'69 Ranchero 302 low compression

[99GTMichael]

Low compression across two cylinders... 6 & 7. Every other cylinder was in the range of 120-140 psi. 6 & 7 were at 50-60 psi. Have not performed a wet compression test yet. Truck misfires noticeable at idle, but you cannot feel any difference while driving down the road even at highway speeds. Also pulls 19" hg of dead steady vacuum. Any guess as to what's causing my low compression & misfires? I was too lazy to perform a wet compression or leak down test today. Adjacent cylinders with equally low compression worries me to be honest. I get the feeling that this engine has never been apart before.

Truck is 1969 302 2bbl C4

[Walking-Tall]

Side-by-side cylinders low is classic blown head gasket between them territory.

Just to verify, I betcha if you install a spark plug in #6 or #7, and check the compression of the other, it might come up some...

[99GTMichael]

When I summon up the motivation, I'll get back out to the shop and tinker with it some more. Thanks Walking-Tall

[cb84capri]

A leakdown tester would be handy to diagnose this. Just putting compressed air in the problem cylinders on the compression stroke could send you on the right path listening around the engine and watching for bubbles in the coolant. Could also be valves not sealing (unsure if hardened seats of not then), or the flat tappet cam may have some bad lobes, bad rings, freak weird stuff etc. If the spark plugs look really clean compared to others the safe bet is it's the head gasket.

The symptoms match what my car did when an intake lobe on the cam went bad, though I only had the issue on cylinder 4.

Cale

[Ethyl Cat]

Side-by-side cylinders low is classic blown head gasket between them territory.

Just to verify, I betcha if you install a spark plug in #6 or #7, and check the compression of the other, it might come up some...

Vacuum would not be steady in that scenario as the intake valve on cylinder 7 is open when 6 is firing.

[Walking-Tall]

Vacuum would not be steady in that scenario as the intake valve on cylinder 7 is open when 6 is firing.

Vacuum? Please elaborate.

We're talking about multiple crankshaft rotations while doing a compression test where the compression gauge gathers and records the highest seen cylinder pressure. I meant, with a spark plug in either of the OP's #6 or #7, the side-by-side leakers, the cranking pressure should come up somewhat with the other, indicating their unfortunate connection through a head gasket that should be separating them.

... and for that matter, with 1-5-4-2-6-3-7-8 (and I think with 1-3-7-2-6-5-4-8 too), I think #7's exhaust valve might be open in the vicinity of #6 TDC compression.

[99GTMichael]

Engine is in process of coming out. Hoping just to do a quick re-gasket & seal. It's nice to have the Ranchero as a toy to mess around with when I'm in the mood. Bit by bit, it gets attention.

My favorite picture. Sold the Mach 1 wheels with my last Mustang which was a mistake. I don't really care for resto-mod but I think that they looked great on it.

[Ethyl Cat]

Vacuum? Please elaborate.

We're talking about multiple crankshaft rotations while doing a compression test where the compression gauge gathers and records the highest seen cylinder pressure. I meant, with a spark plug in either of the OP's #6 or #7, the side-by-side leakers, the cranking pressure should come up somewhat with the other, indicating their unfortunate connection through a head gasket that should be separating them.

... and for that matter, with 1-5-4-2-6-3-7-8 (and I think with 1-3-7-2-6-5-4-8 too), I think #7's exhaust valve might be open in the vicinity of #6 TDC compression.

Michael99V6 wrote in the original post "Also pulls 19" hg of dead steady vacuum."

[Ethyl Cat]

... and for that matter, with 1-5-4-2-6-3-7-8 (and I think with 1-3-7-2-6-5-4-8 too), I think #7's exhaust valve might be open in the vicinity of #6 TDC compression.

If #6 is at tdc compression then #7 is 180 degrees away from tdc compression (bdc intake stroke) with the first firing order.

If #7 is at tdc compression then #6 is 180 degrees away from tc compression (bdc intake stroke) with the second.

Both send cylinder pressure through an open intake valve in the other cylinder if the head gasket was blown.. This would not result in a steady manifold vacuum.

I agree that 99% of the time it is the issue when two adjacent cylinders are low, but the data does not back it up IMOP.

[Walking-Tall]

If #6 is at tdc compression then #7 is 180 degrees away from tdc compression (bdc intake stroke) with the first firing order.

If #7 is at tdc compression then #6 is 180 degrees away from tc compression (bdc intake stroke) with the second.

Both send cylinder pressure through an open intake valve in the other cylinder if the head gasket was blown.. This would not result in a steady manifold vacuum.

I agree that 99% of the time it is the issue when two adjacent cylinders are low, but the data does not back it up IMOP.

Yes, the brain fog has cleared and you're absolutely right, commonly the tail end of the intake cycle. For the record, my confusion stemmed from your reply to my quoted words as opposed to the OP's.

What's the other 1%, coincidental and adjacent cylinder worn/broken parts? I suppose that's a possibility, though I don't see steady and high vacuum readings with worn or broken rings or valves or seats either.

Anyway, good luck with it OP, and let us know what you find.