Low cylinder presure

[jamesr1]

My Mercrusier 140 has one cylinder that registers 25 pounds. The other cylinders register 135-145. Any help would be greatly appreciated.

 

[jamesr1]

Re: Low cylinder presure

Would a cracked head gasket create such a variance in presure?

 

[Don S]

Re: Low cylinder presure

Could be a gasket, a valve, or a piston with a hole in it, or stuck rings because of water getting into one cylinder, (especially #4)<br />You need to do a cylinder leak down test to locate exactly where the problem is...... or pull the head off and look.

 

[rodbolt]

Re: Low cylinder presure

at 25 psi I would not waste time on any futher testing. its time to remove the head and inspect for damage then find the cause and correct it. usually its caused by a bad water pump or a leaking manifold/riser.

 

[jamesr1]

Re: Low cylinder presure

OK, I removed the head. It appears as though I had a stuck valve. One of the valves on that cylinder is chewed up and actually sets farther up into the head than all of the rest. The top of the piston is scared for life, but does not appear to be cracked. I will be taking the head in and having it checked. If it is OK, I guess I will have a valve job done, and have the head cleaned. <br /><br /> Anything else I should be concerned with at this point?

 

[trog100]

Re: Low cylinder presure

yes.. why the valve stuck down in the first place..<br /><br />rodbolt has mentioned some possibilities..<br /><br />trog100

 

[KaGee]

Re: Low cylinder presure

I would also be concerned about that piston.

 

[waterone1@aol.com]

Re: Low cylinder presure

I would suggest a complete rebuild. Deep scratches in a piston can create hot spots. Depending on what the valve looks like, it could have been damaged by a cylinder full of water. If that is the case, there could also be some damage to the connecting rod.

 

[jamesr1]

Re: Low cylinder presure

OK, the head checked out, they replaced the intake and exhaust valves, as well as seats for the cylinder that had low pressure. At this point I want to put it back together and re-check my cylinder pressure. My repair manual has got me a little confused. <br /> Are the torque specs the same for the 4cyl and 6cyl. My manual only shows one set of torqe specs for both 4 and 6cyl. Head bolts are 93ft lb, maifold to head at 23, and and does not specify torqe on the the water seperation tube.

 

[rodbolt]

Re: Low cylinder presure

head and manifold torques look ok. the engines are of the same design family. whats a water seperation tube ?

 

[jamesr1]

Re: Low cylinder presure

It is like a coupler that comes off of the top of the exhaust manifold, and connects to a 4" id rubber hose. I believe it connects to the outdrive. It is described as a water seperater in my Clymer repair manual.

 

[rodbolt]

Re: Low cylinder presure

ohhh the exhaust elbow I gotcha now. some manuals call it a riser. <br /> if its a onepiece unit torque the bolts to 25ft pounds. DO NOT use silicone on the gasket.

 

[jamesr1]

Re: Low cylinder presure

well, i put it all back together and i still only register 60 psi on that cylinder.

 

[Bondo]

Re: Low cylinder presure

well, i put it all back together and i still only register 60 psi on that cylinder.

No Surprizes There..................<br /><br />You Know the Piston is BEAT ..............

 

[jamesr1]

Re: Low cylinder presure

The piston is beat. <br />Could this also be a cracked block? If so, how do I verify it?<br /> I was hopefull when I found the valves to be bad, and not sure what steps to take next.

 

[jamesr1]

Re: Low cylinder presure

How about this? Putting some oil into the cylinder and checking to see if there is a difference in pressure will tell me if it is rings, but will it tell me that a new pinston and rings will fix it, or could I still have a cracked block and not know it?

 

[newport dave]

Re: Low cylinder presure

The engine really should be rebuilt, but it couldn't hurt anything to do a "wet" compession test and a compression leak-down test while the engine is still together. That may give you more info on the source of the compression loss, but you may be "flogging a dead horse" at this point.<br /><br />Dave

 

[jamesr1]

Re: Low cylinder presure

What is a compression leak down? sorry I am still a little wet behind the ears, lol

 

[Don S]

Re: Low cylinder presure

You should have looked for cracks while you had the head off of it. But if the piston is beat up, the compression rings are probably not seating.<br />But here is some information you might like on the leak down test.<br /><br />Leak Down Test

 

[rayjay]

Re: Low cylinder presure

Hook up some muffs and run the motor some and hot lash the valves. <br /><br />Then check the compression.<br /><br />If it doesn't then come up I bet that the piston is either cracked or the ring land is pinched down and has the ring trapped.