20 psi on 4th cylinder

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superjuice

superjuice

10 Year 15 Year
Joined
Apr 1, 2007
Threads
95
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926
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CT
I have owned a 60 for ten years and recently acquired a 40. after changing the fluids and driving about 80 miles around town i thought there might be a problem. for starters it is de-smoged and sports a stock aisiin carb, the only vacuume line runs to the timing advance diapram and the engine dies if it is removed even at idle. I just checked my compression and found all five cylinders read about 150psi...that is except #4 reads around 20psi...bummer. From what i know this could be due to a stuck valve or perhaps a blown ring. there is a hose running from the valve cover to the air intake which is constantly smoking and smells like fuel. i poured some marvel in and let it sit overnight but to no avail. Is it time to do a valve job? Has anyone seen this before? Any insight would be helpful...Thanks
 
I had a similar problem.. What I did after the compression check was make an adaptor from an old spark plug and an air line fitting.. I injected compressed air into the cylinder via the spark plug hole, ensured both valves were closed and listened for where the air exited from.. Poor mans leakdown test I guess.. With this test you can determine what the problem is.. If air comes out through the air cleaner, its an inlet valve.. If it comes out through the exhaust, its an exhaust valve.. If it comes out through the oil filler its likely rings.. If there are bubbles in the radiator, it could be bad head gasket or cracked head.. Looking at the symptoms, sounds like rings.. Either way youll need to tear it down to fix it..
 
The air hose

I think the air hose is just there to ventilate. Air is sucked up from the crankcase via the PVC valve and burnt but fresh air is vented off from the intake into the rocker area and then into the crankcase and it keeps cycling.

If the PVC is blocked then air pressure will back up through and go into the air cleaner area via this hose. Some cars have a little pad there that gets oil soaked when this happens.

Or maybe it could do it if excess pressure was in the case due to ring blow-by. In either case the leak down test is the way to go to pinpoint the cause.

With the compression that bad it should be obvious. You could pull the dipstick and see if there is air coming out---I once did a leakdown and blew the dipstick out of the motor I was pretty sure I had bad rings then!;)
 
superjuice said:
I have owned a 60 for ten years and recently acquired a 40. after changing the fluids and driving about 80 miles around town i thought there might be a problem. for starters it is de-smoged and sports a stock aisiin carb, the only vacuume line runs to the timing advance diapram and the engine dies if it is removed even at idle. I just checked my compression and found all five cylinders read about 150psi...that is except #4 reads around 20psi...bummer. From what i know this could be due to a stuck valve or perhaps a blown ring. there is a hose running from the valve cover to the air intake which is constantly smoking and smells like fuel. i poured some marvel in and let it sit overnight but to no avail. Is it time to do a valve job? Has anyone seen this before? Any insight would be helpful...Thanks
Click to expand...

Have you checked the valve adjustment?
 
no, i have not checked the valve adjustment yet. I will try the leakdown test on sunday, im at work right now. So i guess i'll just break an old plug apart and fit it with an air hose. That sounds like a great idea to see whats going on in there. Should the #4 cylinder be at top dead center when i to the test? Thanks for all the help guys, Its greatly appreciated.
 
TDC for a leak down to check valves or a hole in the piston.

If you want you can also check with the piston part way down the bore, good for checking wear but if you do that release the pressure, move the crank around, lock the motor in place somehow (in gear with a good hand brake perhaps) and reapply the pressure. 100psi takes a bit of holding!
 
So i got around to performing the "poor mans" leak down test. At first i could not find any air escaping while inducing about 100 psi to the cylinder. So i turned the engine over and tried again. Low and behold i could feel the air escaping from the dip stick tube. If i hold my finger over the tube it escapes from the valve cover. So i guess this means the rings in the #4 cylinder are shot. Is there anyhing else that could cause this?
 
I had 2 cracked pistons in a 2f once. A cracked piston or broken ring(s) makes more sense to me than one cyl that just wore out before the others...my .02
 
leakdown

Yep,


If air is coming out the dipstick it's a pretty sure bet you've got a broken ring

see if it comes out the pcv too

if air is coming out the valve cover then either a valve is leaking or you dont have it at TDC on the compression stroke with both valves closed--the air pressure can make it rotate off tdc try a lower pressure you should still see the leak

Sorry for your troubles
 
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I feel like i was just diagnosed with a heart condition, not immediatly gonna die but definatly need surgury. Thanks for the help doc, happen to know a good surgeon:mad: I guess i'll just drive my 60 untill I get up enough nerve to rip the whole thing apart.
 
I was talking to a gearhead at work and he offered that he once changed piston rings by dropping the oil pan and working from the bottom of the engine. is this possible in a 2f? could i actually just change the rings on the #4 cylinder without tearing the head off? maybe it's just wishful thinking but is it possible that it can be done?
 
Normally the crankshaft is in the way no matter how you rotate it, but I could be wrong on the f/2f. Also, you would need to hone it so new rings would seat. Pulling the head is actually fairly quick and easy and you will be working rightside up and be able to see the bore. Well, easy except for broken manifold studs and exhaust studs and those regular nightmares. You could do what my neighbor did back in the sixties to finish out his senior year driving to high school in a dodge flathead six. His dad told him to just pull the bad piston out and run on 5 cyls. He ran it another year like that.... But really, pull the head and fix it right. 150 lbs is excellent and you'll have all six like that.
 
piston change

I did a one cylinder piston swap on a 3.8 Rover in the UK I think that equates to a buik v-6.

I purchased the car extremely cheap to get by a few months till I left.

There is possible and there is right. The danger is that the ring wont seal from not cross hatching the cylinder. Even if it is a cracked piston then you'll need new rings and should (even if the bore was perfect) cross hatch the cyl for ring sealing.


If I didnt have the funds and just wanted a temp fix I would certainly try it but at that point I would start buying parts as I could afford them for a proper rebuild.
 
i bought the truck cheap from a buddy that was moving, and its has more rust than i can fix. CT dmv let me register it without inspection or emissions so i really just want to cruise it around town, maybe some light wheeling. If i wanted to restore it i wouldent think twice about doing it correctly. I just dont want to sink too much time and money into this particular rig. But well see, its hard not to fix broken stuff when you know you "can" do it. Thanks for all the posts guys, im still trying to decide. I could probably get another 2f from a friend for a couple of hundred. Not sure what shape its in though, its been sitting outside under a hard top for a few years. Ahh, decisions decisions.
 
superjuice said:
So i got around to performing the "poor mans" leak down test. At first i could not find any air escaping while inducing about 100 psi to the cylinder. So i turned the engine over and tried again. Low and behold i could feel the air escaping from the dip stick tube. If i hold my finger over the tube it escapes from the valve cover. So i guess this means the rings in the #4 cylinder are shot. Is there anyhing else that could cause this?
Click to expand...

Rings are never a perfect seal, there is always some blow by, so this test is always subjective. Did you test any of the other cylinders to compare the amount of air they pass?

bsmith123 said:
...
If air is coming out the dipstick it's a pretty sure bet you've got a broken ring
Click to expand...

It could be a broken or worn rings, hole/crack in a piston, blown head gasket between the cylinder and crankcase, etc.

bsmith123 said:
... if air is coming out the valve cover then either a valve is leaking or you dont have it at TDC on the compression stroke with both valves closed ...
Click to expand...

Nope, valve cover equals crankcase. If an intake valve is leaking, the air will escape out of the intake/carb, exhaust valve leak out the exhaust system. The big problem is hearing/detecting them, these systems have bends, mufflers, etc that mute the leak sound. On the intake the best bet is to remove the air cleaner, open the throttle plates and listen close.

Have you checked the valve adjustment? This can help in finding the problem, if a valve tight it will be held slightly open, causing the compression loss. If a valve adjustment is way loose, it probably stuck and maybe able to be worked loose. If a valve is way loose don't try to adjust it, attempt to work it loose first.

What is the history on the motor, has it been stored? What your describing is common on motors that have sat without running for a while. Some moisture gets into a cylinder, causes some surface rust on the rings and/or a valve stem, sticking them. Most of the time this can be freed up without taking the motor apart.
 
If you want to reduce your downtime, I have a cherry 2F head that will work for that truck. I had it magnafluxed several years back and it was clean (no cracks) I kept all the valves, springs, etc.....to make it easier for rebuilding. You can have it for my cost on the magnafluxing...which was $45 if I recall.

If you have any friends going to the Fall Gathering (Yankee Toys), I can probably get the head to you that way.

Just a option for you.
 
Quote:
Originally Posted by bsmith123
...
If air is coming out the dipstick it's a pretty sure bet you've got a broken ring

It could be a broken or worn rings, hole/crack in a piston, blown head gasket between the cylinder and crankcase, etc.


True, any of the causes here mean getting the head off and having a look
Quote:
Originally Posted by bsmith123
... if air is coming out the valve cover then either a valve is leaking or you dont have it at TDC on the compression stroke with both valves closed ...

Nope, valve cover equals crankcase. If an intake valve is leaking, the air will escape out of the intake/carb, exhaust valve leak out the exhaust system. The big problem is hearing/detecting them,

Good Catch! of course-- intake air-intake valve, Exhaust valve--exhaust, my bad on late night postings:rolleyes:


But put it at TDC when you do the check, put your thumb over the hole and you will feel compression on the power stroke, manually rotate it till you feel a "dead spot" between going up and going down--center it in the dead spot to do your checks

any evidence of oil contamination like whitish oil from water?
 
Just pull the head, that way you can see what is going on. If it is rings, mucho easier to do from the top, if it is valves, you need the head off anyway. Just don't break your back lifting the head off, get a buddy or a cherry picker.

GL

Ed
 
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