RacingTiger04

only checked one cylinder cause thats all i had time for.. (my dad found his tester lyin around so i put it to use ) but how is 120 for an LT1 with 150k miles, front driver side cylinder was only one checked gonna check em all when i change the plugs and put headers on next week

Injuneer

Doing a check on a single cylinder isn't really very meaningful. The pressure will be dependent on too many factors.... how many cranks, was the cylinder wet, engine temperature, were all the plugs removed, were the throttle blades blocked open, how strong is the battery, etc.? Generally you test all the cylinders and look for consistancy, which is more important than the absolute pressure numbers. With a warm engine, plugs pulled, open throttle blades blades, dry cylinders and maybe 4 rotations, I would expect something in the range of 180-200psi.

RacingTiger04

O_O wow, well the engine was slightly warm, but the throttle blades were closed far as i know and only turned it over once. but whats the difference with having all the plugs removed?? (im really new to all this stuff so more detail would be nice lol) this was one of those that screws into the spark plug hole, maybe im missin somethin and theres different ways to check it *shrug* like i said i dont know much about engines aside from what i read on here occasionally.

cjmatt

you have to hold the throttle all the way open and pull the coil wire so that it doesnt start. when the throttle is closed, you have a vacuum on the intake side of the motor. let it turn over at least 4 times.

Injuneer

Removing the plugs takes all the compression load off the starter and lets the engine crank a lot faster.

Pulling the coil wire off still leaves the injectors dumping fuel into the cylinders. By the time you get to the last one, you'll get a huge pressure, because the cylinder will be filling up with fuel. Disable the injectors.

LiENUS

Isnt the best way to do a compression check on a lt1 to pull the pcm fuse?

jay_lt4

my stock lt1 with 50,000 miles had 185-195 psi on all cylinders, my 383 has 220-235 psi,

i would say 120 psi is rather low

RacingTiger04

so considering i didnt do any of that or know anything about it, that probly made the number a lot lower huh. oh well i know what to do now when i check all 8 just one question now.. whats the best way to disable the injectors?

1racerdude

Unplug the electric wires on each one. Not a big deal or disconnect the opti. The 8 on top would probably be easier for a first timer,ya can see what ya are doing.

LiENUS

Wouldn't pulling the pcm fuse on the drivers side of the dash between the door and the dash in that fuse box be a lot quicker/easier than unplugging each individual injector?

1racerdude

Probably so if he can locate it. Sounds like it is his first time.

RacingTiger04

yep first time with anything engine related, but ive been around the fuses quite a few times just foolin with the fog lights and stereo.. so if the pcm fuse is the easiest way then thats what ill do so then, hold throttle on the floor and turn it over 4 times for EACH cylinder? or 4 times then just to each cylinder.. well, thanks for the info everyone now im curious to see what its gonna read on all 8. so i should be lookin for around 180 under those conditions warmed up right?