Steps in compression test?

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J

JozefJ

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I'm looking to do a compression test on my 318 '73 dart sport.
Now I read all the basic steps, from removing all the spark plugs to unplugging the main coil wire and setting the carb to WOT.

Only.. is it neccessary to take extra steps to disable the fuel flow? And if so what tools do I need? Or does unplugging the coil take care of that?

Can anyone give me a detailed step by step guide on how to perform a compression test, so I can get into it with confidence?

Thanks for thinking along!

JJ
 
Do you have a remote starter button and a good battery?
 
I do just what you said, and I don't worry about the fuel flow. Throttle all the way open or halfway open is fine. I haven't seen a difference. Crank it about 5 revolutions
 
318willrun said:
I do just what you said, and I don't worry about the fuel flow. Throttle all the way open or halfway open is fine. I haven't seen a difference. Crank it about 5 revolutions
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Thanks! So I won't have to do anything extra in terms of closing off fuel flow or disconnecting fuel lines? Sounds like an even easier job then :).

Big fan of your channel by the way. Keep up the good work :thumbsup:
 
JozefJ said:
Thanks! So I won't have to do anything extra in terms of closing off fuel flow or disconnecting fuel lines? Sounds like an even easier job then :).

Big fan of your channel by the way. Keep up the good work :thumbsup:
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Private Message me your address and I'll send you a note and some stickers. I love my stickers going overseas. I'm not in the Netherlands yet (If I remember correctly)
 
Crank it as many times as is required to get two successive same-ish readings. A throttle on the fast-idle cam, will get you there.
If the engine pulls in fuel, that fuel will wash the oil off the cylinders, and it is possible that each successive cylinder will produce less pressure. To ensure this has not happened, after all eight are done, go back and test the first one again. You're looking for the same or a very similar number than was the first one.
Make sure that your helper, if he is holding the throttle open with his foot, that he never lifts until the test is complete.
Some cylinders will come up to peak pressure, with fewer compression cycles than others, which seems kindof odd but is normal.
Make sure your compression gauge is not leaking.
Write your result s down, together with the date and mileage, for future reference.
BTW
the Schroeder valves in those things are NOT the same as cores from tire valve-stems. Tire valves will take more cycles and peak lower.
 
318willrun said:
Private Message me your address and I'll send you a note and some stickers. I love my stickers going overseas. I'm not in the Netherlands yet (If I remember correctly)
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I pin the throttle open with the throttle return spring
 
I never open the throttle. Never. It doesn't make a huge difference. What you are looking for are for the numbers to be as even as possible. Throttle open or closed will not affect that. The percentages of difference from one cylinder to the next will still be the same. In most instances where I have blocked the throttle open, I've seen very little to no difference at all. That's why I don't bother with it anymore. Spin the engine over on each cylinder until the gauge stops climbing. That's usually about 5 or 6 revolutions. Then move on to the next, recording each result. Any more than about a 10-12% difference from one cylinder to another usually indicates an issue.
 
RustyRatRod said:
I never open the throttle. Never. It doesn't make a huge difference. What you are looking for are for the numbers to be as even as possible. Throttle open or closed will not affect that. The percentages of difference from one cylinder to the next will still be the same. In most instances where I have blocked the throttle open, I've seen very little to no difference at all. That's why I don't bother with it anymore. Spin the engine over on each cylinder until the gauge stops climbing. That's usually about 5 or 6 revolutions. Then move on to the next, recording each result. Any more than about a 10-12% difference from one cylinder to another usually indicates an issue.
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I've never opened the throttle, no need to.
 
yeah i quit the throttle thing too
i record 1st pump then the last
a round cam might pump-up but the first pump will be low
 
If it has a carburetor, should not have to block the fuel flow. The needle and seat and float should stop the fuel. If it is fuel injected, I would stop the fuel flow and injector pulse.
 
AJ/FormS said:
Crank it as many times as is required to get two successive same-ish readings. A throttle on the fast-idle cam, will get you there.
If the engine pulls in fuel, that fuel will wash the oil off the cylinders, and it is possible that each successive cylinder will produce less pressure. To ensure this has not happened, after all eight are done, go back and test the first one again. You're looking for the same or a very similar number than was the first one.
Make sure that your helper, if he is holding the throttle open with his foot, that he never lifts until the test is complete.
Some cylinders will come up to peak pressure, with fewer compression cycles than others, which seems kindof odd but is normal.
Make sure your compression gauge is not leaking.
Write your result s down, together with the date and mileage, for future reference.
BTW
the Schroeder valves in those things are NOT the same as cores from tire valve-stems. Tire valves will take more cycles and peak lower.
Click to expand...


There isn’t an engine on the planet that can pull fuel from the booster or the idle circuit with the throttle wide open.

If your theory was correct if you hold the throttle WFO when it’s flooded it would never clear up and start.

I reserve the right to use the ❌ for more egregious errors.
 
I was always told to check it warmed up. Been doin that a hundred years. Maybe not really important, but that's what I do.
 
Ironmike said:
I was always told to check it warmed up. Been doin that a hundred years. Maybe not really important, but that's what I do.
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That's the proper way. Otherwise, the rings aren't warmed up and there's no oil film for ring seal.
 
Think it depends on what your trying to diag
Most cases cold will give you what you need to know
 
Dan the man said:
It doesn't matter
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Really? This is why you get nailed.

I never do a leak down or a compression test cold.

Why you ask?

Because as the engine heats up the bores expand, the pistons expand and **** moves around.

That means the ring gaps close up.

As the piston gets hot, it’s designed to expand in areas to run the minimum clearance without sticking.

Taking all that into consideration, do you really think a piston flopping in the bore is an accurate test?

Of course the ring gap cold being wide like a gate is not conducive to accurate testing.

You do it hot, like it runs or the numbers are at best suspect.

This is why any testing without a valid protocol is garbage.

You wouldn’t like my dyno numbers if when testing your engine the coolant varied between 120 and 180 would you?

How about if I let the oil temp vary say, 20 degrees. That would be impossible to use any of the numbers because they’d be inaccurate.

So Dan, you do compression and leak down tests with the engine hot.
 
Like I said it depends on what your testing
Will it run ,will it miss fire cold is ok
Is the engine 100% are you getting all the power out of it then hot is a must
 
nutz said:
Like I said it depends on what your testing
Will it run ,will it miss fire cold is ok
Is the engine 100% are you getting all the power out of it then hot is a must
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The only one time I would agree cold is on an unknown engine that does not run. There, you have no choice.
 
Newbomb Turk said:
Really? This is why you get nailed.

I never do a leak down or a compression test cold.

Why you ask?

Because as the engine heats up the bores expand, the pistons expand and **** moves around.

That means the ring gaps close up.

As the piston gets hot, it’s designed to expand in areas to run the minimum clearance without sticking.

Taking all that into consideration, do you really think a piston flopping in the bore is an accurate test?

Of course the ring gap cold being wide like a gate is not conducive to accurate testing.

You do it hot, like it runs or the numbers are at best suspect.

This is why any testing without a valid protocol is garbage.

You wouldn’t like my dyno numbers if when testing your engine the coolant varied between 120 and 180 would you?

How about if I let the oil temp vary say, 20 degrees. That would be impossible to use any of the numbers because they’d be inaccurate.

So Dan, you do compression and leak down tests with the engine hot.
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I learned something new, we always did a compression test cold and we would squirt oil down the bore too. The way you explain it really makes sense and I'll remember it.
 
Dan the man said:
I learned something new, we always did a compression test cold and we would squirt oil down the bore too. The way you explain it really makes sense and I'll remember it.
Click to expand...
And yet you gave WRONG advice based on some bull crap THAT YOU DIDN'T KNOW.
 
At least a warm engine, I was taught the 2nd tic on the gauge would show valve sealing and 4th is the ring sealing. More than 4 not needed. Just look for wide variation in 2nd or 4th with each other.
 
When ill doing compression check it normally just to see if it has it
Don't work on many car older the 20years
So just to know if it will fire up or have a missfire
Is not necessary telling how good the engine is
 
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