What next..?

[Goose]

There was some missing from my 350 Chev engine on long trip. Did a compression check when I got home, and all cylinders except 6 & 8 are within 5 psi with an average of about 160. Except for 6 & 8 which came in at 120 each. Plugs for 6 & 8 were badly fouled as well. I obviously need to fix something but where to start? Will a simple valve job and new seals be enough with new head gaskets or is it more serious as in potentially new rings. Engine has approx 137,000 miles. A complete new long block seems most likely, but significantly more expensive than just a valve job if that would cure the ills.

[Richard]

Squirt some oil into the low cylinders though the spark plug holes and redo the compression check. If pressure goes up- rings, if it stays the same-valves or head gasket.

[sick472]

^^^^^
This

And, I always squirt in about a spoonful of oil and turn the engine over a couple times without the sprarkplugs installed and the ignition disconnected so the engine does not try to fire. This way if you put in too much oil, it pushes it out of the plug hole and you don't get hydro lock in the cylinder and pop a piston.

[RustyPile]

Before we jump to conclusions, let's think about this a little bit.. You said you experienced "some missing"? Was it an ongoing steady miss or just an occasional miss? What does the engine oil dipstick tell you?? How's the oil consumption between changes? A couple cylinders have lower compression.. Fouled spark plugs, how many miles on those sparkplugs?? An engine with 137K miles will use a bit of oil.. I wouldn't be concerned unless it's more than a quart every 1,000 - 1500 miles.. 20#s down on compression isn't bad enough to warrant major repairs on an engine of that mileage.. Fouled spark plugs could simply be from too many miles between changing them..

If it were my truck, I'd do a proper tune-up using nice quality parts and procedures and drive it a few hundred more miles and then recheck compression and check the sparkplugs for fouling..

[LT7A]

Originally Posted by RustyPile

Before we jump to conclusions, let's think about this a little bit.. You said you experienced "some missing"? Was it an ongoing steady miss or just an occasional miss? What does the engine oil dipstick tell you?? How's the oil consumption between changes? A couple cylinders have lower compression.. Fouled spark plugs, how many miles on those sparkplugs?? An engine with 137K miles will use a bit of oil.. I wouldn't be concerned unless it's more than a quart every 1,000 - 1500 miles.. 20#s down on compression isn't bad enough to warrant major repairs on an engine of that mileage.. Fouled spark plugs could simply be from too many miles between changing them..

If it were my truck, I'd do a proper tune-up using nice quality parts and procedures and drive it a few hundred more miles and then recheck compression and check the sparkplugs for fouling..

I like this approach. I would include plug wires in your tune up. Those blasted things have a way of surprising you. They are good until they are not...

[Carl Spangler]

I would suspect “stuck rings”.

A Hail Mary approach would be an engine flush with one of the commercial fluids designed to free up stuck rings.

[geezer#99]

Usually two adjacent equal pressure cylinders indicate a blown head gasket.

[Goose]

Thanks for all the good input. I did shoot some engine oil into one of the bad cylinders and got a 20psi increase in compression (120 to 140). That small amount of increase didn't immediately say to me bad rings. Since the two low cylinders are adjacent I'm thinking maybe a breach in the head gasket and valves/seals. I put new plugs in and truck runs fine. Time between plug changes is 2700 miles. We used to change them every 3000 anyway. The other eight plugs show no significant fouling. Did compression check with all plugs out and coil wire pulled. Oil usage is more than a quart per 1000 miles, but good Gen 1 350 probably leaks as much as it burns. Don't know about "stuck rings", but a cleansing flush maybe beneficial. If the rings are still acceptable my initial thoughts to fix and upgrade is a set of vortec heads.

[57taskforce]

I would Try a leak down test next on those two cylinders. That will point where the lost compression is going, intake,exhaust,or crankcase. Are the plugs oil fouled? Coolant fouled? Fuel fouled? I assume oil fouled.

[Richard]

Originally Posted by Goose

Thanks for all the good input. I did shoot some engine oil into one of the bad cylinders and got a 20psi increase in compression (120 to 140). That small amount of increase didn't immediately say to me bad rings. Since the two low cylinders are adjacent I'm thinking maybe a breach in the head gasket and valves/seals. I put new plugs in and truck runs fine. Time between plug changes is 2700 miles. We used to change them every 3000 anyway. The other eight plugs show no significant fouling. Did compression check with all plugs out and coil wire pulled. Oil usage is more than a quart per 1000 miles, but good Gen 1 350 probably leaks as much as it burns. Don't know about "stuck rings", but a cleansing flush maybe beneficial. If the rings are still acceptable my initial thoughts to fix and upgrade is a set of vortec heads.

Wow, I think I change plugs every 50-70 thousand miles. Even on my old rigs. 100+ on the new cars with platinum plugs. Something is wrong with the motor. You need to fix or maybe think about a refit.

[Goose]

Thanks. Exactly the two options fix it or replace the entire engine.

[franken]

Nobody is going to figure out what's wrong with your engine but you. You can read posts with ideas forever but your engine is still yours and what to do with it up to you.

[BigBird05]

Goose, Is that the original engine in your truck? Back in the 70's if you got 100,000 mile on a engine you were living on borrowed time. They just didn't last any longer that that? That engine is more than likely worn out. Pull the heads and (or use a bore scope ) and look at the cylinder bore to see how bad it is worn.

[rsgt]

My truck had similar issues. I just fix it, Here is my old head gasket.

[Goose]

I've change the engine three times in this truck over the time I've owned it since '75. I agree, a Gen I 350 with more than 100,000 miles is suspect. I was using the collective experience on this website to see if others had experienced similar conditions as I'm seeing, and then what they did about it before I tore things apart. I appreciate the feedback. I got it from here.

[toolboxchev]

Originally Posted by 57taskforce

I would Try a leak down test next on those two cylinders. That will point where the lost compression is going, intake,exhaust,or crankcase. Are the plugs oil fouled? Coolant fouled? Fuel fouled? I assume oil fouled.

X2, its more exacting in the diagnosis process.

[slikside]

Originally Posted by BigBird05

Goose, Is that the original engine in your truck? Back in the 70's if you got 100,000 mile on a engine you were living on borrowed time. They just didn't last any longer that that? That engine is more than likely worn out. Pull the heads and (or use a bore scope ) and look at the cylinder bore to see how bad it is worn.

So, where does that put my original 350 with 250K+ on it?