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  1. #1
    Join Date
    Aug 2017
    Location
    Lake Tahoe, California
    Posts
    27

    Default 1986 AQ131 Engine Failure (?)

    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.

  2. #2
    Join Date
    Apr 2016
    Location
    barnegat NJ
    Posts
    3,144

    Default Re: 1986 AQ131 Engine Failure (?)

    get a camera in spark plug hole and take a look

  3. #3
    Join Date
    Jun 2008
    Location
    Island Heights, NJ,
    Posts
    2,921

    Default Re: 1986 AQ131 Engine Failure (?)

    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.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  4. #4
    Join Date
    Aug 2017
    Location
    Lake Tahoe, California
    Posts
    27

    Default Re: 1986 AQ131 Engine Failure (?)

    I took some good advice from johnnygjr and put a boroscope in cylinder 4:

    Click image for larger version. 

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    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.

  5. #5
    Join Date
    Apr 2016
    Location
    barnegat NJ
    Posts
    3,144

    Default Re: 1986 AQ131 Engine Failure (?)

    Sorry to hear the bad news, Best of luck with "her"

  6. #6
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    5,049

    Default Re: 1986 AQ131 Engine Failure (?)

    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?

  7. #7
    Join Date
    Aug 2017
    Location
    Lake Tahoe, California
    Posts
    27

    Default Re: 1986 AQ131 Engine Failure (?)

    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..Click image for larger version. 

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    Last edited by djhinca; 07-29-2018 at 12:49 PM.

  8. #8
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    5,049

    Default Re: 1986 AQ131 Engine Failure (?)

    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.

  9. #9
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,576

    Default Re: 1986 AQ131 Engine Failure (?)

    Any thoughts on how/why this might have occurred?
    Ayuh,..... Most likely detonation from a lean condition,......
    Any Grease is Better,... Than No grease at All....

  10. #10
    Join Date
    Aug 2017
    Location
    Lake Tahoe, California
    Posts
    27

    Default Re: 1986 AQ131 Engine Failure (?)

    Quote Originally Posted by kimcrwbr1 View Post
    .. How much damage was done to the cylinder walls? ..
    Both visually and by the "feel" tests, the walls of all four cylinders are perfect. Unfortunately, the same can't be said for the #4 piston crown:

    Click image for larger version. 

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  11. #11
    Join Date
    Apr 2016
    Location
    barnegat NJ
    Posts
    3,144

    Default Re: 1986 AQ131 Engine Failure (?)

    Been there seen that!!

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