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  View original topic: 72 FI smoke exhaust
yaknuts81 Sun Jan 31, 2016 6:03 pm

Last year, I burned an exhaust valve on cylinder 2 on a fairly new rebuild (~1000 miles). After parking the car for a while, and a lot of cursing, I pulled the engine and had two spare cylinder heads rebuilt (cleaned, blasted, new valves, guides, etc.). The engine is now back in the car with the rebuilt heads (that is all I changed after burning the valve) and I have a new problem--intermittent smoking exhaust.

The car idles fine and drives strong. I get zero smoke when I am idling and zero smoke when accelerating. When I let off the accelerator pedal, however, and let the engine slow the car I get quite a bit of smoke out the exhaust--so much so that people behind me change lanes or drop way back. The smoke is whitish in color.

I'm afraid the most likely cause is worn rings. Before going down that road and pulling the engine again, is there anything else that can cause this symptom and are there any tests I can perform to try to hone in on my new issue?

The car is a stock 1972 FI Squareback that was (best I can tell) running great before I burned the exhaust valve last year.

Donnie strickland Sun Jan 31, 2016 6:18 pm

You could do a compression test (warm engine, throttle wide open).

Is the car an automatic?

raygreenwood Sun Jan 31, 2016 8:56 pm

Smoke on deceleration only. ....is a classic sign of one of two things or a combination of both.

1. Very excessive case pressure in the rocker box pushing oil past the valve guides into the combustion chamber

2. Excessive valve guide ro stem clearance or worn guides.

If you burned a valve......did you figure out why?..... usually its incorrect valve lash......or in rare cases if the guides were not replaced and they had slop.....since they use the valve guide as a pilot for grinding seats......the seat area may have had a thin apot that caused a burn or may not have had concentric contact.

Did you have tbe guides replaced the first time. ...and did you have them checked or replaced when you had the burned valve replaced?
Ray

yaknuts81 Sun Jan 31, 2016 9:45 pm

Donnie: I plan to do a compression check next time I take it out for a spin. I have a MT.

Ray: I am not sure why I burned a valve the first time around. I had it built by a reputable engine builder/machinist who used a new crank, cam, new P/C, etc. and rebuilt my old heads (new valves & guides).

To have a failure after 1000 miles was devastating (the one I built myself in high school lasted nearly 20 years and I just followed the Muir book back then) and I suspected the culprit was simply a defective valve. It probably didn't help that I took the car on a long drive on a hot day including a couple steep inclines (Sacramento to San Jose). My old engine builder passed away, so I couldn't take it back to him.

When my new guy cleaned up the old heads, he noticed a crack internal to the 1/2 head and the guides were very loose. So I brought him two spares I had and he cleaned them up and installed new valves and guides.

Assuming my new guy knew what he was doing, that should rule out #2. Excessive case pressure would be from what, blowby? I should be able to detect that with a compression test, right? Can I still have excessive case pressure in the rocker box with vented (1972 T3) heads? My heads are the kind that are supposed to pull air from the air cleaner.

raygreenwood Sun Jan 31, 2016 10:03 pm

With any kind of venting on the rocker boxes there should nlt be enough pressure ro cause blow by past the guides.

Also.....it would be a rarity that a defective valve cause a "burning" of the valve. It can cause a head to snap off....valves to stretch.....a tip to crater. .....but burning is a valve to seat contact issue or a stress riser issue on the seat of the valve where it was ground to fit the seat.

It can happen if a valve is cocked and does not fully seat.....or is kept open from incorrect lash.....but usually that last one causes issues with running.
.
Lots of things to check. Now.....this is all if you are sure that its ONLY on deceleration.

It could be blow by from rings not seating. Run a compression and leak down test....of those are good.....its most likely stem to guide clearance. Ray

yaknuts81 Sun Jan 31, 2016 10:25 pm

I am positive that I see no smoke on idle and none on acceleration. It is only on deceleration and then it is very noticeable.

I'll run the compression test and see what that reveals. I don't have the tools to run a leak down.

I'd love to narrow this down to a cylinder head issue, but before I point the finger at my new guy I'd like to rule out other possible issues. I'll report back once I can check the compression on a warm engine, plugs removed, at WOT.

yaknuts81 Mon Feb 01, 2016 10:31 am

Compression test results are in, now I'll need some help deciphering and suggestions on what to do next.

Cylinder 1: 125
Cylinder 2: 100
Cylinder 3: 110
Cylinder 4: 125

These were all taken on a warm engine, all plugs removed, and wide open throttle. I also have pictures of the plugs--which don't look great considering they were all replaced when I installed the rebuilt heads (not even 10 miles ago).




A few other observations:
(1) I removed the oil bath air filter to access the plugs on the 1/2 side and noticed smoke/steam coming out of the AID. Is that normal or anything I should be concerned about?

(2) It also looked like the AID was pulling in at least a little oil, as I saw some oil residue on the intake elbow. Again, is this normal or of any concern?

raygreenwood Mon Feb 01, 2016 1:43 pm

Yes....both things are relatively normal. Steamy looking vapor coming from the intake manifold on a warm engine is usually fuel evaporating from the ports.

If its really exceasive you may be running rich.

Yes.....its common that a small amount of lil valor gets pulled from the oil bath air cleaner.

Check your oil level in the air cleaner. Id it was overfilled. ..mor if soke of the materials were missing inside this could. Be your problem. .

But just looking at your plugs...in my opinion .....the low variances don't support that its rings. I still think its valve guides. Ray

yaknuts81 Mon Feb 01, 2016 3:13 pm

Thanks, Ray.

I also noticed while driving that when decelerating, the engine didn't provide as much braking as I was used to previously. Also, when I give it a little gas and let off the accelerator, I notice that the RPMs take a little longer than usual to drop down to idle.

Looks like I'll have to call up my mechanic and tell him I suspect a valve guide issue on the guides he just installed. Should be a pleasant call . . . .

yaknuts81 Sat Feb 06, 2016 8:46 am

Probably no surprise, but when I talked to the guy who did my heads, he suggested it was the rings. He maybe has a point, because I very well could have damaged the rings and realized that was a possibility. Before I pursue the issue with him, I thought I'd pull the heads again and see if I can get proof that this is either a rings issue or a valve guide issue.

With the engine out, and the intake runners off, I see oil in the intake:




Am I right that this suggests a valve guide issue (unless I am really pulling in a ton of oil from the oil bath air cleaner)? If I had leaking oil rings, then wouldn't that only show oil on the exhaust side of the valves? Assuming my rings are shot, is there any way for oil to get from the combustion chamber "upstream" into the intake like this?

raygreenwood Sat Feb 06, 2016 9:16 am

yaknuts81 wrote: Probably no surprise, but when I talked to the guy who did my heads, he suggested it was the rings. He maybe has a point, because I very well could have damaged the rings and realized that was a possibility. Before I pursue the issue with him, I thought I'd pull the heads again and see if I can get proof that this is either a rings issue or a valve guide issue.

With the engine out, and the intake runners off, I see oil in the intake:




Am I right that this suggests a valve guide issue (unless I am really pulling in a ton of oil from the oil bath air cleaner)? If I had leaking oil rings, then wouldn't that only show oil on the exhaust side of the valves? Assuming my rings are shot, is there any way for oil to get from the combustion chamber "upstream" into the intake like this?

Yep...looks like guides to me. Also.....with rings.....its usually....not always.....burning oil all the time. Not just on deceleration.

Ask your head builder what he set valve guide to stem tolerance at. If you does not know....thats an issue.

Usually the best mechanics will pull the spec from the book....and use a small bore micrometer to check. The problem with using a small bore micrometer is that its accuracy is either directly related to feel....or in some cases to how stiff the ratchet thimble is.

An ACVW mechanic may start with a small bore mic.....but then do at least one quick test with the bare valve in the guide, a dial indicator and do the rock test at full lift. You sn do that at home if you have a valve spring compressor. ..and it will tell you a lot.

Also....if this did not tart rigjt away.....its possible you have big geometry problems with your valves and they wore the guides quickly.

Ray

yaknuts81 Sat Feb 06, 2016 9:58 am

I'll take these back to my head builder and have him take a look. If he is clueless or if I get a bad feeling, then I'll likely be out the money I paid him (because I may not want him to fix them) and will need to find a new head builder. If I get to that point, any recommendations in the SF Bay Area?

Regarding your question about this not starting right away, do you mean the engine not starting or this problem not starting? The engine fired right up away (first time once I got the timing set) when I installed these rebuilt heads, they have maybe 5 miles on them since then, and I have never seen oil in my intake like this before.



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