leviticus88

I did a Dry Compression Test last night.
2006 LSJ
63,800 Miles
2.8 Pulley
Last Dynoed at only 230whp (2006 Dyno)

The car was at operating temps.

Results:
Cylinder 1=155
Cylinder 2=140
Cylinder 3=155
Cylinder 4=145

Note: I did forget to hold open the throttle body somehow and/or hold the throttle down when cranking the motor. Is there an easy way to do this with the Fly By Wire Throttle and how badly can this effect my compression numbers?

I was a little disappointed with the results, especially Cylinder 2.

Any thoughts on how these look? They all seem to be relatively even but also pretty low.
I was expecting 180-200.

scotch89

iTrader: (1)

i think there suppose to be around 150. how many times did u roll it over?

Bl4des

iTrader: (4)

when i did mine it was 200 across the the board but your numbers seem fine

elecblue06

iTrader: (1)

they should be within 5-10% of each-other or so if i remember correctly.. that 140 one is kinda low ..

buellfooll

You're good to go. As long as each cylinder is measured under the same conditions. You said a "dry" test. I'm assuming you mean cold. Let the engine run to normal operating temp and do your test again. The pressures should come up some and as long as they're within, like the op said, 10% you're good. There was a very recent thread about this subject that covered just about every base on the subject. Give it a quick search.

leviticus88

I ran the test at operating temps.
Thanks for the information about the recent thread! Appreciate it!

Bl4des

iTrader: (4)

how many times did you crank it and i might do it again by keep you foot on the gas when you crank it to keep the throttle body open

leviticus88

I cranked it about 5 times.
I wasn't sure if the throttle body would stay open or if the computer would close it at start up? Not used to fly by wire throttle.

Jazer

My LSJ read:

1= 200... 2= 205... 3= 195... 4= 200

on 10.5:1 compression.

rico

on stock compression i heard atleast 190 or higher

Jazer

What I heard, if I remember correctly.. ideal is 180-200 on a stock healthy LSJ.

rcorona8

Im new to this compression test, what does the compression test tell you? how much did you pay for the test, Ball park it. thanks.

Dainslaif

This is what I had heard as well. If you're at 140 across the board your compression is pretty low.

rico

^ thats what I was told too. so 180 and up to be safe

hatrickstu

iTrader: (1)

seems low. mine is 185-195 across

crazywhiteboy89

iTrader: (1)

^^same here^^

leviticus88

Thats what I was afraid of...

I will re-do the test pushing the throttle down as I crank. Hopefully this helps?

I just bought a 29$ cheap tester and it seemed to work ok.

Also, I'm at 6,000 feet above sea level, can this have some affect?

Either way, If the re-done test shows low numbers but across the board, what does that mean for the motor? Head gasket?

I am also considering doing a wet compression test and a leak down test.
Any pointers for doing these?

Thanks!

ryan0132

iTrader: (2)

different testers can give different readings, but as long as all your numbers are within 10% from each other you are usually ok

leviticus88

Thanks for the information guys!
I'm new to Compression tests and this has been really helpful!

Frew

iTrader: (9)

make sure you pull the plug for the wiring harness to the injectors

insylem

I always wondered something
I found in the manuals a conterdiction. How did you overcome it:

To do a compression test engine needs to be at opertaing tempature.
Then you need to remove a sparkplug...
but! Do NOT remove the spark plug while the engine is warm, doing so can crack the head.

So HOW in the name of Zues' butthole does one do a compression test?

Killa702

lol good question

ItalianJoe1

iTrader: (10)

It's the fact that you're at altitude. Less dense air in will give you less pressure when you compress it a given amount.

What? I've pulled plugs outta many motors that were warm, including my own cobalt a bunch of times.

ecot3c inside

iTrader: (11)

wait, why would you have to hold open the throttle? I've done plenty of compressions and never opened the throttle. Try it with no throttle and post those compressions. You can't be doing it right, somethings wrong...

ItalianJoe1

iTrader: (10)

You don't "have" to, but it's suggested. The idea is that you don't want any restriction to the airflow into the cyl. Although, as long as it's the same on all 4, you don't open the throttle on 3 of them only or something, it won't matter.

The actual number you get is pretty irrelevant, as long as they are reasonably close between all 4, no more than 10-15% variation from highest to lowest is considered normal. If you see one of them down 30% or so, you have a problem.