Compression Test

[RUSH2112]

I would appreciate if anyone would reply with the correct procedure on how to perform a compression check on my car. I have received conflicting instructions and want to make sure I get an accurate proper reading.

I know I need to remove the coil wire, but my first question is should I remove ALL the spark plugs first, or only remove 1 plug, then test that cylinder, then replace it and move to the next plug?

Also, I have heard different stories about whether or not the fuel pump relay should be disconnected so no gas is being forced into the cylinder(s). Also, I have read articles about performing the test and some state to perform the test under WOT. Should I open the throttle while cranking the engine over a few times (or have someone push the gas pedal down)?

[SonofaBish]

Umm... i'm trying to think back to when i did it. I had a friend help me out with it. If i recall correctly, it doesnt matter whether you take all the sparkplugs out, or 1 at a time. We pulled the relay for the fuel pump so that it didnt turn on. Since there were two of us, i sat in the car, with the gas pedal to the floor, and cranked the motor over a few times. However, i'm sure it really doesnt matter whether you hold it open with your hand or you have someone in the car doing it. The TPS will get a WOT reading either way.
Good luck! Btw, why are you doing the test? from what i've gathered in the past, you're probably better off just doing a leak-down test from the get-go!

[Domination]

ALL plugs out.
Throttle wide open.
Fuel pump disabled.

[RUSH2112]

Umm... i'm trying to think back to when i did it. I had a friend help me out with it. If i recall correctly, it doesnt matter whether you take all the sparkplugs out, or 1 at a time. We pulled the relay for the fuel pump so that it didnt turn on. Since there were two of us, i sat in the car, with the gas pedal to the floor, and cranked the motor over a few times. However, i'm sure it really doesnt matter whether you hold it open with your hand or you have someone in the car doing it. The TPS will get a WOT reading either way.
Good luck! Btw, why are you doing the test? from what i've gathered in the past, you're probably better off just doing a leak-down test from the get-go!

Just want to get an idea on how 'old and worn' my engine is. I am burning oil and coolant and want to see if it is the head gasket and check for possibly cylinder failure(s). I will also have a leak-down test performed to coincide with the data from the compression test. I just need to see if an engine re-build kit is warranted for my spring project. Thanks for the info.

[RUSH2112]

ALL plugs out.
Throttle wide open.
Fuel pump disabled.

Again, this is contradictory to what others have said. Others have made no mention as to disable the fuel pump, in fact, the state to keep it on. Also have heard no mention about WOT, just to let the car crank 'naturally'. I have also heard others say take 1 plug out. So WHO is right?

[Viper_ed]

Again, this is contradictory to what others have said. Others have made no mention as to disable the fuel pump, in fact, the state to keep it on. Also have heard no mention about WOT, just to let the car crank 'naturally'. I have also heard others say take 1 plug out. So WHO is right?

OK, then leave the fuel pump enabled and fill the cylinders with gas! :thmbsup:


:biglaugh:

[87notch]

:yikes:
OK, then leave the fuel pump enabled and fill the cylinders with gas! :thmbsup:


:biglaugh:

[Pure Stock]

ALL plugs out.
Throttle wide open.
Fuel pump disabled.

:iagree:

[Martin0660]

ALL plugs out.
Throttle wide open.
Fuel pump disabled.

This is the only way to get an accurate test.

Should also be a engine warm. but not a necessity. After you run the test, you can give the cylinders a SMALL shot of ATF, and see if the numbers go up. If the do, its typically rings, if they dont, its usually in the valves / heads.

Bob Myers

[Domination]

Again, this is contradictory to what others have said. Others have made no mention as to disable the fuel pump, in fact, the state to keep it on. Also have heard no mention about WOT, just to let the car crank 'naturally'. I have also heard others say take 1 plug out. So WHO is right?


I am right!

hehe.

I don't give advice over the web for stuff I don't know about. (like most people) Check the credentials....

ASE master certified

[Rodeheaver's]

OK, then leave the fuel pump enabled and fill the cylinders with gas! :thmbsup:


:biglaugh:
I don't think it would FILL the cylinders with fuel but i like to unplug the relay fuse to just plain rule that out of the equasion,

[RUSH2112]

I am right!

hehe.

I don't give advice over the web for stuff I don't know about. (like most people) Check the credentials....

ASE master certified


I appreciate the advice. I am not doubting you, I am just getting conflicting stories and want to make sure I do this correctly and get an accurate reading without messing something up. Here is a quote from someone else.

"I hate to disagree with ya but if he removes all the plugs first,when he cranks the car over,it will leave a mess in the engine compartment.....The fuel pump needs to be left on,as gas is part of the fuel mixture,no gas=doctored results as 5.0 intakes do not flow perfect to all cylinders...meaning #7,#8 will normally be lower than the rest. Taking the fuel out of the equation will result in these cylinders to appear really low.....As far as the throttle,this varies,from person to person,but as an engine builder,the way to seal the rings in a new or rebuilt motor,is to have the motor pull against itself, driving the car rev to the moon,close throttle blade,and let motor pull its self down thru the rpm's...This creats what is called top end vaccumm.it increases the pistons ability to pull the air into the motor(efficienty)....Doing a compression check,measures the efficienty of a motor to draw air,and compress it.......Having the throttle blade open,the results will be less than satisfactory as to say your giving the motor a helping hand,in drawing air into the combustion chaimber..... Normal compression reading should be within 120psi-155psi............I could be wrong,but this is how I have always done it..........or Just my $.02"

[coralcoupe1993]

Again, this is contradictory to what others have said. Others have made no mention as to disable the fuel pump, in fact, the state to keep it on. Also have heard no mention about WOT, just to let the car crank 'naturally'. I have also heard others say take 1 plug out. So WHO is right?


Dont leave your fuel pump on! When I was taught, it was told WOT. I have done it both ways and got the same PSI. :goodidea:

[Domination]

"I hate to disagree with ya but if he removes all the plugs first,when he cranks the car over,it will leave a mess in the engine compartment.....The fuel pump needs to be left on,as gas is part of the fuel mixture,no gas=doctored results as 5.0 intakes do not flow perfect to all cylinders...meaning #7,#8 will normally be lower than the rest. Taking the fuel out of the equation will result in these cylinders to appear really low.....As far as the throttle,this varies,from person to person,but as an engine builder,the way to seal the rings in a new or rebuilt motor,is to have the motor pull against itself, driving the car rev to the moon,close throttle blade,and let motor pull its self down thru the rpm's...This creats what is called top end vaccumm.it increases the pistons ability to pull the air into the motor(efficienty)....Doing a compression check,measures the efficienty of a motor to draw air,and compress it.......Having the throttle blade open,the results will be less than satisfactory as to say your giving the motor a helping hand,in drawing air into the combustion chaimber..... Normal compression reading should be within 120psi-155psi............I could be wrong,but this is how I have always done it..........or Just my $.02"


Um, I really don't think that guy is an engine builder. He might have seen one appart once, but not a builder. Let me pick threw this one.

"I will leave a mess in the engine compartment" um not really.

"gas is part of the fuel mixture" What the heck is that supposed to mean. Gas IS the fuel mixture. A mixture that rinces oil off of you cylinder walls if it isn't lit on fire. Thats just great.

"#7&8 will be lower cause there is no fuel" Even if he was on to something, there would be a consistant result for removing fuel to all cylinders. Therefore not making any alarming results.

"As far as throttle....." If you ever have an engine builder do that to your fresh motor hit him, and don't pay a dime. "top end vaccum" doesn't seat rings. cylinder pressure seats rings. You want your starter motor as unloaded as possible when doing this test. Vacuum is load. Compression tests aren't for vaccum, their for compression. Checks for Valve seal, head gasket seat, ring seal, cracks in the head, cracks in the block, and damaged pistons. It could also indicate unrelated valve train failure.

[Rodeheaver's]

this isnt as hard as people are trying to make it, when i am in my shop on a normal busy day i ensure to something so the car doesnt start then i just start pulling plugs and checking the pressure and recording it if i find something abnormal with a cylinder i check that one further but if they are all close to one another and close (depending on mileage) to spec there is no problem there are certian factors on a newly built engine to think about in that case but in this case there are not...

[RUSH2112]

Um, I really don't think that guy is an engine builder. He might have seen one appart once, but not a builder. Let me pick threw this one.

"I will leave a mess in the engine compartment" um not really.

"gas is part of the fuel mixture" What the heck is that supposed to mean. Gas IS the fuel mixture. A mixture that rinces oil off of you cylinder walls if it isn't lit on fire. Thats just great.

"#7&8 will be lower cause there is no fuel" Even if he was on to something, there would be a consistant result for removing fuel to all cylinders. Therefore not making any alarming results.

"As far as throttle....." If you ever have an engine builder do that to your fresh motor hit him, and don't pay a dime. "top end vaccum" doesn't seat rings. cylinder pressure seats rings. You want your starter motor as unloaded as possible when doing this test. Vacuum is load. Compression tests aren't for vaccum, their for compression. Checks for Valve seal, head gasket seat, ring seal, cracks in the head, cracks in the block, and damaged pistons. It could also indicate unrelated valve train failure.


I am not discounting your advice, I am merely showing the different results I am getting from several different people.

OK, so it sounds like the procedure is to:

Remove all plugs, disconnect coil, disconnect fuel pump relay.

insert gauge into 1 cylinder, and with WOT (or gas pedal down) crank motor 4-6 times and record number. Repeat for all other cylinders

[Rodeheaver's]

I am not discounting your advice, I am merely showing the different results I am getting from several different people.

OK, so it sounds like the procedure is to:

Remove all plugs, disconnect coil, disconnect fuel pump relay.

insert gauge into 1 cylinder, and with WOT (or gas pedal down) crank motor 4-6 times and record number. Repeat for all other cylinders
yes that is fine...

[Domination]

Here is an illistrated instruction....
http://www.automedia.com/Compression/CheckUp/res20010301ct/2

[RUSH2112]

Good stuff. :awsome: Thanks I'll post my numbers in the next few days and I'd appreciate feedback about them

[RUSH2112]

Here is an illistrated instruction....
http://www.automedia.com/Compression/CheckUp/res20010301ct/2

Just a side note: I know you said to disable the fuel pump relay, so no gas is being squirted, but this article makes no mention about doing that step. This is why I was having some much difficulty trying to understand the proper procedure, with so many "different" ways that people had said how to perform a compression test.

[coralcoupe1993]

Just a side note: I know you said to disable the fuel pump relay, so no gas is being squirted, but this article makes no mention about doing that step. This is why I was having some much difficulty trying to understand the proper procedure, with so many "different" ways that people had said how to perform a compression test.

Its up to you. Unburned gas in your cylinders or no gas. Pick one. As for the article...um the car has carbs. No fuel will be injected unless you hit the gas. Just put the gauge in the spark plug hole and crank motor.....that is it.

Gas in cylinder = Don’t take 1 minute and disconnect fuel pump
No gas in cylinder = Take 1 minute disconnect fuel pump

That is the only difference!

:goodidea:

[wick]

Is this your first time working on a car? Its pretty much a straight forward task. The people on this forum have it nailed down where it needs to be. That guy you qouted, well, he needs slapped. Unless you are starting the engine, you dont need fuel. You dont start an engine during this procedure. Just pull the coil, pump relay, insert the compression tester into the plug hole and go. Then if you feel so inclined, do a "wet" test. This is when you do everything over again but with a few squirts of oil in each cylinder. Very simple, dont be scared.