Compression Test

[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.