5.7 oil mystery

[Wozny]

I asked this question six months ago, so apologies for repeating it but I’m using a quart of oil every 60 miles and would like to get my 1992 Chevy pickup back on the road. It doesn’t smoke or leak oil, does have a little blow by, but not a lot. So one question I have is, if it were using that much oil due to just blow by, wouldn’t it also be blowing it out the tailpipe? I think blowby can be from either worn rings or valve seals. But I am guessing that only bad rings will smoke out the tailpipe, whereas worn valve seals will smoke out the valve covers, which is where the only smoke I can find is. I’m also thinking that bad valve seals won’t burn as much as a quart in 60 miles, but am not sure. One opinion I got last time was that the catalytic converter could be consuming the smoke, but I didn’t think that likely since that amount of oil burning would create enough carbon to clog a cat pretty quick, it seems. Thanks for any thoughts on this

 

[Schurkey]

1. Cranking compression test on all eight. Inspect plugs for signs of oil burning as they're removed.

2. Cylinder leakdown test on all eight.

3. Verify no oil in PCV suction hose from PCV valve to manifold or throttle body. Do you have OEM valve covers, or aftermarket covers with crappy oil baffling?

4. Remove PCV valve from valve cover, look for continued vacuum in crankcase--evidence of failed intake gaskets at bottom of ports (sucking oil mist and air from lifter valley.



I had an engine that used a quart in thirty miles. Smoked like a mosquito fogger (but old enough that there was no catalytic converter.

Yes, too much oil in the exhaust will poison a catalyst.

 

[Pinger]

I asked this question six months ago, so apologies for repeating it but I’m using a quart of oil every 60 miles and would like to get my 1992 Chevy pickup back on the road. It doesn’t smoke or leak oil, does have a little blow by, but not a lot. So one question I have is, if it were using that much oil due to just blow by, wouldn’t it also be blowing it out the tailpipe?

I'd guess ultimately that is where it's going.

I think blowby can be from either worn rings or valve seals. But I am guessing that only bad rings will smoke out the tailpipe, whereas worn valve seals will smoke out the valve covers, which is where the only smoke I can find is.

All that can pressurise a valve seal to leak into the valve cover is exhaust pressure. Smoke out of the valve covers is more likely blow-by caused by poor compression sealing. Inlet valve seals when they leak are most obvious on start up when oil has pooled around them. And on over-run with high manifold vacuum.

I’m also thinking that bad valve seals won’t burn as much as a quart in 60 miles, but am not sure.

I doubt that they would but no certainties here.

One opinion I got last time was that the catalytic converter could be consuming the smoke, but I didn’t think that likely since that amount of oil burning would create enough carbon to clog a cat pretty quick, it seems. Thanks for any thoughts on this

My smart had the classic failed oil control rings and burned oil like crazy but no blue smoke. The cat cleaned that up. The cat did eventually fail - disintegrated during an annual emissions check. Stank like burned brakes/clutch.
Compression rings can be good but still burn oil if oil control rings are goosed. Your blow-by sounds like both sets of rings are past their best though.

 

[Erik the Awful]

The PCV's the easiest to check, and the most likely culprit for that amount of consumption, but it should be smoking like an 1860's train. The rings are the next most likely, and both a compression test and leakdown test are recommended. The valve seals are the least likely, but not out of the question.

Is it leaking any on the ground when sitting? Possibly more when it's running? How's the coolant look? Just tossing some ideas out.

 

[Wozny]

1. Cranking compression test on all eight. Inspect plugs for signs of oil burning as they're removed.

2. Cylinder leakdown test on all eight.

3. Verify no oil in PCV suction hose from PCV valve to manifold or throttle body. Do you have OEM valve covers, or aftermarket covers with crappy oil baffling?

4. Remove PCV valve from valve cover, look for continued vacuum in crankcase--evidence of failed intake gaskets at bottom of ports (sucking oil mist and air from lifter valley.



I had an engine that used a quart in thirty miles. Smoked like a mosquito fogger (but old enough that there was no catalytic converter.

Yes, too much oil in the exhaust will poison a catalyst.

Thanks for the good ideas. I’ve been wanting leakdown kit and will get one and let you know what I found out. I checked the PCV and found it clean and clear. Did not check the down pipe. The truck only has 140,000 on it but the oil was very bad, sludge. Valve covers are original, caked with carbon, I might take one off. There’s no oil in the antifreeze. Doesn’t smoke but one time I was going pretty fast down a long grade and suddenly stopped to check something and oil was pouring off all parts of that engine’s bottom side, and bell. It didn’t use more than the quart per 60 that trip though, and it’s never done that since. Or leaked sitting either. Did a compression test a while ago. I think it was pretty similar on all 8, and I think it was about 90. Like maybe 85 to 95, something like that but I should do it again as I can’t say for sure. Thanks to all. Will be a few days before info the leakdown.

 

[Schurkey]

Best leakdown tester is one that's made to the FAA guidelines, and also Continental/Teledyne compatible.

I got mine from Aircraft Tool Supply (ATS). Has the Master Orifice for "testing the tester".

https://www.aircraft-tool.com/shop/detail.aspx?id=2EM

Normally ships with 18mm cylinder adapter, you'd want to tell them to provide a 14mm adapter instead.

 

[oldbejuanyaknowme]

This is a very dumb questions I know, but I did see it once many years ago: Do you have an after-market dip stick?
Have you done a complete oil change and filled with exactly the amount of oil specified for your engine?
Do you have an after-market oil pan?
You can see where I am going with this. Good luck with your issue!

 

[Wozny]

This is a very dumb questions I know, but I did see it once many years ago: Do you have an after-market dip stick?
Have you done a complete oil change and filled with exactly the amount of oil specified for your engine?
Do you have an after-market oil pan?
You can see where I am going with this. Good luck with your issue!

Thanks, everything is OEM, I can tell by how the whole engine is caked with old oil all over. And I've been running it enough to know that it's losing a quart every 60 miles whether or not the stick and pan are original. I appreciate the question though

 

[Wozny]

Best leakdown tester is one that's made to the FAA guidelines, and also Continental/Teledyne compatible.

I got mine from Aircraft Tool Supply (ATS). Has the Master Orifice for "testing the tester".

https://www.aircraft-tool.com/shop/detail.aspx?id=2EM

Normally ships with 18mm cylinder adapter, you'd want to tell them to provide a 14mm adapter instead.

Thanks, I ordered one. It looks better than the one at NAPA which I almost bought yesterday.

 

[kenh]

I've had the need for a leak down tester on occasion.. Just looked on the web to see.... OTC (a well respected name) has one for $80 ish. Harbor Freight has one for nearly identical price, (slightly higher price) BOTH appear to be identical except for the gauge face color right down to the case. Interesting!

Ken