compression check result and a question or 2

mostsmooth

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Nov 12, 2002
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Hi all,

95 gt.
car hasnt been run in a week or so.
i pulled all the spark plugs, fuse for fuel pump, and disconnected the ignition coil.
then i did a compression check on each cylinder. if i understand things correctly, this is what would be called the 'dry' results.
#1 - 145
#2 - 155
#3 - 150
#4 - 145
#5 - 135
#6 - 150
#7 - 155
#8 - 155

i think there is some 10% rule to tell if things are ok or not, but i dont know 10% of what? the average? the spec? i dont know what the compression is supposed to be if so.
the average is 149. %10 would be 15. 149-15 = 134, so im ok but jsut barely? or do these numbers look fine? i realize theres a 135 in there that seems a little out of place.

the next thing i think i need to do is a wet test? or am i mistaken? i think the wet test is to squirt some oil in each cylinder and do them all again. im worried though that i can cause damage if i add too much oil. is there a particular method to setting up for the wet test? do i even need to do a wet test?

thank all
 
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7991LXnSHO

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A couple of squirts of oil from a flexible spout oil can will not hydrolock it. A couple of table spoons would be too much ant would still not hydrolock the engine.
 

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mostsmooth

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A couple of squirts of oil from a flexible spout oil can will not hydrolock it. A couple of table spoons would be too much ant would still not hydrolock the engine.
thanks
i picked up a can like that, but its a bit difficult getting it down to the plug holes. i may go get some extra tubing.

but, 150ish on all the cylinders, what should i expect to see with the rings lubed? 160? 180? is there a reason to even bother doing the wet? basically is it worth the risk i guess. i guess lubing and checking the 135 cylinder might tell me something.
truth be told, i dont know if i would be able to tell if i have it lubed enough. like, if i lube it and get 150, i wouldnt know if its lubed enough and i still got 150 or its not lubed enough. i dont want to keep adding oil waiting for it to move higher and then i end up with too much and damage things.

thanks again
 

7991LXnSHO

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Test them all to see how much the compression jumps. If your 135 goes to 180, that’s a strong sign for worn rings. If 2-3 squirts do not raise up the compression for the test, then worn valves are more likely. A cylinder leak down test is the next step. With a leaking cylinder, you can usually just listen and feel if the air is going out the intake, exhaust or into the crankcase.
Engines are made to stand up to compressing fuel and air and containing an explosion. This is not so fragile a little oil will hurt anything, besides eventually foul a spark plug.
A rough calculation estimate is it will take 2.3 fl Oz to completely fill the combustion chamber at TDC. (68.6 cc for a 9.1:1 compression ratio) It will take more than that to cause any damage. You are not trying to fill the combustion chamber with oil, just give it some extra oil to seal with.
 
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mostsmooth

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If your 135 goes to 180, that’s a good sign for worn rings. If 2-3 squirts do not raise up the compression for the test, then worn valves are more likely. A cylinder leak down test is the next step. With a leaking cylinder, you can usually just listen and feel if the air is going out the intake, exhaust or into the crankcase.
yes, the leak down, not sure yet if thats in my skill set, but i will look into it.

as far as the valves go, i had the heads cleaned up when i took the top end apart. springs, seals, etc. all new. probably way less than 2000 miles on it since then. i guess the guy could have done a bad job, but heads are is main service i think, so i give him the benefit of doubt. at a minimum he cleaned up the valves, but i seem to recall they were replaced and the angle checked or whatever its called. have to think about this a bit to see if i recall all that was done.

thanks
 

mostsmooth

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If your 135 goes to 180, that’s a good sign for worn rings. If 2-3 squirts do not raise up the compression for the test, then worn valves are more likely. A cylinder leak down test is the next step. With a leaking cylinder, you can usually just listen and feel if the air is going out the intake, exhaust or into the crankcase.
oh, and if the rings are 'bad', that's probably going to be something i will just have to live with because i don't think i will be sending it to a shop for a tear down (which is what would need to happen to change rings, right??)

thanks again
 

mostsmooth

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So what symptoms have you looking into this?
none really, just doing due diligence. learning stuff.
i took the top end apart a few years ago and put it back together, but never really drove it, now im just checking all kinds of things. was going to grease fittings today too, turns out there are no fittings to grease? or at least none that i saw. weird.
 

7991LXnSHO

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no real need to do a wet test.....
Why do you think that? If he’s checking and wants to know if the block is worn or if the heads are not really done, and he’s not familiar with a leak down test, what would you have him do? Can you teach him to mind meld with the car, or become an engine psychic? That would be cool.
 

HemiRick

I'd be looking at jacking under the house
Jun 28, 2020
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If the engine was warm the numbers might be a little higher. I think he doesn't need to do a wet test, because there's no problem....no need to see if its the valves or the rings.
 
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96pushrod

I think they're a little easier to get off
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If the engine runs good, I’d just take the results you got and be fine with it. I don’t really see any use for a wet test either honestly.
 
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MustangIIMatt

Easy there, this ain't a dating site.
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It's within spec, I mean, it's the low end of the spec, but that range exists for a reason (among them that there's no perfectly accurate gauge and variations in mass produced parts and all that).

Unless you're spraying it or boosting it, run 'er till she pukes, that'll take awhile.
 

nickyb

WAIT,you now have a pair?
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I always warmed up an engine and held the throttle wide open for compression test.