ASE Certified Technician
Category: Car
Satisfied Customers: 916
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: 11 months ago.
Category: Car
Expert:  Greg replied 11 months 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 11 months 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 11 months 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 11 months 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 11 months 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