67 L79 Oil Consumption

[John C.]

Just had my original 67 327 350 HP rebuilt and it used one quart of oil in approximately 150 miles. Machine shop replaced the rings and I had the same results. The PCV system and rear oil draft tube are oil free and no evidence of excessive oil in the air cleaner area. They are suggesting I block off the PCV hose and see if the consumption stops but with it currently dry I will be surprised to see any change. I have read about the intake manifold issues allowing oil into the lifter valleys. This could be the issue as all eight spark plugs have approximately the same amount of oil on them when removed. The car runs strong with no evidence of smoke. The engine was rebuilt using a high pressure oil pump and roller valve train asembly. Please comment should you have any other suggestions as they would be greatly appreciated.

Thanks John C

[Joe L.]

Re: 67 L79 Oil Consumption

Just had my original 67 327 350 HP rebuilt and it used one quart of oil in approximately 150 miles. Machine shop replaced the rings and I had the same results. The PCV system and rear oil draft tube are oil free and no evidence of excessive oil in the air cleaner area. They are suggesting I block off the PCV hose and see if the consumption stops but with it currently dry I will be surprised to see any change. I have read about the intake manifold issues allowing oil into the lifter valleys. This could be the issue as all eight spark plugs have approximately the same amount of oil on them when removed. The car runs strong with no evidence of smoke. The engine was rebuilt using a high pressure oil pump and roller valve train asembly. Please comment should you have any other suggestions as they would be greatly appreciated.

Thanks John C

John------


First of all, you can't block off the PCV. The engine has to have some form of crankcase ventilation and, with no road draft tube and vented oil filler cap, you'd be left with none. Plus, if you were not having a problem with the PCV system prior to the rebuild, why should there be one now?

I would not have used the high pressure oil pump. For your application, especially with a roller cam, you absolutely don't need it. However, I doubt that it's contributing to the oil consumption problem.

You could be affected by the dreaded intake manifold oil leakage syndrome. Were the heads or block surfaces milled? If so, that could set you up for the problem. If it is caused by intake manifold oil seepage how do you solve it? I wish I had an answer. I once tried mightily to find the answer but it defeated me.

[John C.]

Re: 67 L79 Oil Consumption

Joe, would any damage occur by blocking off the PCV just to satisfy the machine shop? I agree that it probably is not the problem especially with no evidence of oil. Neither the heads or intake were machined and we used the NCRS type gaskets from Paragon. They did use Permatex on the front and rear of the manifold versus the rubber seals in the kit for what they felt would be a better seal. Would there be evidence of a failed intake manifold gasket if the intake was removed showing oil in the intake ports? Also, would there need to be leak on both sides for all eight cylinders to be receiveing oil? Is there any chance that this is a break in issue since it only has 160 miles on the rebuild? Thanks for your help. jc

[Clem Z.]

Re: 67 L79 Oil Consumption

have you done any full throttle run up to the red line if not do it as you need pressure behind the rings to seat them. run thru the gears!!!!

[Dan P.]

Re: 67 L79 Oil Consumption

First pull all the plugs and see if any have signs of oil consumption . The do a leak down test on the cylinders . Next I would look at the valve stem seals.

[William C.]

Re: 67 L79 Oil Consumption

John..I had the exact same problem on a L79 years ago and it was the valve guides and seals..Bill

[Timothy B.]

Re: 67 L79 Oil Consumption

John,

Find out from the machine shop if the oil pump is high volume and high pressure. With 150 miles, you mention not excessive oil in the air cleaner. IMO this can be caused by excessive blowby in the crankcase forcing it's way back to the air cleaner through the rear breather of the air cleaner or even through the pcv. How much oil pressure does the engine carry?

This could be from rings seating plus excess oil froth mixed in the air and getting sucked back into the air cleaner. Long time ago I had a 350 motor I rebuilt with a GM H/P-H/V pump that used oil the same way. The PCV was installed in the valve cover with a stock oil baffle but now that I think of it NO WAY that baffle was going to seperate the oil mixed in with the air. All the oil was up top with that H/V pump, the engine ran great as long as the plugs were changed every 500 miles and you kept adding oil.

[Duke W.]

Re: 67 L79 Oil Consumption

Oil pressure has little to do with oil consumption. The most likely causes are:

1. Worn valve guides/valve stems and/or poor valve guide sealing

2. Piston rings

3. Internal oil leak such as block valley to inlet ports.

Most Corvette engine PCV systems will not pass excessive oil if they are properly configured as per the OE design, regardless of oil pressure.

In this case the engine was "rebuilt", but nothing was mentioned by the OP about what work was done in the area of valve guide rebuilding and seals or whether the original valves were replaced due to stem wear or reused.

Most cases of excess oil consumption, whether an old or newly rebuilt engine are valve guide clearance/seal related, so that's the first place to look. What, if any, work was done in this area?

Duke

[Domenic T.]

Re: 67 L79 Oil Consumption

Duke,
When you increase the preasure you do increase the volume to the top end (valve covers) and I have seen the oil level get over the guides and get sucked down into the cylinders.

DOM

Oil pressure has little to do with oil consumption. The most likely causes are:

1. Worn valve guides/valve stems and/or poor valve guide sealing

2. Piston rings

3. Internal oil leak such as block valley to inlet ports.

Most Corvette engine PCV systems will not pass excessive oil if they are properly configured as per the OE design, regardless of oil pressure.

In this case the engine was "rebuilt", but nothing was mentioned by the OP about what work was done in the area of valve guide rebuilding and seals or whether the original valves were replaced due to stem wear or reused.

Most cases of excess oil consumption, whether an old or newly rebuilt engine are valve guide clearance/seal related, so that's the first place to look. What, if any, work was done in this area?

Duke

[Ken A.]

Re: 67 L79 Oil Consumption

This is pretty simple to determine the problem.
First, modern moly filled rings seat in about 30 seconds, end of that story.

Second, remove carburetor & using an inspection scope check all the intake runners for oil leakage-which would indicate an improperly installed gasket or did the heads get surfaced and if so, did the M/Shop rework the intake manifold to match?

Third, if the heads weren't inspected, then pull a valve cover & remove a "wet" spark plug. Loosen both rockers to make sure both valves are closed and squirt some like WD-40 in the SP hole. Apply air pressure of 180# & look for seepage at the intake valve guide(the exhaust makes no difference). If you see stuff coming out the guide, then you've found a problem.

Fourth, forget the PCV system-it won't cause wet sp's & finally you could have both a poor gasket seal and worn out guides.

[Gene M.]

Re: 67 L79 Oil Consumption

John,
A long shot, but are the rings indexed correctly?

Do you get any oil in the air cleaner?

[Duke W.]

Re: 67 L79 Oil Consumption

Duke,
When you increase the preasure you do increase the volume to the top end (valve covers) and I have seen the oil level get over the guides and get sucked down into the cylinders.

DOM

Flow might increase somewhat. I believe it's more of a function of restrictor size once oil pressure is over about 30 psi, but 80 psi should not flood the rocker boxes unless the drains are restricted.

Duke

[Joe L.]

Re: 67 L79 Oil Consumption

Joe, would any damage occur by blocking off the PCV just to satisfy the machine shop? I agree that it probably is not the problem especially with no evidence of oil. Neither the heads or intake were machined and we used the NCRS type gaskets from Paragon. They did use Permatex on the front and rear of the manifold versus the rubber seals in the kit for what they felt would be a better seal. Would there be evidence of a failed intake manifold gasket if the intake was removed showing oil in the intake ports? Also, would there need to be leak on both sides for all eight cylinders to be receiveing oil? Is there any chance that this is a break in issue since it only has 160 miles on the rebuild? Thanks for your help. jc

John-----


Temporarily blocking the PCV system shouldn't cause any permanent engine damage, but doing so will likely cause an increase in oil consumption all by itself.

It's sometimes difficult to see an intake manifold gasket oil leak at the gasket. I can tell you that I could never POSITIVELY CONFIRM that oil was passing the gasket.

If the intake manifold gasket is the source of the oil leakage, there would need to be a failure of gaskets on both sides if all 8 cylinders were affected.

I really doubt that the kind of oil consumption you have described could be related to normal engine break-in. Very slightly higher oil consumption can be expected during operation of a freshly rebuilt engine but nothing like what you've described. As Ken mentioned, moly-filled rings are pre-lapped and seat in very quickly.

[Domenic T.]

Re: 67 L79 Oil Consumption

In reguards to what Joe said about the PCV being blocked, Yes it will cause crank case preasure and cause high oil consumption.

I did a mid 60's small block for a customer ( exchange long block) and he came storming back with his installed engine.

It was still blowing smoke and the got a ticket for that with his old engine.
Yes, you don't see that now days but they gave fix it tickets for oil burners then.

I kept the car in the shop and noticed that the PCV hose was cracking. I bent it to crack it in half and found it clogged which was the cause for the oil burning in the first place.

If the PCV valve is working properly you should have a light film of oil in the hose.

DOM

[Timothy B.]

Re: 67 L79 Oil Consumption

On my 350 oil burner, the installed pump was a high volume and high pressure, lots of oil up top for sure and completely unnecessary. It could have overwhelmed the valve guide seals but I believe the pcv did it's share of sucking.

A quick test to check the intake gasket for sealing is to block the pcv and oil fill tube and install a vacuum gauge on the dipstick tube. If there is no intake manifold gasket leakage the gauge should show 1/2-1 lb pressure at fast idle. If there is leakage the gauge should show vacuum.