B20B motor dry compression test, one cylinder low

 
Old 10-14-2018, 05:06 PM
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Default B20B motor dry compression test, one cylinder low

Just pulled a B20B motor from an EF civic from a junk yard kind of place. The car got a B20 swap but whoever did it didn't figure out the wiring... The car has been sitting in the yard for a while..So I did a compression test using a harbor freight compression testing gage. Cylinder #4, #3, #2 all read 90 psi, but #4 reads only 40 psi. The guy who is trying to sell me it told it was normal. He said normal compression test should be done when the car is warm.... Then he added some oil into cylinder #4, and we did the test again, it read 100+ psi....

Should I be concerned? Or should I rebuild / re-ring the motor ?
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Old 10-14-2018, 09:47 PM
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Default Re: B20B motor dry compression test, one cylinder low

Dry, the compression numbers need to be greater than 140 psi to run... so far, you haven't seen any number that exceeds this, so this engine needs internal work. This idea that the compression test must be done "hot" is crap... that guy is either ignorant about engines or a con-man. Buy it cheap (like for less than $100) or walk away.
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Old 10-15-2018, 06:48 PM
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Default Re: B20B motor dry compression test, one cylinder low

Originally Posted by JRCivic1 View Post
Dry, the compression numbers need to be greater than 140 psi to run... so far, you haven't seen any number that exceeds this, so this engine needs internal work. This idea that the compression test must be done "hot" is crap... that guy is either ignorant about engines or a con-man. Buy it cheap (like for less than $100) or walk away.
I've always read you should warm the car and let the motor cool for roughly 15 minutes then do your compression test with WOT.

I was always told a cold engine will have lower than FSM spec numbers.

You are the first person to say otherwise.

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Old 10-15-2018, 09:57 PM
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Default Re: B20B motor dry compression test, one cylinder low

Originally Posted by TomCat39 View Post
I've always read you should warm the car and let the motor cool for roughly 15 minutes then do your compression test with WOT.

I was always told a cold engine will have lower than FSM spec numbers. It has been a long time since I looked in a FSM as to the factory specifications for conducting a compression test. However, I do have a logical question to consider when testing a damaged/warn out engine... IF it doesn't run, how do you get it up to temperature so that a PROPER compression test could be conducted ???

You are the first person to say otherwise.
TomCat, I don't want to give a false impression here, so let me clarify. The results will be different from cold to warm... but the difference in the hot and cold results will still be LESS than the difference between testing with a closed T/B and an open T/B. The OP in this case offered up some really low numbers... so in short, what I was trying to say was if a cylinder pressures 90psi cold, it ISN'T going to come up to 160-180psi warm.

For reference, my personal experience with doing basic compression tests suggests the difference between cold and warm testing is 2-3%. Testing with the throttle blade open typically returns results 5-7% higher than a closed throttle blade.
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Old 10-16-2018, 05:13 AM
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Default Re: B20B motor dry compression test, one cylinder low

Makes total sense JRCivic. I always thought about that too if a car doesn't run (cold testing) but I didn't realize the change was so small even in comparison to open or closed throttle plate.

Great information to know, thank you!
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