Compression Test for Chuff

[Zaggnutt]

I recently purchased a compression test kit from ebay... 74.99 says "Stark" on the label. I like the kit. Fittings for several diesel engines, seems solidly built and the fittings are well sealed. 1000 psi gauge, etc. Unfortunately the only fitting that works is for the glow plug hole so I am going to return it and just buy the gauge and a fitting that fits a gp hole....

So I picked up another truck recently (check sig for details) and found out on the drive home that it has the only buzz word worse than the dreaded "cavitation"---- It has the "chuff".......

I wanted to compression test it for my own information and also because I thought the bad valve would identify itself with this test. I tested on hot engine with glow plugs removed and I tested for first crank pressure (Riot method) and overall (6 cranks). Did passenger side first, starting with #1. Each cylinder was tested a minimum of (2) times to get a repeated reading.

1. 200, 500
2. 200, 400
3. 200, 400
4. 200, 420
5. 200, 450
6. 280, 450
7. 300, 500
8. 200. 350

There was one oddity... When I did #2 it read "0" on first crank 3 times. I wiggled the fitting and it seemed fine, but then it pressurized. My question is this.... I thought that I would find a cylinder that would not hold pressure that would clearly identify the bad valve.... what am I misunderstanding? Is the low reading on 8 an indication even though it held? I have used heads coming today to replace existing so I will get to do an inspection, but please educate me on why this didn't yield a leaky valve, advice, tips, etc. Thanks!

 

[IDIoit]

a compression gauge isnt really the best way to tell whats going on with the engine.
theres 6 ways for the air to escape,
1, through the rings.
2, intake valve,
3, exhaust valve
4, headgasket
5, cavitation
6 a hole in the piston.

a compression gauge only reads how much compression it will pump.
a low reading will tell you that air is escaping, but it wont tell you where.
every cylinder has some amount of escaping air.
usually down the gaps of the rings.

the best way to determine if you have something going wrong is a leak down tester.
this is where you put the cylinder at BDC, and apply air pressure to it.
if the air stays consistant, then youre good.
if it leaks, you can listen through the exhaust, intake, radiator, and crank case.
and pinpoint your problem, now with cavitation and a headgasket, these could get mixed up because most of the time it will leak in the cooling passages.
sometimes....

http://www.ebay.com/itm/TU-21-CYLIN...1233601965&pt=Motors_Automotive_Tools&vxp=mtr

you may want to hold off and keep the gauge for the fittings.

 

[Zaggnutt]

ahh poop. 1 step forward, 2 steps back... Hey, look! I'm dancin'! Well the chuff is certainly there and I figured I would be able to see what is happening when I pull the heads. Good advice though. Thank you!

 

[BDCarrillo]

Pull the valve covers and use a screw type spring compressor to take some pressure off the valve, and wiggle it around. You'll know the chuffer when you find it.

Doing that combined with leakdown will nail it down 100%.

Easy way to do leakdown without rotating the engine is to pull the rocker arms.

 

[IDIoit]

Pull the valve covers and use a screw type spring compressor to take some pressure off the valve, and wiggle it around. You'll know the chuffer when you find it.

Doing that combined with leakdown will nail it down 100%.

Easy way to do leakdown without rotating the engine is to pull the rocker arms.

the engine will rotate some when you add PSI..lol
if you go in the direction of the firing order, you wont even have to rotate it .

id rather spin the crank close and let the air do the work for me...
easier than undoing 10 bolts and 16 nuts!

 

[riotwarrior]

As has been said, the comp test doesn't really give us a true reading of a chuffing engine.

I don't know for certain the reason of this, however I've been around enough to maybe have an idea. It is based on cylinder pressure and harmonics of a running engine vs a static tested engine.

Essentially that chuff shows it self when cylinder pressures in a running engine are extmemely high during combustion cycle, vs just turning over engine with no GP's doing a static compression test.

A running engine has harmonics throughout it and valve train stress is increased, and what that chuff is is the valve wobbling and not sealing due to how those harmonics affect it's action under the stress of running load vs test load.

The Valve in question would have a much easier time of HOLDING some pressure when cylinder pressure is only a few hundred pounds per square inch. If you look at my post mortem video you can see what I'm talking about. Ya my valve broke, but regardless it still held pressure to some degree prior to that KaBoOm...

Yes a leak down is a great idea, however it won't necessarily reveal the answer to the bad guide either if the valve sits ok against seat. It's when that valve is wobbling around in the guide during it's open and close cycle that makes it seat poorly on a running engine.

Best method is to attempt to gently press against valve tips and see how far they move in the head it self. This can be very difficult to do on an engine in a truck. However a bad guide will show up with this test.

I do hope that MY THEORY has helped you understand this further...though I could be wrong....

JM2CW

Oh and check those used heads too BTW hint hint nudge nudge wink wink

 

[IDIoit]

^^ agreed on the running harmonic engine.

you gotta take in account, air pressure like electricity, takes the path of the least resistance.
when doing a comp test, that path is usually the gauge.
if a hole has been cavi'd or even if a seat does not seat, it will still build some kind of compression before it leaks out, and thats all going right to the gauge.

the leak down pressurizes the hole and you can determine where the air is going.
granted this wont be the same readings as a running engine, but have you ever tried to put your finger down a glow plug hole while the engine is running?

 

[Black dawg]

the way that I have found which hole the valve is leaking is, to pull one glow plug at a time, and then start the engine.

When you get to the problem hole, either the "chuff" with be gone (compression going out glow plug hole instead), or you can hear that the timing of the chuff and the air from the glowplug hole will match.
kinda hack, but works.

 

[icanfixall]

If you listen to the exhaust and hear a chuff noise then its an exhaust valve but which one??/ Same with the intake noise. Best to pull the valve covers and check the valves or look for Soot and ruined valve seals.

 

[typ4]

Ok, the reason it holds on cranking compression is because the added push of the lifter oil pressure is not on the offending valve.
It may just need a spring but usually the guides are worn and it heats the spring and it breaks,OR, the worn guide will let the valve hang on the edge of the seat and the combustion pressure makes the chuff.
I would never put on used heads without a grind at minimum, the nice ones I pulled off runners were hammered so bad on exhaust side the valves were not refaceable. Intakes were ok except for the 2 that were sunk due to seat wear, but the engine ran well . SOOOO , Jm2cw.

 

[Zaggnutt]

Thanks guys, I appreciate the valuable info. Makes very good sense and I am slowly going deeper into engine mechanics and my understanding is slowly going with it IDIoit, Riot, Ican, BDawg, and Carillo I appreciate your help very much. Typ4, you are right and I agree with you that they should be gone over by a machine shop and I came to that conclusion today after thinking about what I needed tool wise, etc. I dropped the used heads at a local machine shop referred to me by Penn Diesel to be cleaned and spec'd to determine any further work. $80/head. Turns out the shop is owned by the grandfather of a childhood friend and I got to catch up a little and I walked away feeling much better about my path. No matter what the outcome it was the right move to let the pros tackle this one so I can keep workin' on the oil cooler, dropping the pan, etc on Frank the 4 door.

I'll update on the head project here and the possible spun bearing/diesel in oil/knock project on Frankenstein in the "Coolant in Oil" thread. Thanks guys!
Also want to say thanks to Farmer Frank for the used heads. One way or another that chuff is gettin' gone! Thanks Frank!

 

[riotwarrior]

Ok, the reason it holds on cranking compression is because the added push of the lifter oil pressure is not on the offending valve.

On this one Russ, I openly will disagree with you, as the oil pump if true, would hold ALL valves open as you describe while the engine ran.

I would be happy to hear more on this from you and learn about what you may have for new info.

 

[Zaggnutt]

Yes a leak down is a great idea, however it won't necessarily reveal the answer to the bad guide either if the valve sits ok against seat. It's when that valve is wobbling around in the guide during it's open and close cycle that makes it seat poorly on a running engine.

Best method is to attempt to gently press against valve tips and see how far they move in the head it self. This can be very difficult to do on an engine in a truck. However a bad guide will show up with this test.

I do hope that MY THEORY has helped you understand this further...though I could be wrong....

JM2CW

Oh and check those used heads too BTW hint hint nudge nudge wink wink

Yeah, agreed on checking the heads. Also, I have watched your video on compression testing and the video of the inspection of the heads after the valve issue. I'm curious, what exactly happened to the truck on your way to wherever you were going that you tore it down? Was there a sound? did it run different? You already had the chuff, but what is the "Kaboom" you are describing?

 

[riotwarrior]

I was a little over a mile from home when I heard KaBoOm in a bad way...and lots of smoke and knocking from engine. I pulled over and let it run, I know better n to shut **** down...won't start when away from home = TOW BILL

Here is just after that occurred and on way home...
http://youtu.be/Fx2sHvW75OU

And here is in driveway...

http://youtu.be/JXG2z8oVIcE

Hope that helps....

 

[BDCarrillo]

On this one Russ, I openly will disagree with you, as the oil pump if true, would hold ALL valves open as you describe while the engine ran.

I would be happy to hear more on this from you and learn about what you may have for new info.

Oil pressurized lifters take up slack and exert a small amount of force on the spring, decreasing seat pressure. On an engine that's sat, the lifters can drain down just a hair (to zero pressure) and let off the spring. That small change in seat pressure may mask a bad/worn/gunked valve when static testing the engine. Little spurts of starting won't maintain the pressure in the lifter galley.

It's one of those small factors that won't matter on a totally trashed valve, but may hide one that's got the start of an issue.