Blowby and compression test - 225

[jwicker]

Hey all,

My slant 6 225 is running heaps better now that I've completed a proper valve lash. HOWEVER, it seems that there is still a bit of blowby and I'm smelling gas in the Oil. I haven't confirmed a fuel pump internal leak yet, thought it's possible.

My Hobo Fright compression test revealed about 90-92 PSI across the board Is it time for rings?

A bit of oil in the cylinder didn't seem to improve the compression much. I'm surprised since she starts and runs so smooth.

I really don't want to do a full rebuild but I worry it may be time. No known rebuild in the life of the engine, and the car has 68K original miles (1964).

Thoughts?

 

[RustyRatRod]

Was the compression test before or after the valve adjustment? Was that with the throttle blocked all the way open and the engine warm? All plugs removed? It's a little low maybe, but it's even and that's good. I think the book says a minimum of 100 PSI. However, if all of the above conditions are not met, it can affect the outcome drastically.

 

[GTX JOHN]

Back in the 60s when I was a broke kid - I did a valve
job to a couple of my old worn cars. It seemed to make the
smoking and ring seal even worse after the valve job.
In any of my stock V8 cars = i would be a bit worried about
only 90 PSIs compression pressure.

 

[junkyardhero]

In any of my stock V8 cars = i would be a bit worried about only 90 PSIs compression pressure.

it's on a slant six, and 90 isn't really all that far off of the minimum spec-- especially if the test criteria wasn't followed and the test conducted properly.

 

[junkyardhero]

i'd suggest double checking the process of the compression test and run it again to verify the results. then add leak down test; that would give a little bit bigger picture of the overall condition of the engine.

 

[RustyRatRod]

Keep in mind the stock slant 6 only has like 80 pounds on the seat. lol

 

[66fs]

I'd get the gas "leak" fixed, if there is one. Change the oil and filter with Valvoline racing 20w50 and run it for a while. Then recheck. You might only need valve seals with that low mileage.

 

[Darter6]

but it's even and that's good.

66fs made a point. A few fresh oil changes and a new fuel pump (not in that order) will do allot. Baby steps.
My 63 had 90 psi across the board when I got it. After a tune and couple of fresh oil/filter changes compression came up to 120 psi. The engine was a 170 slant. Putting some miles on it made a difference.

It seemed to make the
smoking and ring seal even worse after the valve job.

I agree with John. Old timers would say a fresh top end will suck the rings out of a old bottom end.

 

[Syleng1]

A few drops of oil in the cylinder and re do the compression test will indicate rings or not. If you got gas in the oil, change that quicker than I can type with two thumbs.
That will do more damage than you know.

Stock style fuel pumps are cheap. Replace that if you even question it one bit.
The price of a new pump is less than bearings and a crank.

SN: if you do not know how to properly do a compression test (some people here have suggested you may not) then ask. If done properly it is an effective way to evaluate the situation. If done wrong it will send you down rabbit holes (cylinder holes ) you may not need to go down)

Syleng1

 

[AJ/FormS]

IMO
I would not suffer an engine that only had a true 90/93 psi; unless it was a stationary engine, or if I only wanted to idle it around for like a couple of miles a week, lol.

To get a better idea of the condition of your engine;
1) if it has a PCV valve, I would flip it out of the valve cover, start it up and see how much blow-by is actually being created, by what is blasting out of that hole. or
2) if it doesn't have a PCV system, then it will have a road-draft tube, so you can just check it, for billowing smoke.
3) slapping the gas pedal to say 3000 rpm should NOT make the smoke come blasting outta either the hole in the VC nor the draft-tube.
Excessive blow-by can only be coming past the rings..... or thru the pistons, lol. But you know, the compression rings could be stuck in the grooves .......... which would give low pressure readings and blow-by, as would;
out-of-round bores, or huge ring-gaps, scratches in the cylinder walls, or maybe, a plugged muffler.
A LeakDown test will give you a snapshot of what is going on at TDC, but not the why of it. On a tired engine, there is no telling what percentage of the LD is from the worn bore, and what percentage is from the rings, nor why. I mean I'd hate to tear off and rebuild the head, with no improvement, just to find out the rings are stuck. That would suck.....
Doing this test with the piston at the Bottom of the bore will give you a better idea of the ring condition, but to do so, the valve gear has to be disengaged.