compression test
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Junior User
Joined: Aug 2008
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compression test
I want to check the compression of one cylinder on my 01 F250. I have the haynes manual but do i really have top take out all the plugs.
Can someone give a quick guide to a fast compression check. Like what to unhook and where to put the throttle. Is it ok if the truick starts while doing it?
Can someone give a quick guide to a fast compression check. Like what to unhook and where to put the throttle. Is it ok if the truick starts while doing it?
Freshman User
Joined: Dec 2008
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You do not want the truck to start, that will more than likely ruin your compression tester. It doesnt matter where you put the throttle, you just want the piston to complete it's compression stroke. You could unhook all of the leads to all of the coils or anything to keep the truck from starting. In the old days you just took off the coil wire. As far as a reading, you just want it to be close to uniform for all cylinders. Hope this helps.
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Junior User
Joined: Aug 2008
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I have what seems to be a misfire at idle only.
My friends scanner says number two cylinder.
I've replaced the coil, plug and switched injectors.
athe number two plud i removed is snow white with a powery dust on it. I removed a couple more that were a tan brown.
So i'm looking for some other problem and i was told to do a compression test.
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
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From: Long Island USA
Yes, it does matter where you put the throttle. The throttle needs to be completely open so that the compression test is accurate. Without it, you don't get a lot of intake air, and the compression will read very low.
If you don't want the truck to start, remove the fuel pump fuse while it's running (might need to clear a PCM code afterwards), OR - remove the cam/crank sensor connectors.
If you don't want the truck to start, remove the fuel pump fuse while it's running (might need to clear a PCM code afterwards), OR - remove the cam/crank sensor connectors.
Elder User
Joined: Jan 2002
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It does matter.
As others stated, you DO NOT want the engine to start while doing the test, so disable either the spark or the fuel. If you want to get an accurate compression test to evaluate the overall condition of the engine, you need to pull all of the plugs and check the compression on every cylinder. If you are only concerned with the one cylinder, you can just pull that plug and test that cylinder. The engine will turn over a lot easier if ALL of the plugs are out, but it is not absolutely necessary. The test will be more accurate if the throttle is fully open during the test. If you are just trying to see if that one cylinder is "in the ballpark" and has a reasonable compression, just pull that plug, insert your guage, disable the spark or fuel so the engine will not start, and turn the engine over 4 or 5 times. Read the guage and see what you have.
