Dead #5 Cylinder. Can I keep going...?

[Don S]

Re: Dead #5 Cylinder. Can I keep going...?

Even on a cold engine you will have some compression.
Personally, I would say keep running it. The more you run it the worse it gets and the more money you get to spend and put into circulation.

If you really want to know, get that cylinder up on TDC on the compression stroke, pressurize the cylinder with 50 to 100 psi of air, and listen to where the air leaks. If you hear it loudest in the exhaust, you have an exhaust valve gone bad, if from the carb, an intake valve, if from the valve cover, you have a hole in a piston.

 

[UncleWillie]

Re: Dead #5 Cylinder. Can I keep going...?

Lacking any other symptoms, the exhaust valve is the most likely culprit.
Leaks are leaks in both directions.
A head gasket, cracked piston, Bad rings will pull in whatever is on the otherside of it on the intake stroke, if it pulls in water or oil, they will appear in the exhaust.
But just don't expect to see them in a boat exhaust.
The oil maybe, the water, forget it.
However the water/oil will also be on the sparkplug.
What does the Plug look like?
A misfiring plug will be nasty.

As others said, plug the #5 plug wire off and start it.
If it sounds worse, the plug was firing before!
Absolute Zero compression is a BIG leak, not a small crack.

 

[kleake]

Re: Dead #5 Cylinder. Can I keep going...?

A buddy of mine has a 350 in a crownline he is selling. We knew it ran rough at idle, and needs a rebuild. A buyer stopped by and did a compression check. 4 of the cyl were in the 150-160 range, 3 were low around 60-80 psi. One had 0,,, the needle never moved. His is definitely rings or a hole in the piston because it runs decent on the trailer with a rough idle, but put that thing in the water and start working it and oil will start pushing up the dipstick tube because of all of the crank case pressure. If you aren't getting crank case pressure, it would be a valve issue most likely. If you are getting alot of crankcase pressure, it's a piston or rings.

 

[04fxdwgi]

Re: Dead #5 Cylinder. Can I keep going...?

Should also check to see it the rocker arm studs are intact. If the heads have pressed in rocker studs, have seen them pull out of the head and the rocker arm **** can to one side, disengaging the pushrod / valve stem, causing a dead silent situation with a bad miss, as long as the lifter doesn't launch out of the bore. If the lifter leaves the bore, then the oil pressure drops to zip and there is plenty of noise. Same holds true of a push rod piercing the rocker arm (have seen them actually come thru the valve cover). All typical SB Chevy problems, but real easy to fix if they didn't cause more damage.

 

[mpdive]

Re: Dead #5 Cylinder. Can I keep going...?

Problem with the keep running it scenario is that you are constantly "washing" the cylinder. This dilutes your oil and thus you lose lubrication quality of all the cylinders. Don's advice on pressurizing the cylinder is a good way to go. You can buy the air line at Napa. Best and quickest way to know what your working with. Ignoring one cylinder will eventually lead to problems with all the cylinders due to washing.

 

[Scott Danforth]

Re: Dead #5 Cylinder. Can I keep going...?

I can use my compression tester to pressurize cylinders. just uncouple the hose from the gauge, remove the schroeder valve and I have a perfect hose to check cylinders under pressure. If your using a hose type compression tester with a quick release, this is a possibility. Just dont loose the schroeder valve, they are not the same as a regular tire schroeder valve