I have a bit of a mystery to solve, and haven't been able to come up with the right solution. Here is what I know so far. I rebuilt a 400 small block with new vortec heads and an Edelbrock 2116 vortec manifold. The engine runs well, but uses an incredible amount of oil, to the tune of 1 qt in 50 - 80 miles of city driving. There is certainly some oil smoke, but not as much as you might think. I pulled the engine to inspect the piston assemblies, and found them to be excellent, rings were installed correctly and they seemed to be seating well all around. I have done a leakdown test, and all cylinders are within 3%-6%. Next, I measured the clearance of the valve retainer to the guide seal and found it to be very close at maximum valve lift. The cam is very mild comp cam 260, with .440 lift on both intake and exhaust. I removed the heads and cut down the guides to provide more clearance, installed new guide seals, but there was no improvement in oil consumption. As I was working on the heads, it was obvious where the oil was entering the combustion chamber. The intake valves were coated with a hard oil residue on the underside of the valve, and up the stem. I was
sure that the seal interference (about .010") was the culprit and that the oil was being forced from the top of the heads through the seals and into the combustion chamber. No such luck. I cleaned it all up and re-assembled, but no improvement. Yesterday, I pulled the carburetor and swabbed a clean cloth through the intake manifold runners, and there was a lot of oil on the rag, even before getting to the head. The oil seems to be coming through the intake. The obvious thing is the PCV valve, but the valve covers are baffled at the PCV valve and the vacuum line is completely clean and dry. Also, the intake manifold is not leaking. There is no vacuum at all when I block the PCV valve and the air intake on the valve covers. I'm using the GM manifold gaskets and when I've disassembled it, it appeared to have sealed well. There are no external leaks at all on the engine. I'm really baffled, and although I'm getting better at pulling the
engine apart, I'd like to figure this one out. Any suggestions on where to look next?

Thanks, and sorry so long!

 

Hmm, I was thinking an intake gasket leak the whole way till you said it wasn't that...

 

There's really only one possibility, and it seems you already know what it is by your line of questioning...

I'd try a different brand / type of valve guide seals, and check them carefully to make sure the retainers don't smash into them... you might even want to get some other springs & retainers.

 

If you have screw-in rocker studs:

Some rocker stud holes go into the intake runner. (My L98 heads do.) The stud threads need sealer on them so oil isn't sucked into the intake runner. Your oil consumption is pretty high. If they were all leaking, that might explain the oil loss.

 

Geez, thats alot of oil....I dont have any suggestions but I will add that my winter beater, an 85 delta 88 goes thru alot of oil too, from Toronto to Cleveland last week we went thru 4 of the small jugs, and the same back, but you would expect that with a beater, not a nice ride that you have done a bunch of work on!


Let us know what it is once you have figured it out, I am certainly interested what it is in yours, since I know mine is bad rins and valve seals, but you would expect that on an engine which hasnt been opened since the factory.

 

Beerman,

The lack of vacuum in the crankcase with the PCV vent blocked seems like a big hint. What is the idle vacuum like?

Can you describe the contact pattern of the intake runner gaskets between the intake flange and heads? (Got any Prussian blue?)

 

Vader,
I'm using the O-ring type GM gasket and I can see contact around each of the intakes and the water passages. The prussian blue is a good idea. I have a can and will try it. It seems like if it was leaking at one of the intake ports, I would see it mostly in that cylinder, and not in all, but I'm obviously missing something basic so I can't assume. I'm really bafffled by the oil in the manifold though. Thanks for all the ideas. I'll let you know what I find out, and I will figure this one out.

 

Vader,
Sorry, I didn't answer your question about the manifold vacuum. It is a bit low, which is a good clue. It runs steady at 18 - 19".
Any thoughts? The car idles great, but has a bit of stumble just off idle. Right now, it seems like that could be related to the high level of oil in the cumbustion.
Thanks

 

Beerman,

That vacuum at idle may not be too low, depending upon your cam profile, valve timing, and idle RPM. As long as it's steady, you should be O.K. mechanically. It probably isn't a valve guide issue any more, but that's just my gut feeling.

My suspicion is that even though you see a contact patch around each runner, there may be some moderate leakage at the lower edges of all the intakes due to machining differences between the heads and intake. Were the heads milled? Regardless, a little Prussian blue between the metal parts should indicate the contact patches pretty well. You'll at least be able to identify or positively eliminate another possibility.

You've apparently already cut the valve guides and installed new seals. Did you check valve stem clearances when the heads were apart? If there was a lot of contact, you may have done something to the stem side clearances/guides, but that is a remote possibility. Are the guides pressed in, by any chance?

The oil in the intake runners should be a good clue, but I can't think of a source other than the PCV or intake gaskets (unless you have a cracked intake casting). And the very good leakdown results (no more than 6%) should indicate that blowby and excessive crankcase pressure aren't an issue.

At least you've got us all thinking....