SILO52

I have a snap-on compression set so I wanted to check my engine just as a baseline and to see if I might have any hidden problems.

Basically I need to know what needs to be done before doing the test. I know the fuel pump fuse needs to be pulled but which one is it? Do I need to disconnect the injectors harnesses? I think I read on this forum to run the car with the fuel pump fuse disconnected to get all the fuel out of the injectors to get a more accurate reading.

Is there anything else I'm missing?

I don't know what to do or don't do....I know someone has all the right answers.

P.S. -- I've been using the search but kept on pulling up people's results from their compression tests and no directions.

pushinlsteg

pull fuel pump and ignition fuses.

pull all plugs out first

put a touch of oil around the o-ring on the part that is going where the spark plug hole is. DO NOT TIGHTEN IT VERY TIGHT!

crank her over 6-10 times, with the gas pedal to the floor, some people say only 4 some say only 5.

I turn it over until it reads no higher.

then hope for better results than me.

oh yeah, do it on a warm engine.

RWD_RCKT

50 people are going to tell you 50 different ways to do it.

Quick and dirty:

Engine off but AT OPERATING TEMPERATURE
Pull the fuel pump fuse (don't know which one it is off hand... sorry)
Don't mess with the injector harness
Remove the spark plug and plug in the gauge of whatever cylinder you want.
Press gas pedal to floor
Crank the engine until the gauge doesn't raise anymore (I would say 5 seconds max)

If your engine starts... you pulled the wrong fuse and you should obviously take your foot off of the gas. When your done, put the spark plug back in and repeat for the other cylinders.

1.)
Some people say to remove all of the plugs and test that way... it is easier on the motor... but it is not that tough in the first place so I wouldn't be too worried.

2.)
I see it is possible that fuel could mess up a reading, but I have never worried about it. I tested mine the way I described and I got 220psi across the board.

3.)
The key with this test is more consistency than anything. You want all of the numbers to be close to the same or within 10 psi of each other... and hopefully above 190psi.

4.)
The real test that you should focus on the numbers for is a leakdown test. But that is whole 'nother story.

SILO52

Thanks so far guys! Anyone know the exact fuse(s) to pull???

folex187

I've got a question. I had a brain fart last time I did this on my car and didn't put the pedal to the floor. I got 210, 210, 211, 210. When I did it last summer with the pedal to the floor I got 220 +-2. Did my not opening the throttle account for the lower compression?

RWD_RCKT

jasonoff's post near the bottom...
CHECK IT...

RWD_RCKT

It's possible. If the throttle is closed, it will take longer to fill the cylinder. If you didn't wait for the gauge to max, that would happen.

But regardless, 210+-1, 220+_2... your a-okay.

GrimReaper

Modifry had a nice DIY for compression testing in the past, but I can't seem to find the link. If you pm me your email address I can send it to you.

-sam

SILO52

PM sent

f20kills

Helms says 28psi within cylinders.
I got 205 215 225 210 (4-1)
Wet I got 245 265 265 270 (4-1)

Car still runs strong, but I plan on getting a dyno done this spring to see what power I am putting down.