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]

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!

[87notch]

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

[Rodeheaver's]

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

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,

[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

[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"

[Domination]

Here is an illistrated instruction....
http://www.automedia.com/Compression...es20010301ct/2

[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.

[Pure Stock]

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

[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

[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.