Basic question on compression test
Thread Starter
Safety Car
Joined: Mar 2006
Posts: 4,448
Likes: 4
From: Sarasota Florida
St. Jude Donor '06
Basic question on compression test
Ok, real basic question. I feel dumb having to ask it, but here goes.
I understand the basics of doing a compression test. You pull all the plugs, screw the gauge into each plug hole, crank the engine, read the results, right?
But how do you keep the coils from firing? Or maybe you don't Do you just let the plug wires dangle? I want to make sure I don't damage anything.
I understand the basics of doing a compression test. You pull all the plugs, screw the gauge into each plug hole, crank the engine, read the results, right?
But how do you keep the coils from firing? Or maybe you don't Do you just let the plug wires dangle? I want to make sure I don't damage anything.
Burning Brakes
Joined: Jul 2008
Posts: 923
Likes: 37
From: Green Bay Wisconsin
Hello,
It's better to ask a dumb question than make a dumb mistake!
I would not allow the plug wires to bounce around and arc because it's just not good policy. You could get a shock, or the arc could be an ignition source for a fire.
With that sad I would unplug the ignition wire from the spark plug and unplug the ignition wire from the coil.
All your readings should be within 10% or less highest to lowest compression reading.
Good Luck,
Douglas in Green Bay
It's better to ask a dumb question than make a dumb mistake!
I would not allow the plug wires to bounce around and arc because it's just not good policy. You could get a shock, or the arc could be an ignition source for a fire.
With that sad I would unplug the ignition wire from the spark plug and unplug the ignition wire from the coil.
All your readings should be within 10% or less highest to lowest compression reading.
Good Luck,
Douglas in Green Bay
Thread Starter
Safety Car
Joined: Mar 2006
Posts: 4,448
Likes: 4
From: Sarasota Florida
St. Jude Donor '06
Thanks for the reply!
Also, since our cars have pushbutton start, how do you turn the engine over? Do you just let it crank until the start algorithm "gives up"?
Also, since our cars have pushbutton start, how do you turn the engine over? Do you just let it crank until the start algorithm "gives up"?
Burning Brakes
Joined: Nov 2011
Posts: 795
Likes: 1
From: tacoma wa
thats a good question, I would short the starter relay to make it crank. this would also not allow the coils to fire. I would also hold open the throttle body if it were me.
post up the numbers when you are done! also why are you doing one?
post up the numbers when you are done! also why are you doing one?
Team Owner
Joined: Sep 2000
Posts: 25,015
Likes: 2,709
From: Far NW 'burbs of Chicago
St. Jude Donor '13
My understanding is that a "leakdown" test with compressed air is more difficult to do, but may provide more accurate information.
Depends on what you are trying to discover.
Depends on what you are trying to discover.
Burning Brakes
Joined: Nov 2011
Posts: 795
Likes: 1
From: tacoma wa
what else are you looking for? un plug the coil pack the center big white plug on each side. for cranking push the start button, when the gauge stops moving up push the bottom of the start button and it will stop cranking.
Le Mans Master
Joined: Aug 2002
Posts: 8,472
Likes: 38
From: Houston TX
St. Jude Donor '11
As mentioned unplug the coil packs at the middle big plug, on both sides. But I would also add removing the fuel pump relay as you don't want the cylinders to get any fuel and skew the results as you continue to crank. Screw in the tester one cylinder at a time and hit the starter button. It will crank and crank. Stop when you see the gauge not moving any higher, about 4-5 seconds.
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Burning Brakes
Joined: Dec 2011
Posts: 876
Likes: 116
From: HOMESTEAD FL
Pull the fuse to both the fuel and ignition system. This will remove any spark or fuel from the equation.
You also want to be holding the throttle body WOT while your turning it over... You want zero restrictions.
Also you want to make sure all spark plugs have been removed.
Doing this after the engine has fully warmed up is not completely necessary but can give higher readings. But its the difference between each cylinder that your most concerned with.
SPEED SAFE, NICK
You also want to be holding the throttle body WOT while your turning it over... You want zero restrictions.
Also you want to make sure all spark plugs have been removed.
Doing this after the engine has fully warmed up is not completely necessary but can give higher readings. But its the difference between each cylinder that your most concerned with.
SPEED SAFE, NICK
Last edited by AIR_RAM; Aug 23, 2012 at 10:09 PM.
Pro
Joined: Jul 2011
Posts: 515
Likes: 1
From: Boca ARton FL
Pull the fuse to both the fuel and ignition system. This will remove any spark or fuel from the equation.
You also want to be holding the throttle body WOT while your turning it over... You want zero restrictions.
Also you want to make sure all spark plugs have been removed.
Doing this after the engine has fully warmed up is not completely necessary but can give higher readings. But its the difference between each cylinder that your most concerned with.
SPEED SAFE, NICK
You also want to be holding the throttle body WOT while your turning it over... You want zero restrictions.
Also you want to make sure all spark plugs have been removed.
Doing this after the engine has fully warmed up is not completely necessary but can give higher readings. But its the difference between each cylinder that your most concerned with.
SPEED SAFE, NICK
Oh one final tip make sure the schrader valve is installed in the air hose, some people take them out to pressurize the cylinders to change valve springs, ect, without it all you'll get is Phffft, phffft, phfft
.
