Need help with diagnosing engine problem
 

Yesterday was our first launch this year and all went well till about an hour after launch. Was just coming up on WOT when all of a sudden a vibration started and the engine bogged down. Throttled back immediately and was able to limp back in to the ramp (10 miles up river) at 1500 rpms (anything above would setup a vibration). Oil pressure and temp remained normal. Oil level was normal and oil color was good. Determined cylinder # 5 was dead by pulling plug wires. Plug 5 looks good and has good spark. No backfiring coming through crab. Does this narrow it down to a bad exhaust valve or hole in the piston? I don't have the equipment to pressure test the cylinders but will see if I borrow/rent one later this week. Any thoughts that you may have would be appreciated?

Re: Need help with diagnosing engine problem
 

check prop for damage

Mike

Re: Need help with diagnosing engine problem
 

put your finger of the open spark plug hole. got any pressure when you crank it over ? a hole in the piston would surely produce no pressure..

did you just foul a plug ? or a bad wire ?

Re: Need help with diagnosing engine problem
 

First thing I checked, prop ok but changed to my spare to be sure, no difference.



Just finished a compression check:

1 180
3 15
5 10
7 175

2 190
4 175
6 180
8 175

Looks like I have 2 dead cylinders not just 1 as I suspected. What is the best way to further narrow down the reason for the low compression? I suspect the head needs come off, is the correct?
Your thoughts are greatly appreciated.

Just occured to me, could this be a blown head gasket between # 3 & 5?

Thanks

Re: Need help with diagnosing engine problem
 

My first thought would be, blown head gasket. But either way, start working your way down to those cylinders. Not to scare you, but it could be a cracked head. Engines are designed for the gasket to be the weakest link (for obvious reasons) but sometimes strange things happen. Are you planning on doing the work yourself, or having it done?

Re: Need help with diagnosing engine problem
 

I was thinking of doing the tear down myself and if it is just a head gasket I would do the reassemble as well. If it is more serious I would need to get it done.

Re: Need help with diagnosing engine problem
 

Well, good luck! It's not a terribly difficult job if you have the tools, patience, and a little know how.... If you are going to do one, might as well do both sides, and get the heads checked and valves ground. Hopefully you are lucky and it is nothing more than a gasket! Let us know if you need advice or help with anything. Don't forget to post your end result so that it may help someone else in the future.

Once again, GOOD LUCK!

Re: Need help with diagnosing engine problem
 

Started the tear down and this is what I found, blown gasket between 3 & 5. My concern at this point is the damage done to #5 cylinder wall. I know that it’s hard to tell in the pic's but what would you do? Give here new gaskets and closer her up or would you pull the engine and get the cylinder honed and new rings?

I bought a complete top end gasket set so will pull the starboard head as well but first I need to decide whether gaskets is all that is needed or do I pull the engine?

Any thoughts would be appreciated.

Thanks

Re: Need help with diagnosing engine problem
 

My 2 cents, it appears from the marks on the top of your pistons "0.030" that your engine has already been overhauled at least once before. Depending on how deep the score marks are, our may not be able to rebuild it again. At this point, I would buy a thick FELPRO marine gasket set (SS compression rings with blue gasket) put it back together and try running it. Do another compression test and if you have decent compression and don't burn any oil...you're golden.
Good luck!

Re: Need help with diagnosing engine problem
 

Thanks for the reply Steve. I had a mechanic come take a look at it and hed said the same thing. By looking at it he detected damage to #5 pistion and figured that the rings probably are seized on it. He thought that it might work just regasket and go but recommended going the long block route seeing as the motor had already seen work some time ago. He will get me some pricing and then I'll need to make a decision. This wasn't in my budget but nor do I want to do a patch job and more then likely have to do it all over again shortly. If I do it right the first time I should be good to go.