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1986 AQ131 Engine Failure (?)

djhinca

Member
We recently experienced a significant engine problem on our 1986 Bayliner with AQ131/275. During our last outing, after running normally for approximately 20 minutes at 4400 RPM's (oil pressure, temperature, power, etc. all normal), we went to idle for a few minutes. When I reapplied the throttle, a loud grinding noise, following by a loud knock was heard, with smoke emanating from the carb spark arrestor. I immediately turned off the engine and we were towed back to port. I've been diagnosing with the aim of deciding how far to go with engine repair. Here's what I've determined so far:
1. No oil, or engine coolant leaks observed.
2. Removed valve cover - no obvious damage to cam or hydraulic lifters. No obvious wear or damage to the timing belt.
3. All four plugs look perfect
4. When turning engine over with plugs removed - a knock is heard on each revolution.
5. Compression test: Cylinder - 1 (130), 2 (130), 3 (75), 4 (0)

At a minimum, it looks to me like a significant valve or cylinder head problem. What concerns me however, is the loud knocking noise when running, indicating perhaps a valve having been sucked into the #4 cylinder? At any rate, given the ominous symptoms, I'm not sure doing a leak-down test would provide further definitive information. At this point, I'm considering pulling the cylinder head and seeing what that reveals.

I've learned a great deal from the expert commentary available here, so I'd be most appreciative of any thoughts, comments or advice regarding further testing prior to pulling the head. Thanks.
 
Re: "1986 Bayliner with AQ131/275. During our last outing, after running normally for approximately 20 minutes at 4400 RPM's"

...not generally conducive to long engine life... unless your particular engine is spec'd
to run up to 5500 RPM by Volvo.
 
I took some good advice from johnnygjr and put a boroscope in cylinder 4:

AQ131-cyl-4a.jpgAQ131-cyl-4b.jpgAQ131-cyl-4c.jpg

It appears the exhaust valve has been dislodged and has been striking the face of the cylinder, the source of the loud knocking sound observed. I'd appreciate opinions on whether the piston damage is bad enough that in addition to cylinder head repair, a cylinder repair would be advised (obviously requiring an engine pull..). Thanks.
 
Ouch your mext step is to pull the head and measure the depth of the gouge in the cylinder. If your lucky it less than .010 and the engine still has the stock bore. It is probably more cost effective to fing a good running used if your determined to keep the boat. In no way is it going to be cheap sorry for the bad news the head is probably shot as well?
 
Well, I got the exhaust manifold and cylinder head removed and I didn't expect what I found. The exhaust manifold for all four cylinders was in near perfect condition; no sign of any exhaust blockage issue whatsoever. These pic's tell the tale though: the face of number 4 exhaust valve has been completely shattered and the debris is embedded around the cylinder head crown and to some degree, into the face of number 4 piston. The cam mechanism appears to be in perfect condition, although I haven't rotated the camshaft yet to see if there is any interference. Any thoughts on how/why this might have occurred? I'm taking the head to a machine shop tomorrow for an opinion..Cylinder 4-2.jpgCylinder Head 3.jpgExhaust Manifold 2.jpg
 
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The cam is probaby good. You may be able to install new valve seats in the head but probably better off getting a good used head. How much damage was done to the cylinder walls? It probably sucked a bolt or nut down the carb to cause that type of damage.
 
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