[dv657172]

Checking the compression on a 65 GTO 389 and the DS bank is #1 185, #3 185, #5 190

When I went to check cylinder #7 on the 6ht and final crank it almost sounded like the car tried to fire and the gauge read 210. All the plugs were out and coil grounded.

Tested cylinder #2 and same thing happened. Tried it again and stopped at 5th crank with a reading of 190.

Tried #4, #6, and #8 and got 0. Went back to #2 and also #1 and they were all zero.

Is the gauge shot? What did I do to it?

[Formulas]

take your dist. cap off crank it and see if rotor turns..

your description sounds like a timing chain jump then break

just a possible guess looking at 3" square screen

[Cammer-6]

check the Schrader valve in the end of gauge
they get stuck and reading stays at zero

[Schurkey]

When I bought my Snap-On compression tester a thousand years ago, they included a baggie of replacement Schrader valve cores. Common problem.

For the record, if you try to use a Schrader valve core scavenged from a tire valve stem in the cylinder-end of the hose, all your compression tests will show low pressure. Compression tester valve cores have very light springs so they don't restrict pressure into the gauge hose.

[dv657172]

Tried the gauge on another car, same result of 0 compression so timing chain not the issue.

Looks like the valve spring has worn out as it basically has no spring to it.
Are these valves replaceable and if so how do you size one?

Thanks for all the help!
DV

[Cammer-6]

contact the manufacturer of the unit to get a proper replacement.
Im betting they are sold on ebay as well.
As Schurkey stated tire valves wont work

[Schurkey]

Originally Posted by Cammer-6

contact the manufacturer of the unit to get a proper replacement.
Im betting they are sold on ebay as well.
As Schurkey stated tire valves wont work

Wild Guess: Any "Tool Truck"--Mac, Snap-On, Matco, Cornwell, etc. would have or be able to get something suitable.

The manufacturer of your tool may be the best/least expensive solution, though.

If this is a Hazzard Fraught tool-shaped object or similar bottom-feeder, might be better to buy a better-quality compression tester.

[gto4ben]

I bought this replacement valve core for my Compression Gauge.
https://miltonindustries.com/product...44e4d8fe&_ss=r

[dv657172]

Thanks but just bought a new one, with an extra valve, as the one I had was over 25 yrs old and no longer made.

DV

[steve25]

Good, at 25 years old that one you retired doesn’t owe you a dime!

[dv657172]

Ok so I bought a new compression guage. Warmed car up to 180 degrees, pulled all the plugs out and disconnected the ignition coil wire from distributor cap and grounded it to the ground strap on the firewall.

Just as in my previous experience, cylinder 1, 3, and 5 all tested fine at about 185-190 after 6 strokes. When I went to do cylinder 7 it sounded like it tried to fire on the 6th stroke and it broke the spring in the shraeder valve of the new compression tester. Thankfully I bought an extra valve.

What am I doing wrong? I have done numerous compression tests on both cold and hot engines but have never had one sound like it was trying to fire

Thanks and I really appreciate all the help.

DV

[Schurkey]

Breaking the valve on two different compression testers while testing the same cylinder indicates that cylinder has problems.

At best, drop an inspection camera into the plug hole and have a look around.

At worst, time to pull the head and see what's happening.

Any chance that cylinder has a fluid leak into it? Gasoline or coolant would be most-likely, and could raise the static compression ratio sky-high. Gasoline might "roll downhill" in the intake manifold so as to flow past the intake valve. I suppose oil could enter the chamber via broken rings or valve guide/valve seal issues, but seems less-likely to me.

I have trouble believing an engine that's been shut-down long enough to yank the plugs out and screw a compression tester into four cylinders could still have a hot-spot igniting fuel/air mix on the fourth cylinder tested.

Yup, you got a real-live mystery. Be sure to let us know what you find.

[steve25]

You must yank that head to find out what’s taking place.
The issue is that for some reason the compression is high enough in 7 to light off any fuel that gets in it and that takes out the valve in the gauge.

I bet when you pull the head that cylinder will be packed with carbon and oil residue.

[dv657172]

Schurkey I wouldnt think there is a fluid leak. Car seems to run fine, vacuum is steady at 15 inches. There is no smoke from either exhuast and I just changed the oil and it looked fine.

[Schurkey]

Originally Posted by Schurkey

At best, drop an inspection camera into the plug hole and have a look around.

At worst, time to pull the head and see what's happening...

...Yup, you got a real-live mystery. Be sure to let us know what you find.

Originally Posted by steve25

You must yank that head to find out what’s taking place...

...I bet when you pull the head that cylinder will be packed with carbon and oil residue.

Originally Posted by dv657172

Car seems to run fine, vacuum is steady at 15 inches. There is no smoke from either exhuast and I just changed the oil and it looked fine.

SOMETHING is going on, and you need to find out what.

[Formulas]

you said on 7th stroke it broke.. whats the reading before it breaks

say on 3rd or 4th stroke i usually stop at 5 as i usually dont get an increase after 5

[dv657172]

Hard to crank and watch but about 150 or 160 on 4th - 5th stroke, I would say. I have generally went to 6 strokes to make sure its close to max and never had an issue in the past. When it did try to start the guage would hit like 210 and then when I tried it again, the spring broke. 1st guage and 2nd guage. I guess I am a slow learner.

[dv657172]

Redid the compression test on the engine cold in order to keep it from attempting to fire and was able to do so without issue. Still not sure why it tried to fire, so I have that yet to determine.

What would you expect difference to be between a cold engine compression test vs. a hot engine compression test?

Cold engine vs. (Hot Engine) PSI

#1 210 vs. (190) #2 215 vs. n/a
#3 210 vs. (190) #4 215 vs. n/a
#5 205 vs. (195) #6 215 vs. n/a
#7 215 vs. n/a #8 210 vs. n/a

DV

[steve25]

Even with 94 octane pump fuel in the tank, if you have more then 195 psi ( which you do for whatever reason) your in a danger zone for pre ignition and knock if your tuning abilities are not top notch!

As I post this, you ( the OP) have never mentioned what octane your running into your carb.

[dv657172]

Understood, the engine was cold when it produced the high compression figures of over 200, when the engine was hot the readings were all lower at 195 and below.

Compression ratio is 10:1 on the car, initial timing is 11 degrees and total is 32 degrees. I honestly am not happy with my vacuum advance as that appears to be able to create 19 degrees of advance. I haven't noticed any knock although admittedly the car hasnt been out much in quite awhile.

If the engine does have some carbon buildup whats the best way to clean it up without tearing it down?