Can anyone give me a basic how-to for a Compression Test?

[Erik]

I am going to do a compression test amongst other things this weekend (what a way to celebrate my birfday huh, fixing my sick, most prized posession...) Anyway if any of you smart folks know how to perform a compression test I'd really appreciate hearing the skinny on how it is done. I know that I need to head down to autozone and buy the compression test "meter" or whatever it is. But beyond that I don't really know.

Also if anyone knows what the normal range on an 80's PCM Ford 351 (right-hand) that would also greatly be appreciated.

Thanks!
Erik

[ohsix]

You screw the compression tester (basically a pressure guage) into the spark plug hole and turn the motor over about 8 times. Look at the pressure on the tester and make a note of it. I'm not sure what the compression should be on that exact motor, but I would guess around 100 psi. Be sure and disconnect the coil wire from the distributor so the motor doesn't fire up.

All of the cylinders should be pretty close in compression, if not you've got problems.

[Erik]

Ok Wes, so one by one I go and remove the spark plugs and run the compression test... I was thinking I take them all off and then go one by one on each cylinder with all of the sparl plugs removed. I was thinking wrong - you are saying I leave the other spark plugs in until I get to "that" plug, then I continue... correct?

[chavez]

Erik Jernberg, correct.

Just make sure to follow that last instruction and remove the coil wire!

[MrBlean]

Erik - a wise and inexpensive precaution. You'll probably find instructions on the packaging for the tester. As Wes says, it's just a pressure guage mounted on the end of a rubber hose with a plug adaptor fitted to screw into the spark plug hole.

There is a technique involving testing the pressure then squiring a few drops of oil in the cylinder and testing again. Only do that if you are concerned about the readings first time around. The idea is that the oil helps "seal" the rings to the bores. If you get a better reading second time around then it's rings/bores at fault not valves/valve seats in terms of where the compression leaks away.

I seem to recall that the rule of thumb is that the cylinders should match within 10%. Rather than rely on my sketchy memory, take a look at:

http://www.georgiazclub.com/tips/ztech/comptest.htm

and also:

http://www.cruzers.com/~twakeman/LR/FAQ_compression.htm

Good luck. Let us know what you find.

[joedirt00]

Be absolutely sure to disconect the main wire from the coil to the distributor. Otherwise the engine WILL fire on 7 cylinders. I've seen a guy forget to do that. Since he didn't get hurt (luckily), it was pretty funny. He about wet his pants.

[Erik]

Thanks Wes, Chavez JoeDirt and Mr Blean.
Very much.

Erik

[tommyadrian5]

if you get frustrated you can usually get a local mechanic to do it real cheap

[SILLY]

im a marine engineer by trade and all i have to add is your cylinders should be with in 15 % of each other.

[SILLY]

the compresion on that motor should be between 120 and 150 psi(g) depending on ring wear and atmoshperic condition

[Steelerguy]

Well we did ours a little different.

1. DISCONNECT the DISTIBUTER WIRE !!!!
2. We took all plugs out.
3. Tested all eight cylinders. Nothing Lower than 86 or higer than 95.
4. Plugs back in
5. check the timing.

Out the door not one problem (knocking on wood) The PSI is low but since all cylinders are close she runs just fine. I have a newer block we will be starting on this winter.

\

[joedirt00]

If you take all the plugs out at once, be sure to mark the wires. That is unless you know the firing order of a Ford. Ican't remember what it is but I can Tell you a SB Chevy in my sleep.

[ohsix]

1-8-4-3-6-5-7-2 is the small block Chevy. I've worked on one or two of those.

I've never done, nor seen a compression test done with all the plugs removed. I would think your pressures would be a little different (lower) because the rest of the crankcase wouldn't be under as much pressure with all the other cylinders venting straight into the atmosphere. Therefore you would lose compression from the cylinder you are testing to the crankcase much quicker.

[Erik]

Thanks for the info guys. I will play it 100% by the book. I have a lot to test out this weekend but I will report back with the findings.

I don't want to just hit the water and go balls out right away. I am too much of a worry-wort for that

[tommyadrian5]

Wes, I think the fact that the valves are closed on the cylinder when it should be "firing" allows the pressure to be read for that cylinder alone. I'm no expert though, but i do know that you do it one cylinder at a time (like women, unless you are extra special).

Erik, if you want, I have a buddy in Brighton, MA who is an engine wiz and an up and coming wakeboarder, he would be happy to help you out if you want it. He's also looking to get an old 2001 so i'm sure he'd be happy to help with anything to learn more about the boat. PM me if you want his #.

[D-GOOSE]

Hi, Sorry to interject this but you do want all the plugs out (yes coil wire off). Why you ask! because of the fact that you over heated the eng and if you blow a head gasket you will still have a ok reading on the gauge if the plug on the other side of the blowen gasket is pluged in. Air will be traped in the other cyl and transfer to the testing cyl. I wish u were around the courner from me because I would do it for free. Or just a ride after finishing the job. Sally is wright after testing all cyl there should only be a 15% difference from the high to the low. p.s. have a battery charger on so the cranking speed do's not slow down as the battery will be drained some during the test. MrBlean
web sites have great info to help.

[twitch88]

definitely all the plugs come out and while cranking the throttle should be wide open.

[tommyadrian5]

i stand corrected, all plugs out

[Wakebrad]

How much is a compression tester?

[twitch88]

not sure exactly but the autozone here has a loan tool program that you could look into. Maybe they do the same thing in dallas?

[wesgardner]

Hey Wakebrad,

Compression gauges seem to run about $40.

Wes

[Erik]

[Wakebrad]

Erik Jernberg, if you're looking to buy one they have some on ebay for like $5. I just looked it up. I'm going to go to autozone to try that borrow a tool thing first.

Yeah there are some $200 ones. The diesels are expensive too.

[rxdog]

All plugs should be IN for this test. (It effects cranking speed).

All cylinders within 15% or less.

High energy igintions only need to be grounded, not just disconnected.

Thats about it.

[ohsix]

[D-GOOSE]

OK. so tell me this. HOW MANY OF YOU TURN A WRENCH FOR A LIVING OR HAVE BUILT A MOTOR. ERIK there is some great info, but there is a ton of crap on here. IMHO

[D-GOOSE]

SO Wes how do you tell if you have a blowen head gasket?? yes I know about a leak down test already.

[Erik]

[aceyx]

of everything i've read/heard/done, plugs out, throttle open, ground the ignition. i was a little hesitant thought since all this is automotive knowledge, and there could be some variation with marine engines. until i found this (looks authoritative enough for me):

http://www.boatfix.com/merc/Bullet/97/97_25.pdf

[D-GOOSE]

[D-GOOSE]

Yes I am refering to i/o's
I have never worked on outboards.

[wesgardner]

Hey gbotech,

Rest assured, boat engines are just car engines that have been "marinized"...go look on the block it'll be stamped "GM" (for chevy based engines). There are some marine components added such as starters and of course the flame arrestor on the carb. (if you have one)...some engines may have a SLIGHTLY different cam installed with a SLIGHTLY larger lobe to allow for the sustained higher reving boat engines are subjected to. Oh yeah, obviously the exhaust system is "wet" but the same back pressure deal holds true (there should be SOME but not too much) but all in all, it's a big hunk of American automotive metal.

Read the second link that Jeff (Mr. Blean) provided...fairly good stuff and agrees w/ the stuff from MerCruiser.

I've not turned a wrench for a living for years but did race bracket cars in the '70's and built up several motors....of course when I look under te hood of my wife's car now, there's this shroud covering everything except for the dipstick - I take that to mean "no user serviceable parts inside"

Wes

[D-GOOSE]

wes
thats the same info that I have heard about the boat motors cam and crank may be different do to high rpms. By the By were is volvo made. I still like the GM power plants.
wes Try to do a tune up on a 04 expedition. the fuel rail needs to come off and the book time for just changing plugs are 3 hours. Yes it take that long. unless you have done a few. Know you now y it cost $350 for a tune up. what a rip off.

[wesgardner]

Hey gbotech,

Gotcha...some of the new "engineering" appears questionable at first as you've experienced with the Exped. and what was that Ford you needed to haul the motor up to change the plugs....on the other hand, I recall when cars didn't even have the sixth digit on the odometer and when/if you reached 100,000, you were done...that said tune ups were done by ear in 10 minutes...go figure


Wes

[ohsix]

Edit: Post deleted.