Compression Testing Method ! throttle open big difference ?

[GBC_069]

hey guys, well i did a compression test on a st185 gt4 this afternoon with 150K on the clock.

i got a reading of 150 odd across all 4. now i know that is a lil low . making me sceptical about this car and whether its getting a little tired.

HOWEVER i didnt do the test with the throttle open ! ? ( i forgot and didnt think it made much difference )

know what i want to know , is will doing it with the throttle open bring it up to ideal compression 170 odd ? !

i ll be going out to do it again properly i'd say, seeing i buggered this one up !

thoughts appreciated..................

[Dale]

Well thinking about it, it probably would make a fair difference... with the throttle closed there's a lot more restriction on air getting into the motor

[Vios-GT_07]

yea it does make a difference..

i did the compression test for my car, and the first cylinder i did it with throttle closed.. it sounded weird and took a long time for the reading to come up.. got 190.. i thought oh okay.. then yea remembered throttle open.. pulled it out and tried again, this time throttle open.. within 5 turns i got a max of 220 psi..

yea do it again properly with the throttle open and see what results u got

[oldeskewltoy]

150 ish for a turbo engine is pretty decent!

4AGE TVIS engines are new of about 170, with a minimum of 140..... Those have CR of 9.4 to 1. The ST185 3SGTE has a mid to high 8 for a CR??

[Vios-GT_07]

4AGE TVIS engines are new of about 170, with a minimum of 140..... Those have CR of 9.4 to 1.

manual lists them as 180 new and minimum 140..

now that i think about it, 150 does sound decent in a turbo motor..

[Jorrs]

Nick having the throttle open does make a difference, when i had the rolla tested the 1st time they did it was with it closed, and it read about 153 across four, then with it open i got bout 170 across four

[o_man_ra23]

Ill 5th? the notion that it makes a big difference. But... if the cams arent standard, or have been adjusted... then that can make a huge difference to your static compression reading. A really high compression engine with huge cams can give a reading of 120psi and still be quite healthy... itll just be doughy down low, and make all its power up high. The important thing to look for is that all readings are within a few PSI of each other. Remember that piston 4 will wear the quickest, so the amount of pressure that this is lower than the other 3 will give a good indication of the health of the motor. If they are all low, but even, it could just be a cam timing adjustment... drive it and see how it goes at high RPM if the readings are even but low.

All things being equal tho, I think the motor you are testing is probably stock and you need to just open the throttle.

Cheers, Owen

[GBC_069]

thanks for everyone so far.

yes the internals are still the same.

cheers again. keep the opinions coming !!

[GBC_069]

well again.
i just compression tested my celica 3s-ge.

doing it properly ( with the ignitor cable removed )
throttle closed
170 psi on all 4
throttle open
180 psi on all 4

making me think this gt4 may have a better compression, because the car kept just starting for a few turns yesterday , and giving us lower readings.

to try again i shall !!!

[Ribfeast]

Mine ranges from 171 to 177 on my 1JZ with throttle open. A mate's is in the 180's.
Opening the throttle made around 5-10psi of difference. If you are really picky you can disconnect the fuel pump so that fuel doesn't affect the readings.

[o_man_ra23]

The other thing to remember is run and warm up the engine first, as this will give the rings a correct seal.

EDIT: also remove all plugs so that only the cylinder being tested is providing compression, this will lessen the strain on the starter and the battery, and provide a quicker reading.

Cheers, Owen

[TooF]

also disconnect your coil(s) as they dont particulary like trying to spark but not being able to earth out the spark.

[mr2gze]

Mine ranges from 171 to 177 on my 1JZ with throttle open. A mate's is in the 180's.
Opening the throttle made around 5-10psi of difference. If you are really picky you can disconnect the fuel pump so that fuel doesn't affect the readings.

Your are suppose to disconnect all injectors including cold start. No need to disconnect fuel pump.

[Ribfeast]

That would work too, just 6 plugs instead of 1 relay though
I just start the car and pull the fuel pump relay, wait for it to stall, then do the comp test.

[Aust162]

Like O_man ra23 said, its most important that the readings are within a few psi of each other cylinder.

Every compression tester (brand/type etc) will vary. You could go and buy a new compression tester, or borrow your dad's 30 year old one for example, and test the same engine twice (one after the other), using the same method and get different readings..