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Compression Test BJ7

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S

shorn

Jedi Knight
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A friend wants to borrow my compression guage and has asked for my assistance in doing the test on his BJ7. It has been years since I used the guage. I think I remember the procedure: 1. warm engine first, 2. remove all spark plugs, 3. remove coil to distributor lead, 4. fully depress trottle while turning engine over for each cylinder test, 5. record results. Any low compression cylinder should be at least 75% of the rest. Two low cyliners side by side suggests head gasket leak. Question? Would it be advisable to disconnect the power to the fuel pump while performing the test?
 
Shorn, you do not need power on to complete this test use the Solenoid button and you will be home free.-FWIW---Keoke
 
I usually check for a 10% differential across all cylinders. If all readings are within the 10% things are usually ok unless you have low compression on all cylinders. More than that I start looking for things. I don`t know what depressing the throttle will do. I have never done it on an auto engine. If you have access to a gauge that uses compressed air you can see how much retention the cylinders have and you can also check for burnt valves (air leaking into carbs or exhaust) and you can listen for air leaking into the crankcase. Skip
 
Skip, depressing the throttle is simply done to open the carb bores{Throttle Disks} they can be blocked open to perform this test.---Keoke
 
So my friend's BJ7 tested out pretty good. All cylinders tested between 147 and 152. His car has about 60,000 miles on it. I guess now that I found my old guage, I should test my BJ8. Keoke, thanks for the solenoid button tip. I obviously was trying to complicate this. No reason to make it a two person task.
 
When performing the test does it matter how many time you let it crank? Meaning if you let it crank for 3 seconds or more is that too long for an accurate reading?
 
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When performing the test does it matter how many time you let it crank? Meaning if you let it crank for 3 seconds or more is that too long for an accurate reading?

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Should be long enough to come up to pressure (ie no increase on the gauge towards the end), but also an equal time per cylinder to get a fair comparison.

I use 6 or 7 seconds but other may have their own formula

I get 160 - 165 psi across the 6 cylinders of my 1964 (unknown engine mileage) BJ8

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Try counting to ten revolutions/ Clynder thats easy to keep track of-FWIW---Keoke /ubbthreads/images/graemlins/patriot.gif /ubbthreads/images/graemlins/patriot.gif /ubbthreads/images/graemlins/patriot.gif /ubbthreads/images/graemlins/patriot.gif
 
I listen for three compression cycles and then record the reading.
 
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