The Stovebolt Site Forums The Shop Area Engine Shop Please help me with my compression readings

Tonight I did a compression test on my 54 235. This was the first compression test on this engine, and my first compression test ever. I've read a lot about this, so I understand what the readings mean, but what I want to know is are my findings acceptable for a 235, or should I be saving for an engine rebuild in the near future?

The dry test showed:
#1 = 115, #2 = 115, #3 = 125, #4 = 119. #5 = 95, and #6 = 115.

A wet test showed #5 = 132, so at least for #5 I can say that the ring is worn to some degree. I didn't do a wet test on the rest since they all had relatively even readings, but their low readings (should be 130 or better) could be rings or valves or both.

The book says they should all be 130 or better and +/- 5-10 pounds for "satisfactory engine performance". The engine seems to have worn evenly across the board, with #5 being the only exception.

The truck seems to run pretty well right now. Maybe it could run more smoothly (it rumbles a bit maybe), but at least now I can attribute the slight rumble to #5. Or can I?

So all this leads to my main question: how bad are my readings. Is 115 (15 down from the perscribed 130) ok, so-so, bad, or really bad? Am I missing gobs of power and fuel economy, or am I not that bad off? As I said, the truck runs pretty well, and I realize I might not be getting optimal gas mileage/power, but I don't want to rebuild the engine right now.

If you had an engine like this, what would you do? My truck is being rebuilt to a safe and mechanically sound state -- I'm not doing a full restoration. So I have tolerance for it not being perfect, but I don't want to overlook a big issue. And I don't have the experience with this 235 to know how big an issue I have on my hands. My instinct tells me just to forget about it and drive the truck...

Greg,

Do you know how many miles are on the engine since its last overhaul? As you have surmised, the rings on #5 may be somewhat tired, and the rest of the cylinders are slightly low. I recall seeing 130 to 140 psig on good 235s, and I get 230 psig on my 261, with 11:1 compression ratio. However, unless you want to rebuild the engine for other reasons, there is no immediate need for an overhaul.

By the way, was the throttle plate blocked full open when the engine was cranked for the compression test? If the throttle is closed, the manifold pressure will drop and the peak compressed pressure will be low. This will probably have not much effect on the difference between #5 and the rest of the cylinders.

Good the see another St. Louisian active with a stovebolt.

Hoyt

Greg,

I agree with Hoyt. I would leave it alone and eventually #5 will probably lead the pack, and if nothing else, start to use a little oil or smoke a tad. The truck is old..the engine is old.. it has to be worn. Those numbers are good for an old engine, they are to be expected. Drive it and enjoy it but I wouldn't rebuild it right now, just on those compression numbers.

That's my unprofessional two cents worth!

Stuart

Thanks for the adivice.

I did have the throttle wide open (using the throttle knob in the cab, which I know is working), so my numbers are as good as they are going to get.

I don't know when/if the engine has ever been overhauled. It has about 48K original miles on it -- or at least that is what the odometer says. Judging from the manifold bolts, I'm not sure it has ever been rebuilt. Those bolts look like they haven't moved since the truck was built.

Well thanks for the info guys. I'll just keep driving it until it clearly needs help -- like big smoke clouds or something!

The next logical step is a cylinder leakage check, where compressed air is introduced into the cylinder through the spark plug hole and you listen for leakage at the intake, exhaust, and crankcase filler hole. Both vakves need to be closed, and the piston at or near top dead center when the air goes in. Put the transmission in 3rd. gear and lock the parking brake or chock the wheels to keep the crankshaft from turning. If, as you suspect, rings are the problem, you'll be able to hear air escaping into the crankcase by listening at the oil filler cap. Unless there's a big leak, I'd suggest driving it, and keep an eye out for blue smoke in the exhaust and/or excessive oil consumption.
Jerry

I would do a wet compression test on all the cylinders so you have a reference for the future. I agree with the others that your situation at this point is acceptable, although the 95 PSI cylinder is lower than desirable. I would also suggest setting the valve clearance, to take out that potential variable for compression. It doesn't seem that your low cylinder would have that trouble, but set the valves anyway: Intake, .008"; Exhaust, .015". These settings are recommended for severe service operation, but were typically used for all service. Standard service settings: Intake, .006; Exhaust, .013".

When you have all the spark plugs removed for a compression check, it is not necessary to open the throttle; there won't be any manifold vacuum during cranking when the spark plugs are all removed, so there won't be any loss of pressure because of manifold vacuum (reduced manifold pressure).

Also, take note of the spark plug color and appearance. Dark brown to black indicates rich fuel mixture, oil consumption, or the spark plug heat range is too cold. Light tan to off-white insulator is a good condition. Pure, clean white insulator and a bluish color and shine on the center electrode indicate hot plugs, for what ever reason they are hot- heat range too high, fuel mixture too lean are possibilities.

Harvester