EMan 928

Could someone tell me what I should expect for compression test values for a 1993 GTS with 70k miles? Doing a test as I write this at an Indy shop and coming up with the following values for 1-4:

#1 160/178
#2 142/160
#3 160/180
#4 150/162

Note that my battery is fairly weak and we're not getting as much ooomph when we crank it over, which may affect the numbers a little bit.

Thanks,

EMan 928

Values for 5-8:

#5 130/140
#6 110/120
#7 160/180
#8 150/170

Kevin in Atlanta

Try again with fresh battery.

Herman K

That looks like you have way to much variance should be around 180 +/- 10% all around.

Did you use a digital gauge (way more accurate and they keep each cylinder in memory) http://www.ebay.com/itm/Mityvac-MV55...c858ff&vxp=mtr

davek9

Just got off the phone w/ Erick, the engine was also stone cold (should be at least warm).
I suggested he also tries a leak-down test, but I'm leaning toward this is carbon on the valve seats.

Dave

oups59

If I remember correctly, mine was between 190-205

depami

Cold shouldn't matter if he does it wet.

But you really want comparison between dry and wet so warm is definitely preferred.

Is that what the 1st and 2nd #s are, dry/wet?

Also, you should have at least five revs for the reading to "pump up" so cranking power could be an issue.

It helps if you pull all the plugs so you’re only cranking against compression of one cylinder.

EMan 928

Thanks, Depami.

For the first and second numbers, indy mechanic cranked 3 times as the low, took the reading, then 3 more times and took that final reading as the high.

Engine wasn't run in 24 hours and the shop was 60 degrees room temp. And he had the 4 spark plugs removed on one bank when he performed the test (other bank still had the spark plugs installed)

GlenL

The important part is to be consistent with each cylinder and then look for consistency across the holes. Remove all the plugs to start with so it spins more quickly. Remove the relay for the fuel pump. Count 5 or 6 cycles with the key on and throttle wide open and then read the value quickly. Re-testing low holes is a good idea as the adapter may not seal well.

Absolute numbers vary with the pre-conditioning, gauge, battery and the technique. Do the first hole over again to see if the battery loss was making the numbers trend down.

John Speake

Was the throttle wide open for the tests ?

depami

As Glen and others have pointed out, consistency is more important than the numbers themselves.
Generally you don’t want more than 10% variation between highest and lowest cylinders. I’m not understanding the before/after meanings but compared to the others, #6 looks to be too low for sure. 160 – 10% = 142. I would test again ensuring 5 plus revs for the reading then do it again with an ounce or two off heavy oil added. If highest and lowest are within 10% of each other when tested wet, I’d think you should be good.

The WSM says 8 bar or more new, 6.5 bar wear limit. That is 116 PSI or more new, 94 PSI wear limit.

And as John points out, the throttle should be wide open during test.

Imo000

+1^^^^ and a fully charged battery. Also, remove all the spark plugs. I think it should be done with a cold engine but a leakdown test would be much better.

Hilton

Also it needs to crank over around 7 times per cylinder to for the tester needle to stabilise in my experience.

I have two testers - one gives 170-180 and the other gives 190-200 results, on the same car. So as noted above, worry more about variation between cylinders than absolute numbers.

The absolute pressure can be affected by pinholes, cracked o-rings, loose seals etc. on the tester - its not like they're highly-calibrated tools

EMan 928

Thanks for the feedback everyone. No, I don't believe the throttle was wide open for the tests. I may end up buying my own compression tester and do it myself next time. Right now I think I'm going to put some sea foam in the fuel and see if I can burn a little carbon...there was some present when we scoped it. Then, a leak down test.

depami

What is the issue that led you to a compression test?