V8 compression - Chevy 350 (1 Viewer)

[Norcalcruizer]

Hello all,

I picked up a 1976 FJ40 a few weeks ago, it has a stock SB 350, carbureted with HEI (accel coil- smog legal - passed CA smog test), the motor runs great, plenty of power, never seems to miss a beat. The PO had no receipts on mileage, just maint and oil change receipts . I have noticed when I hit the throttle or rev it up I do get a small plume of gray/blue smoke coming out the exhaust. I know this is a sign of burning oil. I did a compression test on the engine I got the following:

(psi)
8 - 145 7 - 135
6 - 135 5 - 130
4 - 135 3 - 130
2 - 145 1 - 130

Is this within normal limits for a 1976 SB 350 block? When the engine starts up, or is just idling I get no smoke at all, only when I hammer the throttle hard. Anyone have any ideas if this would be a ring issue or valve seals? Someone even mentioned a PCV valve, I did check this (at least it rattles) so assuming it is not the pcv valve. I was also told valve seals can be replaced with just removing valve covers. If it is the rings I would be more than likely just buy a rebuilt block and start new. Any thoughts to what I should do next?

I attached a pic of the rig, have been doing some basic exterior clean up on it the past few weeks, really enjoying getting my hands dirty and watching it clean up.

 

[RWBeringer4x4]

What's most important is that all your cylinders are within about 10% of one another. The physical numbers may vary gauge to gauge. If everything is within 10%, but universally a bit low, it could indicate normal wear, or just that the gauge is off a bit. Also - was the engine warmed up for this check? A hot reading is more accurate than a cold one.

I don't think that compression is awful. It's pretty close to what I see in my 283 with similar symptoms. I'm not running a PCV valve at all - but I should be. Compression numbers are also sort of dependent on the engine setup - whether it's a high performance, high compression 350 (i.e. A camaro or corvette) vs a lower compression, lower horsepower version like one out of a truck.

Usually if it's your valve seals, you'd see a puff of smoke at start up from oil leaking past the seals when the engine was shut down. The seals can be replaced by removing the valve covers.

If you really want to check your valves and rings - you can perform a leak down test. This measures how well the cylinder holds pressure, and also lets you listen to where the air is leaking from. A brand new engine will still leak about 5% or so. Generally, unless you're a racer and concerned with every last smudge of horsepower, 10-20% could be considered normal wear, but serviceable. Once you start getting into the 20-30% leakage range and beyond your talking a pretty tired engine.

As a point of comparison, my 1967 283 SBC leaks between 9 and 19%. Not great, but not worth it to me to replace the engine just yet.

your compression numbers are pretty decent. If it's not smoking too badly I'd just run it.

 

[Tex40]

Nice looking rig - really like the color combo! Might be on the lower end of the normal compression, but there is no big variation between cylinders - it's just a little tired but if your happy with the power then personally I would't worry too much and just enjoy if for a while.

If you want get one of those old time oil squirt cans and put a squirt in a cylinder and see if the compression jumps up - that's a sign of worn rings. Could be valve seals but usually you get the smoke on startup with those (still could be intake valve seal)

***DOH! Beat me to it!*****

 

[bikersmurf]

I'd say it's within range... On the low side, but not terrible considering it's likely only 8.5:1.

 

[Norcalcruizer]

Hey, thanks guys for the feedback... the test was done after the engine had been sitting for about hour, I believe the temp gauge was around 130 when I looked at it. So no, it was not at full normal operating temperature. I would like to get the smoking issue resolved, infact I am surprised it would pass a smog test if it was puffing out smoke after they revved it up. It did have high HC readings on the test, barely passed. It had low CO (%) and NO (ppm) reading but the HC (ppm) was at the limit. I am not sure if it is worth taking to someone to have valve seals put in if it is not definitely the problem. With new rings I would be looking at a complete rebuild. The engine runs great, power seems as good or better than previous cruisers I have owned. I do not hear any missing at idle if I put my ear near the exhaust.

If I squirt oil in the cylinder and do another compression test what happens when I start the engine back up? Will it smoke like crazy for awhile or foul a plug?

 

[Solace in Solitude]

No, you are only squirting a couple of squirts in each cylinder, as you compression test them... Just to see if your compression numbers improve markedly due to oiling the rings.

I would bring it up to operating temperature, then compression test each cylinder DRY.

Then squirt a couple of drops of oil in each cylinder, as you compression test it WET, while still at operating temperature.

Then compare DRY vs WET to determine the percentage of improvement, when WET.

 

[shawnfj40]

If the numbers stay aout the same, change your valve seals.

 

[Norcalcruizer]

No, you are only squirting a couple of squirts in each cylinder, as you compression test them... Just to see if your compression numbers improve markedly due to oiling the rings.

I would bring it up to operating temperature, then compression test each cylinder DRY.

Then squirt a couple of drops of oil in each cylinder, as you compression test it WET, while still at operating temperature.

Then compare DRY vs WET to determine the percentage of improvement, when WET.

Just to confirm a couple squirts from oil can or a couple drops? What is going to happen when I restart the vehicle with oil in the cylinders? will it fowl a plug or just smoke a little? and what is your thought about running a heavier weight oil? I just changed it with 10w-40, but I have read some will run 50w to help reduce blow by.

 

[Solace in Solitude]

Just to confirm a couple squirts from oil can or a couple drops? What is going to happen when I restart the vehicle with oil in the cylinders? will it fowl a plug or just smoke a little? and what is your thought about running a heavier weight oil? I just changed it with 10w-40, but I have read some will run 50w to help reduce blow by.

HERE is some wet compression test specificity... It says 2 oz of oil and explains how to read the results.

I ran 20w-50w before the rebuild, but I don't think it helped the low compression... Just tried to ensure components were well oiled.

I doubt you will see more than a puff of smoke when you start it back up. If you inject oil into the cylinders and do the testing, by the time you get the plugs back in and start it, the oil will likely be gone from the cylinders.

I never fouled my plugs doing wet testing.

 

[Howard705]

Drive it and see how much oil it consumes over several k miles. On older 350s-from the 70s-I beleive around 1 quart/1000mi is acceptable. When you hammer it snd see smoke it's probably from gas- from a big squirt of the acceletator pump. Valve guide seals will seep a little on old 350s but it won't hurt anything. Like Tex said-mostly on startup.

 

[Norcalcruizer]

Drive it and see how much oil it consumes over several k miles. On older 350s-from the 70s-I beleive around 1 quart/1000mi is acceptable. When you hammer it snd see smoke it's probably from gas- from a big squirt of the acceletator pump. Valve guide seals will seep a little on old 350s but it won't hurt anything. Like Tex said-mostly on startup.

Being winter now, its pretty cold driving every day without a top. I am trying to get out with it on nice sunny days, this is my plan, drive it and see what the oil consumption is like. Can excess fuel cause a blue/gray smoke plume? I always thought this was a sign of oil burning? fuel related was more black smoke?

 

[Howard705]

Being winter now, its pretty cold driving every day without a top. I am trying to get out with it on nice sunny days, this is my plan, drive it and see what the oil consumption is like. Can excess fuel cause a blue/gray smoke plume? I always thought this was a sign of oil burning? fuel related was more black smoke?

fuel more black. guess it depends if when you're hitting the throttle is in neutral or driving and under load. In neutral it will have sucked the oil in the guides and burn it. Under a load same thing somewhat if you've let off it in a lower gear and coasted somewhat first before romping it. Coasting down a long hill under engine braking will suck in oil too.

 

[Norcalcruizer]

I just got some good news from my mechanic, I drove my cruiser to his shop on the freeway majority of time, about a 20 min drive. Pulled into his shop and zero smoke at all, even with high revs. He said motor is running perfect. We let it idle for about 15 mins and it started to begin to smoke again, he is convinced some new valve seals will do the trick which only take him about 2 hours to install. I am keeping my fingers crossed this does the trick.

 

[Tex40]

Keep us updated if the valve seals fix it!

 

[Norcalcruizer]

Umbrella seals were replaced yesterday, unfortunately I drove home in the dark so I never got a chance to verify exhaust. Good news is I did not smell any exhaust on the way home, so that is a step in right direction. He did save all the old seals and they were dry and cracking, definitely needed to be replaced. You could squeeze them and they would break apart. Wont be driving for a few days but hoping this ends up resolving the issue.

 

[Norcalcruizer]

Some follow up on this. The valve guide seals definitely helped the initial smoking at start up, however I was still getting a puff of smoke inbetween high rev shifts. I ended up replacing the valve covers because they had no baffle in them. I was sucking oil up into the PCV valve going to intake, valves, pistons, etc. After I installed the baffled valve covers smoking has completely gone away. Oil consumption seems to be zero now. I was surprised how much oil was up inside that PCV valve from those non baffled covers.

 

[Tex40]

Good news! Now go drive it!!!