"Blown Engine" Update

Since it's near the end of the boating season here at lake Tahoe, I thought I would post an update on an earlier post regarding engine trouble with my 1986 Bayliner with AQ131A/275. In early June, after a nice 30 minute run to one of our favorite coves, the engine suddenly quit. We got a tow from TowBoat US back to our launch site. I strongly recommend the Boat US towing insurance service wherever it's available - it provided a free tow service for a normally $500+ tow.

Inspection with a boroscope in cylinder #4 revealed a near-complete disintegration of #4 exhaust valve crown and significant damage to the piston head.

When I originally posted about this problem here, the prevailing educated opinion was "it's a major rebuild - probably not worth it considering the age of the engine/boat." In any event, +$$$ ahead for a repair. I pretty much agreed and initially decided to sell it as a project boat. To make a long story short, I eventually decided to do a minimal repair to the cylinder head, do nothing to the piston other than grinding way any impact-induced projections, and see what happens. I did the following:

1. Replaced exhaust valve #4
2. hand-lapped all the valves that showed leakage during a water-retention test.
3. Since the cylinder walls of #4 were not visibly damaged, I simply cleaned and deburred the piston crown.

I reassembled with new gaskets etc. Compression test after the reassembly showed: #1: 135; #2: 130; #3: 130; #4: 120.
To my delight, the engine fired up after a prime and ran as smooth as ever. (!!) Here's a pic from the first day's "sea trial."

To date, we've have about 25 hours on the repaired engine and it continues to run well: no oil usage, no water or exhaust in the oil, fresh water clear and clean. Will the "good luck" last?? Time will tell. On a "cost versus benefit" basis though, I'm currently ahead. I added up the repair parts /fluids cost recently: $250.

I would never have attempted this repair if I hadn't gained tremendous knowledge and insights from the posters here. Thanks to you all for a great service!

Congratulations on a job well done! I lived in Meyers for about 8 years and loved taking my boat out onto Tahoe, but only if it was not going to be windy. I got surprised by the wind once while we were beached in a nice cove on the east shore about 5 miles north of Cave Rock. Once I saw that the wind had came up, we got out of there as fast as we could. Needless to say, it was a very scary trip back to the Cave Rock launch ramp in 3-4' swells in my little 17 foot runabout where I had to put all my 30 years of boating experience to the test. At one point, a swell came over the front of my boat! We made it back safe but humbled.

Congrats on your repair. Just want to mention I have two AQ151 engines which I completely rebuilt in 2015 for sale. They're in great condition in case you have problems (and if I still have them) I'm asking $1,500 each for them but will consider a reasonable offer.

djhinca, I hope that you replaced the exhaust manifold.
#4 cylinder can be problematic and typically relates to exhaust manifold issues.



And by the way....... great job on the repair!

RicardoMarine said:

djhinca, I hope that you replaced the exhaust manifold.
#4 cylinder can be problematic and typically relates to exhaust manifold issues. ..

Click to expand...

Ricardo, I was aware of multiple of your previous posts regarding the exhaust manifold and prior to removing it, I was expecting the worst. I was very pleasantly surprised when I examined it after removal - it was in excellent condition.


The pic's are prior to any cleaning efforts. I probed the water ports with a wire brush, flushed extensively with water, etc.; no signs off obvious corrosion. So, I reinstalled with new gaskets and so far, so good..