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Old 08-08-2017, 03:09 AM
Wrsteinesq Wrsteinesq is offline
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Effects of Broken #1 Exhaust Valve

My A4 was rebuilt by a reputable engine shop in 2011 and has been installed in the boat, which is on the hard. The whole boat project has been quite lengthy, and the engine was just installed last year. In trying to tweak the installation we could not get it to idle decently. The engine would die under around 1150 or 1200 rpm. Checked the plugs: Nos. 1 & 2 were somewhat fouled and smelled of fuel, while Nos. 3 & 4 were fine, that tan/cocoa brown color. Ignition tested fine, as did all plugs, so fire didn't appear to be the issue; and swapping the plugs for each other resulted in the new Nos. 1 & 2 (the old Nos. 3 & 4) becoming fouled and smelling of fuel, while the new Nos. 3 & 4 (the old Nos. 1 & 2) improved their appearance.

Thought maybe a stuck lifter, but pulling the valve cover, the valve train seemed to be functioning properly, from what we could see. The lifters appeared to be rotating. Compression testing yielded 105 pounds on Nos. 2, 3, and 4, but No. 1 wouldn't rise above 30 pounds, although briefly up to 60 pounds on application of Marvel Mystery oil. Uh-oh. Trying to feel for a problem through the sharp plug hole on #1, the 'weapon of choice,' an Allen key, didn't seem long enough to feel much; but using a longish pair of offset tweezers I could feel the No. 1 exhaust valve, and it wasn't solid but yielding and made a hollow noise.

Pulling the head confirmed a broken No. 1 exhaust valve, broken about an inch down the shaft. It was a new valve in 2011 and didn't have two hours running time. Not what I wanted to find, but better than broken rings or a hole in a piston or anything else that would require pulling the whole engine, I suppose.

I know what I need to do (pull the probably-mangled No. 1 exhaust guide; install new guide and new lapped exhaust valve) but am seeking confirmation of my theorizing regarding the fouling of No. 2 plug, which had fine compression and spark. It looks like No. 1 and No. 2 cylinders have separate exhaust ports, but appear to share a common intake. Since the broken No. 1 exhaust valve wasn't closing, it was never getting decent compression, but why the fouling on No. 2 as well? I'm guessing that with a broken exhaust valve, No. 1 was 'backfeeding' on its exhaust stroke and maybe pushing back against the intake flow to No. 2, through that common intake port?

One thing I did note was that the oil level was a little low, just above the 'add' mark, but there's no evidence of leakage. Maybe there's some connection here between that and the broken valve? Thoughts? Thanks--
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Old 08-08-2017, 01:16 PM
Dave Neptune Dave Neptune is offline
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W, your diagnosis seems spot on.

The fouling of 1&2 is mostly because they exhaust together and breath from the same end of the plenum so the fuel does not stay mixed with the air very well. #2 was probably not as "fouled" though.

Since you caught it so fast there may be no real damage in the guide.

Dave Neptune
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Old 08-08-2017, 08:58 PM
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romantic comedy romantic comedy is offline
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I am always curious as to why things happen. Why is definitely the question with the broken valve. My main concern is about the other valves and what may happen to them.
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