Tips on doing Compression test
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Freshman User
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Tips on doing Compression test
So I’m gonna do a compression test on my motor because after I did spark plugs I noticed I had oil on the bottom half of spark plug. The cop is dry by the way.
so to do compression test, I know need to take all plugs out but someone told me I have to disconnect my ignition system, or hold the gas pedal all the way done to the floor. And I don’t under stand why I need to poor a little oil in the cylinder head before I do it.
and can’t I buy a switch so I can start the truck with out being in the cab?
so to do compression test, I know need to take all plugs out but someone told me I have to disconnect my ignition system, or hold the gas pedal all the way done to the floor. And I don’t under stand why I need to poor a little oil in the cylinder head before I do it.
and can’t I buy a switch so I can start the truck with out being in the cab?
Lead Driver
Joined: Sep 2006
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No need to put oil in the cylinders because you want to know the pressure as it is, not help it with an oil seal.
Pull all fuel injector connectors off of so fuel is not sprayed and blown out of the plug holes lest you have a fire situation from spark.
The throttle has to be opened at least 3/4 to allow enough air into the cylinder under test to compress and free up the other 7 for faster spin.
If any cylinder is Low >>>>> then put a small amount of oil in to see if the compression pressure increases significantly. That is the value of putting the oil in in that circumstance. It would suggest a ring seal issue.
if the compression is low, listen in the intake and exhaust for leakage. This would suggest valve seal leakage.
An air leak heard in the crank case is rings/and or a piston issue.
A blown head gasket will influence the pressure reading. Into the next cylinder or into a coolant passage.
Use common sense on these possible results..
All this make sense to you?
Good luck..
Pull all fuel injector connectors off of so fuel is not sprayed and blown out of the plug holes lest you have a fire situation from spark.
The throttle has to be opened at least 3/4 to allow enough air into the cylinder under test to compress and free up the other 7 for faster spin.
If any cylinder is Low >>>>> then put a small amount of oil in to see if the compression pressure increases significantly. That is the value of putting the oil in in that circumstance. It would suggest a ring seal issue.
if the compression is low, listen in the intake and exhaust for leakage. This would suggest valve seal leakage.
An air leak heard in the crank case is rings/and or a piston issue.
A blown head gasket will influence the pressure reading. Into the next cylinder or into a coolant passage.
Use common sense on these possible results..
All this make sense to you?
Good luck..
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Thing is modern manufacture engines aren't worn out after 75k like they were back in the day. Not sure that a compression test has all that much utility on anything made since the 1980s or so. Could be wrong.
Technically speaking the engine is supposed to be fully warmed up, by oil temperature indication. This is widely ignored because nobody likes working on a hot engine. Usually checked first without adding any oil to the cylinders, and then about a tablespoon or so if low, to see if the problem is rings or valves.
Yes, you'd want a remote start switch, unless you've a helper.
Technically speaking the engine is supposed to be fully warmed up, by oil temperature indication. This is widely ignored because nobody likes working on a hot engine. Usually checked first without adding any oil to the cylinders, and then about a tablespoon or so if low, to see if the problem is rings or valves.
Yes, you'd want a remote start switch, unless you've a helper.
Fleet Owner
Joined: Apr 2008
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From: 0,0,1
Cross-Country
Joined: Jul 2019
Posts: 76
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From: Texas
Learned a quick & easy compression test to do. Used it during pre-firing my engine. Whew...it was good.
Unplug the crank sensor connector so the engine won't start
Turn the ignition key like trying to start it and listen carefully for maybe 20-30 secs
duh unna unna huh duh unna unna huh <-----is the sound of one or more cylinders having less compression than the others.
unna unna unna unna <-----is the sound of cylinders all being the same.
It's OK to say that I may need a doctors office visit for diagnosis
Unplug the crank sensor connector so the engine won't start
Turn the ignition key like trying to start it and listen carefully for maybe 20-30 secs
duh unna unna huh duh unna unna huh <-----is the sound of one or more cylinders having less compression than the others.
unna unna unna unna <-----is the sound of cylinders all being the same.
It's OK to say that I may need a doctors office visit for diagnosis
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Freshman User
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Thanks for all the tips/ advice I’m about to do it when I get home.. and clutch5sp your speaking my language lol.
And is the crank sensor behind the ac pulley that you can see from passenger wheel well?
And is the crank sensor behind the ac pulley that you can see from passenger wheel well?
Logistics Pro
Joined: Jan 2001
Posts: 3,740
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From: Central NYS
Instead of pulling all the fuel injectors just unplug the crank sensor. That will keep the ignition from firing and keep the fuel injectors from firing.
The oil you found on the bottom half of your spark plugs could be from a bad camshaft cover gasket letting oil leak down into the plug well.
The oil you found on the bottom half of your spark plugs could be from a bad camshaft cover gasket letting oil leak down into the plug well.
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Freshman User
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Instead of pulling all the fuel injectors just unplug the crank sensor. That will keep the ignition from firing and keep the fuel injectors from firing.
The oil you found on the bottom half of your spark plugs could be from a bad camshaft cover gasket letting oil leak down into the plug well.
The oil you found on the bottom half of your spark plugs could be from a bad camshaft cover gasket letting oil leak down into the plug well.
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Cross-Country
Joined: Jul 2019
Posts: 76
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From: Texas
The crank sensor is at the harmonic balancer 7 o'clock next to the AC comp. Get it from under the truck. Typical Ford connector that will fight you to unplug. Find the clicky tab and pull it loose making sure it doesn't get caught up in the belt while testing.
Fleet Owner
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From: 0,0,1
If it's easier you can also pull the fuel pump relay to keep the engine from starting. This has the added benefit of turning off the fuel supply. In fact I usually start the engine and then pull the relay so the fuel pressure bleeds off and you're not working around or on a pressurized fuel system. I'll keep the relay out until it's time to start the engine after the work is done.
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