Is it valve job time?

Will N

Will N

Well-known member
Had some spare time today, so I pulled all the plugs on my '15 and did a quick compression test. The car runs pretty good on the flat terrain where I live. But it looses steam on even the smallest of hills.

Engine is cold and it's put away for the winter in a closed area, so I can't start it to warm it up. Opened the throttle fully, and hand cranked it over several times. Did it dry first, then pumped some oil in. I got around 32 lbs dry and 35 with oil in all the cylinders. Those are low, as it should be around 50-55, right? The cold engine might be contributing to the low readings.

But what I'm seeing is that the cylinders aren't holding the pressure. The pressure builds on the upstroke, but bleeds off immediately. When I do a compression test on my Model A, the pressure stays and I have to release it by pushing the button on the gauge.

I'm thinking the valves aren't sealing, and I'm looking at a valve job at minimum. Do you agree? Thanks!
 
Before you pull the head check the valve clearances. You will need to open the covers anyway to do the valve grind. Remember, the T when new was only about as powerful as a good riding lawnmower.
 
Franklin, are you suggesting that the valves may have receded in their seats so much that there is no clearance left, which is preventing them from closing completely? That could explain the fast bleed off of the compression pressure. But wouldn't that condition lead quickly to burned valves?
 
If the valve clearance is too close, yes you could get burned valves. If there is too much clearance the valves may be sticking, but also they may not be opening enough to get best breathing.
 
While you're on the low side it's not tremendously bad. The thing is when you add oil and the pressure goes up it usually indicates ring/cylinder wear as the oil seals the rings for a few cranks. This doesn't affect the valves at all and is no indication of their condition. As to the gauge not holding pressure: if it held on the A and you had to release it on the gauge and it doesn't hold on the T the release valve on the gauge is shot. It should hold it's reading until you release it no matter how low it is. So here's the thing; the gauge is NG. You need to repair or replace it and re take the comp test because if it bleeds off as soon as you stop cranking there's no way you're reading it accurately. I always take low pressure T motors cold for comp tests. It gives a better indication of what the cold start compression is.
 
All compression testers that I have used in the last 50 years always has a schrader type valve to hold the reading until released. As the piston comes up top for each of the 5 "hits" the reading stays or rises just a bit for the final reading. If there is rust or carbon lodged in the schrader, remove the valve and clean or replace it. I remove my valve when I use it as a cylinder leakdown tester but compressed air is introduced at 35 # and the piston is stationary at TDC.
 
While a leak down test will tell you where air might be leaking from in the valve train short term, all three rings have gaps that allow the air to get past for long term test. In other words, all compressed air will leak out eventual. With the engine valves both closed, add compress air (not too much you are checking for leaks not trying to turn the motor over) to cylinders and check for leakage at the intake or exhaust manifolds.
 
Huh, I never thought that there might be something wrong with the gauge. I used it on a friend's "A" back in September, and it worked fine then. I'll check out the release valve to see if it's sticking open. Thanks guys!
 
Sure enough, it was the way I jury rigged the gauge to fit the pipe thread spark plug holes. I cobbled together some fittings and I took the hose off the gauge and connected the gauge right to the fittings. Well, the check valve is in the hose! I ordered the 14MM adapters so my gauge will fit. Thanks again guys!
 
Thought I'd post an update if anyone is interested. Got the sparkplug adapters so my gauge is assembled completely and working correctly.

Dry
1 35'
2 25'
3 35'
4 25'

Oiled
1 40'
2 40'
3 39'
4 39'

These readings are on a cold engine. The bad news is it looks like my rings are kinda weak. The good news is that the oiled numbers are pretty consistent across the cylinders. Like I said earlier, the car runs good on flat ground, and I don't think those numbers are bad enough to justify tearing apart the engine. Next spring, I'll take readings after the engine is all warmed up to see what impact that has.

Thanks everyone for your input and help!
 
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