Blowing oil update #3. NO broken rings

[cstreit]

Awwww Hell!

I thought it would be cut and dry...

I felt my case vent and there was a definite pulsing (One per revolution, and a steady "blow" at 3000 RPM. I then compared against a stock SC with 75k miles... it just had a slight back and forth feeling, normal, even at 3K RPM... Plus I smelled raw fuel vapor coming out of the case.

So I figured, must be a cracked ring...

Pulled the motor and the heads, cylinders, NO CRACKED RINGS, no upside rings...

They all looked good, motor looks perfect. I noticed that one cylinder MAY have had the compression rings and oil ring gaps almost lined up...

So NOW WHAT?

(se previous threads in this forum for more discussions... Newly built motor 400 miles, 10:1 compression and boattailed case, new valve guides, etc..)

[dad911]

Did you run a leakdown test before pulling the heads? If so, was the cylinder with the lined up rings leaking?

[john walker's workshop]

a bad guide(s) can blow pressure into the crankcase. they would have to be pretty worn.

[cstreit]

JW. EBS did the valve guides, they are brand new so I doubt that this is it.

I was not able to do a leakdown, just a compression test. Cylinders came out 152, 153 on all of them...

[WERK I]

Chris,
Aligned ring gaps can definately cause blowby. Gaps should be staggered 120 degrees with no gaps near the exhaust side of the barrels. I think you found the problem, especially if the two aligned gaps were the compression rings.

[cstreit]

They were...

[MotoSook]

Alright Chris, I have too ping the collective knowledge of the BBS (mostly for my own education).

Even with newly machined and assembled heads, could the valves allow blow-by (blow-back?)? On a motor that has less the 1K miles, is it normal to see any carbon residue behind the intake valve?

I would expect to see fuel residue on the back of the valve, but with such low miles, could there be any carbon buildup behind/upstream of the valve, on the port walls?

Is it possible to set the valves too tight such that the valves are not seating properly at high temps, thus allow blow-by back behind the valve?

[jluetjen]

Souk;
Given that Chris's cams have a significant amount of overlap, I wouldn't be surprised to see carbon buildup such as you described. At least as long as it wasn't excessive.

[cstreit]

I'm going to post a picture later. Hadn't thought of that john. I may try and set the valves that have this (not all do) just a hair looser to be sure...

[911pcars]

"...could the valves allow blow-by (blow-back?)? ..."

The only path from the combustion chambers to the oil tank is through the crankcase (oil supply and breather hoses). The intake tract is on the other side of the engine and thus provides no pressurized connection to the oil tank. However, I believe it's venting to atmosphere in his engine. Besides, if a valve isn't closing for whatever reason, a compression/leakdown test would show that.

Perhaps a leakdown test might have revealed a compression loss path through the ring gaps, but combustion pressure are much higher than leakdown pressures and may not have shown much. As Werk I suggests, make sure the gaps are staggered during reassembly. You may wish to check/verify the piston-to-cylinder wall clearance as well (despite them being new) and make sure the rings aren't hanging up on their groove.

As an added suggestion, you may want to go through an engine break-in procedure once more.

Sherwood

[snowman]

Without leakdown, compression test data you have to many possible causes for this problem to be properly analyzed.

Note: Rings constantly rotate while the engine operates. Its unlikely they would all align, but possible. Even if they did it would only be temperary.

Could be a whole lot of things, even improper break in in which case NOTHING would look wrong. (unless you have a metalurgy lab at your disposal)

[cstreit]

I talked to too many shops (including Kelly Moss and Cox) that said unless I had a broken ring, leakdown wouldn't show anything, nor would compression... Compression I did because I wanted the guage.

The cylinder pressures in combustion are so much higher than in either test, they told me that neither were likely to show much.

All gaps and clearances were checked before the engine was assemebled, and all parts were new. I'm going to reassemble, reset the valve clearances, and hope for the best.

Since there are no overt causes (tolerances, breakage, etc...) there is little else I can do... Anything more would require some pretty extensive and expensive measures.

I will go through the break in procedures again...

[snowman]

Just in case the rings failed to seat properly buy a grape type hone (about $50) and hone the cylinders for about one minute each. Use transmission fluid for a lube. Dip the hone into a can of tranny fluid, hone for about 15 seconds redip and continue honing. Use a variable speed drill running at about 300 to 400 RPM and stroke it at about once a second. The hone will put a new finish on the cylinder that should allow the rings to seat. No significant material is removed so you don't have to worry about taking to much out. OR have a shop do this for you.

[john walker's workshop]

don't hone any "sil" style cylinders. the exposed silicone molecules will be removed in the process.

[cstreit]

JW, that's what I've heard... I haev Nikasil Mahle Cylinders. They only haev 500 miles on them, so I would guess they are still okay.

One question. Is it normal to see some vertical lines on the cylinder walls? I can see where the pistons are going up and down. I can't FEEL the marks, even with a fingernail, but I can see some slight wear. It's not the piston skirts, looks like it's from the rings...

[john walker's workshop]

kind of normal, as long as they are scuffs and not grooves.

[911pcars]

Chris,
Double-check your fuel delivery system. Excess fuel can wash away enough cylinder wall lube to create this wear.

Sherwood

[cstreit]

Sherwood... That would also coincide with the fuel smell from the case, so I've pondered that... but the plugs are clean above idle... Any way to really check that?

JW, they are scuffs, definitely not grooves...

[911pcars]

"Any way to really check that?"

On a chassis dyno with appropriate A/F sensors in the tailpipe.

On-board: Wide range O2 sensor and accompanying circuitry.

Running carbs? Check float bowl fuel level; verify correct fuel pressure.

Sherwood

[cstreit]

Well I know the fuel pressure is 4 psi, but haven't checked the float level though, worth looking into, thanks,