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Old 05-12-2019, 09:29 PM   #1
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Default Mercruiser 4.3LX Hesitation and loud click?

Hi Everyone,

I知 now the proud owner of a 2001 Maxum 1800SC (Engine is a 1996 4.3 LX/LXH ALPHA AND BRAVO (GEN +). Took it out for the first-time last Sunday and ran like a dream.................. until things got a little weird.

After a few runs with water skiing, I noticed some hesitation on full acceleration.. Eventually it mustered up the courage to get to plain and continued as normal for another 1 or two runs between skiing. Then hesitation occurred again and heard a click (backfire?) noise from the engine compartment it continued but stopped on idle.. took 1 or two attempts to get it going again however eventually got it back to the slip on low speed.

Previous mechanic (who was also the broker) asked me to check the fuel pump. Started engine with cups in my back yard with no issues in idle or in throttle/gear. With the engine off I could hear pump going like a flushing cistern . Next he asked me to bypass the oil pressure sender (pic attached) and it also worked without issue. He said it could be a coil issue and might need replacing.

History of the boat.. Previous owner hit the outleg and prop.. Had a complete engine rebuild and new outboard leg 2017. Got a mechanic to give it the once over before I purchased it.. Unfortunately, he didn稚 do a compression test but said everything else looked in order. The boat ran perfectly on the water during the two tests we did before purchasing and we did put it through its paces.

Previous work done before I bought the boat:
  • Fully reconditioned engine with New exhaust manifoulds all work completed 2017
  • Alpha one gen 2 drive. new gearbox, propeller, impeller kit, gimble bearing, drive & exhaust boots & control cable kit, new gauges fitted all work completed 2018.
  • fully serviced 2018.
  • New starter motor/battery/battery switch fitted 2019.
  • fully serviced 2019.

I知 tempted to try bringing it out on the water again to see if it reoccurs but a little apprehensive in case something goes wrong. I知 a complete newbie when it comes to boating and I知 learning fast as I go. Any advice would be appreciated.

Thanks everyone!!
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Old 05-13-2019, 10:18 PM   #2
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Do you have a mechanical fuel pump and a carb? If so, there may be a semi-clear tube running from the fuel pump to the carb. Not the fuel line. Trace this semi-clear tube to where it enters the carb, is there milky brown stuff coming out?
You'll need to remove the flame arrestor and look into the carb.
I'm thinking the cistern sound you hear could be a failed fuel pump.
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Old 05-14-2019, 12:14 AM   #3
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The need for an oil pressure switch it has an electric fuel pump. Backfiring is the sign of either timing or backpressure. Is this a salt water boat and how old is the exhaust?

Do you have the Thunderbolt V ignition? Timing should be verified.
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Old 05-14-2019, 02:07 PM   #4
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Thank you both..

It's an electric fuel pump.. It's freshwater only.. Exhaust manifolds were replaced about 2-3 years ago by the mechanic I believe.. Other than that I don't really know.. If there is anything I can do/check please let me know and I can send on some pictures..

I'm now not 100% certain it was a backfire... My brother just pointed out it could have been a life jacket ring hitting off the side of the engine cover. However, the hesitation and stalling was definitely there.. He also mentioned oil pressure and temperature gauges were normal. Thunderbolt V is written on the Flame arrestor. See pics of the engine attached.

What else can I try?
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Old 05-14-2019, 04:07 PM   #5
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How old is the fuel? Possibly water in it.
If you are just cruising around does this happen?
Sounds like the fuel may be vapor locking as the engine is hot from water sports then shut down to let rides on/off. When the engine is shut down the temps rise as no cooling water is flowing, this can result in vapor lock. Fixes have included a spacer under the carb to wrapping the fuel line. I think there may be a mercruiser service bulletin on this.
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Old 05-14-2019, 04:26 PM   #6
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Thanks again Mike.... That's two things you have helped me out with.. If you ever make it to Ireland IOU a pint! (beer!)

Don't know too much about the state of the fuel.. We did fill up the tank before we went out on the water and took toughly 74 Litres (I think its 100L tank?).... so thats 26L of fuel I don't know how long has been in the tank..

Quote:
If you are just cruising around does this happen?
It was fine for the first hour or so.. and it was started/topped 2-3 times before the hesitation started.. It started out of the blue when I was about to tow my brother and got progressively worse.

I've googled around to find out more on the vapor lock.. Some mention this shouldn't be an issue if its fuel injected or has a fuel pump on the line? Is this true? If you have a link to that mercruiser bulletin that would be great.

Thanks!
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Old 05-15-2019, 12:23 AM   #7
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Beer! I might have to swim across the pond for that!

I値l see if I can find the bulletin. Ignition coils can act up when they get hot but need to cool down before they can produce a good spark again.
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Old 05-15-2019, 01:13 AM   #8
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Don't shut it off between pulls... let it idle for a while.
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Old 05-15-2019, 02:15 AM   #9
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Coming at this from another angle. Check/replace the fuel/water separator. If fuel was iffy, filter may be "full" and passing a bit of water which can cause these symptoms.
Also, check the purple/white wire next to the carb an be sure it's not grounding out. It should not be connected to anything. It's used when setting the base timing only and if it grounds, timing will be retarded and can cause hesitation, etc.
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Old 05-16-2019, 01:32 PM   #10
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Thanks again everyone for all the advice.

Attached is the service bulletin from Mercruiser I found regarding the vapor lock. Is there anything a newbie can do to verify its definitely a vapor lock issue? I would like to take it back out on the water to verify (at the risk of using paddles to get it back to the slip!). I could leave the blower running on full time and see if it helps. I normally run the blower only when starting the engine.

I'm also tempted to buy a clear marine grade tube and install it between the pump and the carb.. That should show me if its an airlock issue if it contains bubbles or air? Also thinking about investing in a fuel PSI gauge that I could install permanently. That would help diagnose a fuel pump issue and also teach me some plumbing :-)

Quote:
Don't shut it off between pulls... let it idle for a while.
Great suggestion but I would be discouraged as I'm 110% committed to safety.. I would hate to think an engine could become a 200HP meat grinder for a skiier

@jparsons121 Thanks for those suggestions.. I'll give the fuel filter a shot. Is it considered OK to reuse the same one?.. I'll probably dump whats in it into a clear container, observe, fill with fresh fuel and rescrew the same one back on?. Or should I just buy a new one? It was "fully serviced in 2019" by the previous owner so I'm guessing it was also changed during that time. Will also look for that wire and isolate it with insulation tape if necessary.

I'm learning! :-)
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