68vert

What should the cylinder pressure readings be for a 68 350 2barrel?

Six cylinders were 118-120psi, one cylinder was 110psi, one cylinder was 100psi. The 100psi cylinder gets an oily-gooey spark plug after 300-500miles.

I put a little oil in the 100psi cylinder retested and got the same reading-100psi. I also put a little oil in the 110psi cylinder retested and got 125psi.

Does this mean I have valve problems on the 100psi cylinder and piston ring problems on the 110psi cylinder? Is 110psi low enough that I need to do something with it?

olds 307 and 403

Your engine looks worn out by those numbers.

68vert

Yea it's tired 140k on it. What's the cylinder pressure of a stock 350? My 13yr old son and I are just trying troubleshoot things and learn a little before we bring it in for rebuild. It runs ok and wife n kids like to take it for ice cream.

Any info or insight is greatly appreciated! Thanks

don71

By all means keep taking the family out for ice scream. Not everyone gets to to do that in a 68 vert.

You have a pretty good idea your engine is worn out. Make plans and a budget now. You probably have a few more miles in it before it gets worse. Enjoy it.

The piston rings are probably stuck with carbon to the ring lands. The valves probably aren't sealing too well and you might have a vacum leak which equals worn out.

If it were me...I'd get a can or two of sea foam. Follow the directions. See what happens.

When in doubt keep the family happy.

MDchanic

Definitely SeaFoam the bejeebers out of it - pour a can into the motor oil, run the engine for a few minutes, then shut 'er down, next remove the plugs and squirt SeaFoam into all the cylinders, then let it sit for a week or two, and finally give it a crank with the plugs out to expel any excess liquid, then start it, run it, then pour a bottle of SeaFoam slowly down the carb while holding the engine speed to about 2,000. Once it's warm, drain and change the oil and filter, then go out and do a whole bunch of driving with your foot all the way to the floor, then all the way off the gas, so that the engine repeatedly goes to maximum pressure and maximum braking (this will load the rings in both directions, and may wiggle out any carbon that was softened up).

You'll loosen all kinds of things up, which will probably be a good thing.

As for your cylinders, the compression is somewhat low all around, but it's fairly even. Those numbers are within the allowable variation between cylinders.
I usually figure a good, stock 10.25:1 motor to be in the 170-210psi range (probably 180-190), and a good, stock 9:1 motor like yours to be in the 130s to maybe 150s.
You're around 120, with one at 110 and one at 100, which, as I say, is within spec.

The dumb questions:
Was the engine fully warmed up?
Was the throttle 100% wide open?
Did you crank until the gauge stopped going up (may take up to 4 or 5 cycles)?

To answer your questions, Yes, the 110 is losing a bit of compression past the rings - as noted, the SeaFoam might clear that., and, Yes, the 100 is probably losing compression through a valve (head gasket leak or a crack are not out of the question).

The next step is to do a leakdown test - your compression gauge may have a snap-off hose that allows you to thread into the spark plug holes and connect the other end to an air compressor.
Place each cylinder at TDC compression (both valves closed) and add compressed air (maybe 60-100psi), and listen at the tailpipe, the carburetor, and the oil filler.
You should hear a bit of leakage past the rings through the oil filler.
You should NOT hear any leakage past the valves to the carburetor or the tailpipe.
If you have a leaky valve, then it's time for that head to come off (at which point, you Might As Well...).

- Eric

Fun71

Don't do this in the garage.
Or the driveway.
Or anywhere in your neighborhood.
It will put out more smoke than you think could ever come out of your car's tailpipe(s).

MDchanic

And... ?



- Eric

jcdynamic88

140k is overdue for the timing chain to fall off the cam and make a mess imo..i wouldn't try to thrash the carbon out of it.

MDchanic

A fine point.

I guess I kinda ASSumed that he'd changed that by now.

However, I didn't mean to actually thrash it, just to do a lot of loading and unloading of the rings to rattle them around and loosen the carbon.

If the timing chain is the original, I would go so far as to advise changing the timing set as a first priority, before doing anything else.

- Eric

jcdynamic88

i believe WEBSTER'S dictionary defines "thrash" as a whole lot of gas pedal all the way to the floor and then all the way off