Diy Compression Testing Qn

[errno]

Just want to confirm something about doing a compression test - havent gotten an answer in the ancient DIY thread i bumped in the tutorial section (i guess under general maintenence is a bit more browsed than there) so hopefully I can get a faster answer here as i plan to do this tomorrow.

Basically just want to double check something with the procedure. One user suggested that unplugging the crank angle sensor will prevent spark AND fuel injection - is that right?

So would i be right in assuming I could

- disconnect CAS + coilpack loom (which will be out because the spark plugs are out anyway i guess)

- depress accelerator fully for WOT

- turn key as if i was starting the car until gauge needle stops moving

- repeat with small amount of oil for a wet test

Or will I have to unplug other things/remove fuses etc?

[StealthX]

just start up your car with the fuel pump fuse disconnected, then wait for it to stall.

crank it for a few seconds after that.

now remove your coils and spark plugs.

stick your tester in cyl 1 and crank for a few seconds under full throttle and get the reading.

repeat for cyls 2-6...

[errno]

just start up your car with the fuel pump fuse disconnected, then wait for it to stall.

crank it for a few seconds after that.

now remove your coils and spark plugs.

stick your tester in cyl 1 and crank for a few seconds under full throttle and get the reading.

repeat for cyls 2-6...

ok sweet, fuel pump fuse is the top fuse in the boot of an r33 right

err 5th one i mean

Edited March 26, 2010 by errno

[Duncan]

and if you are genuinely worried....just save yourself time and start at cylinder 6 not 1....

[errno]

and if you are genuinely worried....just save yourself time and start at cylinder 6 not 1....

not really worried, just changing the spark plugs so i figured id do this at the same time and make sure everything is actually sweet before i start spending money on other mods, then only to find out something is going to go wrong

Cyl 1 is the front of the engine where the radiator etc is right?

[andy95'R33GTR]

when compresstiontest all the cylinder are open? spark plugs removed? right? i thot you need only to remove the sparkplug hole you wnt to test

[Duncan]

yeah cylinder 1 is at the front, and cylinder 6 is where at least 50% of failures happen if you suspect a problem.

it doesn't matter if the other plugs are in or out (there is no connection between individual cylinders except the crank shaft) but with them out watch out for the 150psi air shooting out the spark plug hole!

[mad082]

yeah cylinder 1 is at the front, and cylinder 6 is where at least 50% of failures happen if you suspect a problem.

it doesn't matter if the other plugs are in or out (there is no connection between individual cylinders except the crank shaft) but with them out watch out for the 150psi air shooting out the spark plug hole!

the air coming out of the hole won't be at 150psi. that is just the pressure that will build up if there is something blocking it.

the reason for removing all the spark plugs when doing a compression test is so you get an accurate measure of each cylinder individually. if you have spark plugs in the other cylinders then they will be building pressure on their compression stroke which may lower the reading you get because the engine will be turning over somewhat slower.

[andy95'R33GTR]

the air coming out of the hole won't be at 150psi. that is just the pressure that will build up if there is something blocking it.

the reason for removing all the spark plugs when doing a compression test is so you get an accurate measure of each cylinder individually. if you have spark plugs in the other cylinders then they will be building pressure on their compression stroke which may lower the reading you get because the engine will be turning over somewhat slower.

ok got it =)thanx

[MrStabby]

Dont forget the engine should be warmed up.

[errno]

sweet thanks for the help guys

got it done, decided to do 2 dry tests (just incase i did something wrong on the first one because ive never done it before) and a wet test

on first test got

1:174 2:172 3:171 4:171 5:171 6:171

second time got

1:172 2:172 3:170 4:170 5:169 6:168

tried to to a wet test, put a tiny pit of engine oil into cyl 1, put in the tester and turned it over but the guage started to read up to like 200 psi so i crapped myself and stopped - then tried to crank it again and it wouldnt crank over at all only the fan would start up, nfi what happened so i just threw in the new plugs put it all together.

it started up ok after this but it did blow white smoke out the exhaust for a while, hopefully it wont blow smoke next time :S

[mad082]

the smoke would probably be the oil you put into number 1 cylinder.