I have read posts in the past on this topic but can't seem to find what I need in the "search". I have a 71 350 modified engine. Between 3-4,000 miles it started significantly burning oil (1 qt in 2-300 miles). Performed a compression test and all cylinders read between 155-160 psi. The oil burning is strictly from the RH bank. At least thats where blue smoke can be seen after decelerating and taking off. This would seem to indicate valve guide or seal problem. The heads had new valves and guides installed. The spark plugs look good on #1,3,5,7 and 8. #6 has about 1/16" deposit buildup around the insulator and is completely black. Also, there was oil around the base of the plug threads even though it took a lot of force to break it loose. #2 & 4 plugs have significant deposits and are black.
Even though the compression test is good is it still possible that the oil rings are bad? Is there any way to determine the real problem; valve seals, valve guides or oil rings?

 

Not that I'm an expert, but a couple of things spring to mind:

• valve seals
• worn valve guides

First engine (and the last so far) that I rebuild had the same problem.
It still runs though 15k miles later and still uses oil at the rate of 1 qrt/200miles

 

I don't know who assembled the engine or it's history of abuse in the last 4000 miles, but it is very rare that parts go bad in so few miles if it is not an all out race motor turning the limit.

Sure sounds like the oil scrapers were put in upside down on those bad cylinders. Compression check etc won't show it either.

 

The compression test is a good place to start, but it did not give the answer you were looking for. Did you do it with the motor at operating temp?
With a consumption rate like you describe it won't be hard to find the problem, especially if you have wet, oily plugs and you see smoke. Oil can enter the cylinder from only three places. From the top - around the valve guides, from the bottom - up past the rings, and from the intake manifold (either a bad gasket or, via the carb if you are running a pcv system and no baffles in your valve covers). You are running some sort of breather system, right? It can also enter from the exhaust during overlap - but that's getting it from another cylinder. As bad as yours is there should be some associated symptoms - miss fire, rough idle, running hotter than normal, minor stumble off idle, etc.
Here's what I would check:
1) With the engine idling at operating temperature, remove the oil filler cap or breather hose from the valve cover. You should see nothing. If you see vapors or hear the engine "woofing" out of the holes you have compression entering the crankcase.
2) If you do not have one, invest in a vacuum gauge. Read the diagnostic guide that comes with it and go to work.
3) If you do not have a leak down tester get one. You can get one at Harbor Freight for $35.00. Of course you will need compressed air to use it.
4) If you do not have a leak down tester or an air compressor, remove the valve cover from the suspect back and remove the vlave springs in order to check the valve guides and seals. You can use a length of small diameter rope through the sparkplug hole to fill the cylinder and hold the vlaves closed. You will need a valve spring compressor and any parts store has these on the tool rack ($25 or so ). Bad valve seals will cause smoke on start up only. Bad guides will do so on deceleration as well.

 

The car had been setting in my workshop for three weeks before the compression test. The engine still runs pretty strong; will pull to 5800 rpm with no miss. Idle is somewhat erractic but I'm running a Comp Cam Extreme Energy 274 cam. Vacuum was OK (10" at 750rpm and 18" at 3000rpm) before the oil consumption but I probably hadn't checked it since. Running stock PCV (LH bank)and rocker cover breather (RH bank) to air cleaner. No unusual noise or smoke with air cleaner off. However I will check this again after installing new plugs and firing it up. I have an air compressor but not a leak down guage. I can get one but can this test indicate bad oil rings?

 

Engine has never been raced. I do of course get on occassionally but certainly has not been abused. The first 3,000 miles it didn't use but a half quart which would be pretty normal for break-in. I changed the oil after 300 miles.

 

The leakdown test will determine the sealing ability of each invidual cylinder by filling them with compressed air. It is a somewhat tedious chore because you have to do it while the engine is warm - just like the compression test. And just like the compression test you have to do ALL cylinders and then compare the readings. It will not tell you if you have bad valve guides. You will still need to remove the rocker arms and springs to check them. Valves and guides are one area where it's real common to get crappy parts and crappier work. Are you sure the guides are new and not just knurlized? Your mileage is about how long that process lasts.

 

Check for oil in the intake runners for the suspect cylinders (look under the carb). Could be bad intake mani gasket - My guess since it is only effecting one side. Where is the PCV plumbed (to manifold or carb)?

Not likely that valve seals would go bad that quick unless they are wrong - or you have insufficient clearance at max valve lift.

Single or dual plane manifold?

 

The engine builder does a lot of local circle track race engines so I'd like to think he put in new guides as he said. I also had larger valves, hardened seats and guide plates installed. However anything is possible I guess.
When I did the compression test the engine block was only 54 degrees. I don't know how you'd get a compression test or leak down test done with a warm engine on this car. Even with my own lift it takes me 1 1/2 hours to get plugs removed and get a simple compression test on all cylinders. With the lack of clearance I can't imagine how many burns I'd have trying to do this with the engine hot. A leak down test would take significantly longer.
I'm tempted to just pull the heads and have the guides and seals re-done.

 

It's a stock dual plane manifold. Intake runners are clean just looking down with the carb off. However it seems I could have a leaking intake gasket along the bottom and it's jsut pull oil into the intake runners in the heads from the valley, right? If this were the case I should see a loss in vacuum from my previous readings right?

 

Machine work or lack of it to the heads should show up as more even failures, not usually on select cylinders. Other than screwed up seals.
Even if knurling would go bad after 4000 miles (which it doesn't), a quart/200 is more than that would cause.

Doing hot tests is ideal, but not always possible.
You are looking at consistancy or lack thereof.
Last engine I tested was only off by 2 psi total on any of the cylinders.

Simple stuff first.
Did you replace the pcv valve and test to see that it sucks. Your intake runners are clean so you aren't pulling ooil in by that.

 

That is what I'm thinking, though vacuum may not change enough to notice if the leak is small.

Other possibilities that I have witnessed:

Valve spring pockets cut too deep - penetrating the intake runners

Valve seals hammered by valve spring retainers due to insufficient clearance

Incorrect valve seals chaffing on springs

Cracked head


PS: I don't think a leak down test will indicate an upside down oil control ring if the top ring is in good shape. If none of the above you will probably just have to pull the pistons for a visual.


Stock iron heads or aluminum - with or without screw in rocker studs?

 

Stock iron heads with screw in studs, roller rocker arms. The entire valve train was a Comp Cams kit, at least thats what I paid for. I'm wishing now that I'd assembled the engine myself. As a side note two cams have been replaced (by the engine builder) due to a lobe going flat. These were both Comp Cams Extreme Energy. The second I used Rotella diesel oil for break-in and am now adding ZDP. I am about ready to redo the complete valve train with a roller lifter setup. I just don't want to go thru all this if the oil comsumption is due to ring problems.

 

I realize it is work, but if you perform the tests in my original post you will verify the cause. And knurlized valve guides can fail in as little as 4,000 miles - I've had it happen on a Pontiac back in '04. Same symptons as yours. Good luck!

 

Thanks!

 

Check that the studs are sealed in the bosses (GM gasket seal,locktite, something). Many times these penatrate the intake runners.

 

Just to concur with Capella, methinks you're looking at an intake joint sealing issue. Having suffered the same problem myself (with its attendant 150 mile-per-quart rate of oil consumption) I can personally vouch for the fact that many 'experienced' mechanics will seem incredulous when confronted with this situation. When I purchased my '70 11 years ago I decided to tear it down to a) see if the [low] indicated mileage was genuine and b) have it de-compressed by milling the pistons so it'd run on our crappy CA gas. My machinist (a very reputable San Diego shop who had been in the hi-performance race engine machining biz since the early 60's) inadvertently angle-milled my heads, and what began as an econo-job turned into a two year-long running nightmare where I wound up pulling the engine two more times, having the bottom end bored and fitted with new pistons (again being in denial about the fact that a leaky/mis-machined/otherwise mismatched intake joint could cause such an insane rate of oil consumption), and finally having my heads re-machined to correct the angularity issue. It has now run like a tame kitten for over 30,000 miles with hardly a drop of oil usage. The Chevy smallblock was designed to be simple and therefore CHEAP, hence no valley cover a la Pontiac, et al, and to make matters worse the intake bolts on one side pull against those on the opposite side (by contrast those icky old Fords have them pulling the intake straight DOWN vertically against the heads) which tends to pull one side of the intake away from its respective head while the other side is being tightened. It is therefore critical to make sure that the sealing surfaces are properly machined and sealing properly, and that good gaskets are used. I would also use a very thin bead of Permatex Grey hi-temp RTV to seal around both sides of the gaskets at each intake port, and be sure to seal the intake bolts and torque them in three steps IN THE PROPER SEQUENCE with a KNOWN GOOD torque wrench. Sorry about being so long-winded; but I do have a little personal experience with this kind of frustration...hope you get it sorted out.

 

Thanks! This sure makes sense to me. My heads were definitely milled and may have be done angular. I'm going to pull them off, find a reputable machine shop and have everything checked.