Compression check
Cargo Master
Joined: Nov 2011
Posts: 3,354
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From: Great American SouthWest
I don't know the reason, but I think whatever method is employed, its most important that one is consistent across cylinders
Fleet Mechanic
Joined: Sep 2014
Posts: 1,626
Likes: 78
From: NE Texas
Just got back onto the forums today... think the question has been answered pretty thoroughly by Tedster and Celtic1. By the way, FB, for folks who stop by infrequently, four days may indeed not be sufficient time, especially since if fte doesn't tell someone there is a specific question pending.
Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
I would think there would be a better definite reason as to why pulling the plugs, disconnect the coil (understood for safety), carb at WOT and counting the engine's revolutions other than, "This is the way I was taught."
No offence, Houston Dave and I'm not trying to be a wise apple...but as in checking/setting, say, the gap on pistons rings, the why would be "If the rings aren't gapped to specs, they'll be too tight and you run the risk of broken rings, broke ring lands, etc,"....."not gap the rings to specs."
This is just a curiosity on my part....
[QUOTE} By the way, FB, for folks who stop by infrequently, four days may indeed not be sufficient time, especially since if fte doesn't tell someone there is a specific question pending.[/QUOTE]
Understood and agree....no problems
Fleet Mechanic
Joined: Sep 2014
Posts: 1,626
Likes: 78
From: NE Texas
Been a VERY long time since I was taught to run a compression test but I was told that consistency in testing was important, and you only want to turn so many times for each reading. Having the engine running gives you an indeterminate amount of turns. I suspect having the engine running wide-open with the tester screwed in could potentially damage the tester, as the diaphragms in them probably only take so much pressure and maybe that could be exceeded. You might re-read Tedster's comment, he actually has more 'why' than 'how.' I am far from an authority.
Depends to some extent what info you are looking for. You could probably run several tests: with the coil wire off and removing only one plug removed at a time, both hot and cold. All plugs removed, hot and cold. Squirting oil in each cylinder to see whether any low readings were due to rings, hot and cold. As you said, a compression test is a rough test and other diagnostics like a bleed down test will probably give more complete data: but an initial compression test can give surprisingly accurate indicators of where to go next.
Depends to some extent what info you are looking for. You could probably run several tests: with the coil wire off and removing only one plug removed at a time, both hot and cold. All plugs removed, hot and cold. Squirting oil in each cylinder to see whether any low readings were due to rings, hot and cold. As you said, a compression test is a rough test and other diagnostics like a bleed down test will probably give more complete data: but an initial compression test can give surprisingly accurate indicators of where to go next.
Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
Thanks for the response, Dave.
But I'm not saying to run the engine at WOT....just running for a few seconds. The WOT is for all plugs pulled and coil disconnected.
I agree that consistency - within 10%-20% is ideal and a good starting point. I'm partial to a leak down test initially, though.
Thanks again
But I'm not saying to run the engine at WOT....just running for a few seconds. The WOT is for all plugs pulled and coil disconnected.
I agree that consistency - within 10%-20% is ideal and a good starting point. I'm partial to a leak down test initially, though.
Thanks again
Fleet Mechanic
Joined: Sep 2014
Posts: 1,626
Likes: 78
From: NE Texas
was referring back to "carburetor choke and throttle plates blocked open."
At an idle you would not be WOT, obviously... I have seen a running compression test referred to as a tool, but like I said, when I was taught a running test was considered a no-no. YMMV. I still like to change dinosaur oil every 3,000 miles, too.
At an idle you would not be WOT, obviously... I have seen a running compression test referred to as a tool, but like I said, when I was taught a running test was considered a no-no. YMMV. I still like to change dinosaur oil every 3,000 miles, too.
Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
Ahh, hehe....I change mine once a year whether needed or not...generally 5 - 6K miles. Amsoil synthetic with the ZDDP in it.
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