The Stovebolt Site Forums The Shop Area Engine Shop Low compression number 3 cylinder

Hello everyone, my 1950 chevy 3100 with a 235ci seemed to be lacking in power so I did a compression test here are the results:
1. 106 PSI
2. 109 PSI
3. 36 PSI
4. 107
5. 102
6. 101

I poured some oil in number 3 to see if that would seal a ring and improve my compression, but it only came up to 44 PSI. So, I am thinking it is a valve or a head gasket does anyone have any further trouble shouting suggestions?

Mike,

Have you pulled the valve cover and checked to make sure the valves for #3 aren't stuck open???

I would be inclined to think more of a valve issue than a head gasket. Have you checked your oil and coolant for cross contamination???

Dan

Check for a broken valve spring if this happened suddenly. I've had 3 valve springs break in 235s and what you described is typical. This will also cause them not to idle worth squat. Good luck.

Blow some compressed air in the #3 spark plug hole and see where the air is leaking out. Both valves need to be closed. I have a compression gauge which has a short removable hose with spark plug threads on one end and a male quick disconnect fitting on the other, which fits my air compressor hose. I use a regulator between the air hose and the short hose to manage how much air to deliver. Too much air pressure will turn the engine over.
It will either be coming out of the
carburetor. intake valve is leaking. Pull the valve cover and hit the rocker arm right above the valve stem with a dead blow or rubber mallet and listen for a resounding clack when the valve hits the seat on the rebound. (hit one of the closed valves of a good cylinder so you know what a good one is supposed to sound like.

the exhaust. exhaust valve is leaking. Same as above.

the crankcase. broken or gummed up rings. If the valves aren't leaking from the valves, you can try adding 2 quarts of diesel to your oil and run it at higher than idle (by pulling out the throttle cable) for 20 minutes. Do this in the driveway. Do not drive the truck! Your crank case will be 2 quarts overfilled which will allow the crankshaft journals to sling that solution around like a carwash.
Even if it does not solve a stuck ring problem, when you are done, at least you have a squeaky clean crankcase to deal with when you tear the engine down.
(I stole a good bit of this material from HRL, so you know that its the right thing to do. )

I am with the others. Sounds like a valve issue. Pull the valve cover and look at the rockers and the valves for No. 3. If the exhaust valve got stuck, you might just be able to squirt some penetrating oil around the guide and tap the valve with a block of wood and hammer and get it unstuck. If the exhaust valve does not seem stuck, then it might be the intake valve and that might be a speck more complicated. The exhaust valve is in the chamber and the design makes it impossible for the piston to hit it no matter what you do. On the intake side, its different because the intake is pretty close to the piston. So if it was a stuck valve and it kisssed the piston which unstuck the valve, you likely have a bent intake valve head which will never seat flat. That will require a little remedial work at the machine shop.

Could also be just a lot of crud and junk on the seat and valve preventing the valve from fully seating and sealing. If noting evident from a visual inspection, you may have to pull the head and take a look and see what is going on.

thanks, everyone I am going to take all of these suggestions in, also, all the cylinders seem kind of low, is around 110 PSI normal PSI for a 235?

Depends on a host of factors, Mike. Age, mileage, care it has been given, if it has ever been rebuilt, etc. 110 isn't terrible, really. I like to see 120 to 130 in each cylinder, but you're not all that far off percentage-wise, you know. I've seen many engines with only 100 psi in each cylinder that ran just fine.

So, I finally got the head off and took it to the machine shop for them to run a Vacuum test on it. The issue was in the head, and all the exhaust ports and valves were worn and needed to be replaced. I am looking forward to seeing how much that improves compression once I have the head back on.

Mike, first of all Good Luck! In my experience if the head is freshened up, it's a good policy to do an in-frame on the rest. Drop pan, cut the ring ridge if necessary, remove rod caps, drive pistons out from bottom with hammer handle, hone cylinders to get a proper crosshatch pattern, new rod bearings or not after inspecting the crank and bearings, new rings on pistons and reassemble. Not as good as a proper full rebuild but I've driven many 10s of thousands of miles with no issues using the shade-tree freshening just described.

PS, proper break-in technique to seat rings is very important. Lots of discussion on subject, Hotrod Lincoln is a great source. Been using same for over 50 years.

RonR

Speaking of a quick and dirty in-frame refresh. Let’s say I just wanna throw some bearings in a 235. Rod bearings obviously not much issue.

Can you get enough drop on the crank to replace upper main shells…without removing the crank?

After mid-year 1955, 235s have an anti-rotate tab at the parting line of the upper main bearing shells. Those mains can be swapped by loosening the other main cap bolts 2-3 turns. 54 and early 55 engines need the crankshaft to be dropped about an inch to change the upper bearings. That requires removing the harmonic balancer, the timing cover, and the transmission to get that much drop on the crankshaft.
Jerry

Mines a 62 model. I suppose it’s possible, then.

At one time, there were tools that could be inserted into a crankshaft oil passage to roll the upper main bearing shells in and out by turning the crankshaft. I've usually been able to do it by inserting a cotter pin into the oil gallery and flattening the ends down against the crankshaft surface. Turn the shaft, and the bearing rolls right out. It installs the same way.
Jerry

I've heard of the cotter key trick, too.

I don't know that you will see a big drastic overall bump in compression, but the valve job will certainly help, and the resolution of the bad cylinder is a terrific outcome. I will take the other side of the argument on the rest of the motor. Put the head on and see how it runs. If it runs decent, you are all set.

I'm with Dragsix. If it ain't broke, don't fix it.

You'll be opening a can of worms that leads to a full engine overhaul which will lead to a freshening up of the engine bay which will eventually end up as a total frame off restoration.

Do you want to drive it or work on it?

I will “third” DragSix and Otto’s advice here for your situation. One possible caveat….

I think avoiding “while I’m at it” snowball effect is pretty good general advice, but obviously varies a lot situation by situation. Case in point, I recently did a front motor mount replacement, but it would have been fairly absurd to not replace the front crank seal while I was in there. The leak there is what made the motor mount fail to begin with.

In your case, while you have the head off, might as well take a good look at the condition of the cylinder walls, how much of a ridge there is at the top, etc. You could even get a quick and dirty idea of ring seal by applying some liquid to the top of pistons….might be good to do it in pairs with pistons somewhere near top of cylinder where taper is likely to be the worst. Then, time/compare low long each cylinder holds that liquid before it leaks down. Look for outliers. If all do a reasonably good job of holding liquid, call it good enough.

Someone else can probably do a better job of detailing the “test” I’m describing. Basic idea is to lend some confidence the engine will be a good runner when you button it back up.

Check the cylinder taper. Turn a piston to the bottom of its travel, and insert a piston ring into the cylinder, up against the ridge at the top of the ring travel. Stack up feeler gauge blades to measure the ring gap. Write that dimension down. Push the ring down into the cylinder about 2-3 inches, making sure to keep it square with the top of the block. Measure the gap again. Subtract the bottom gap from the top and divide by 3. The result is the taper wear in the cylinder in thousandths of an inch. Any taper exceeding around .005" indicates it's time for a rebore and oversize pistons- - - - -a re-ring job with tapered cylinders is a waste of time and money.
Jerry