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Mercuiser 4.3 popping, hesistation and cuts after 45 mins

C

cuan

New member
Hey Everyone,

Disclaimer: I'm a complete boating/engine newbie! however have a inquisitive mind and passion for technology!

I bought a wonderful boat with a 1997 Mercruiser 4.3LH/LHX 443LC00JT Engine. I get this strange issue where after 40-45 minutes I hear popping coming from the engine (Backfire?), then starts to give hesitation and will cut out under load. If I leave it for ~10-15 mins it will start again and get me back to the slip.

The last thing a mechanic changed was a new oil pressure sender switch. We ruled out a pump / oil pressure switch by bypassing it using a small wire.
I have posted this problem originally on another forum and feedback is it could be moisture in the distribution cap or possibly a coil issue.

Would anyone have any other insights or have experienced something like this before? Since I'm starting out on this venture I'm trying to gather as much help as I can. Would like to try fix this myself but without experience its difficult to diagnose and will probably end up purchasing parts I don't need..

Any help or advice would be very much appreciated.

Thanks Everyone :D

The engine was completely reconditioned by the previous owner.


  • 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.
 
Thanks Chris, Never thought of that.. We ruled out vapor lock as we kept the engine going for 45 mins without stopping... I fill the tank each time before we go out so it could also be something to do with the tank.. :D

Any more suggestions?
 
Probably not the coil. Not a bad idea to replace dist cap/rotor.

Sounds like its going lean/staving for fuel after running for awhile.
 
cuan said:
Thanks Chris, Never thought of that.. We ruled out vapor lock as we kept the engine going for 45 mins without stopping... I fill the tank each time before we go out so it could also be something to do with the tank.. :D

Any more suggestions?
Click to expand...

Not thinking about Vapor Lock. I was thinking about clogged tank vent.
 
Thanks again Chris. Might give the boat a try tomorrow and see if the loosening the cap works.. Do you recommend doing this from the top of the tank itself or the side fuel intake of the boat?

Also looking at purchasing a coil if it doesn't work.. Would this one be ok? https://marineparts.ie/engine-parts...underbolt-mercruiser-electronic-with-bracket/ .. Its not a sierra or quicksilver they seem to be the premium brand.. however it matches the part number 8M0079202.

Also any tips on replacing? or should it just be (disconnect battery first) then plug and play?

#LearningEveryDay! :)
 
Last edited:
Put your serial number in the mercruiser parts search here on marine engine and it will take you to the correct coil. Link is in the orange at top of page
 
I finally got some time to check out the boat in my yard with the cups.

I disconnected the vent tube at the tank and blew through it without any issues.. The port for the tank looked clean so I don't think its a clogged tank vent?

I also did the same with the tube from the tank to the fuel filter.. No issues with clogging there.. I took the fuel filter off and letting it drain overnight to see if any water shows up. Interestingly the O ring for the filter is larger then the inside ring size of the filter.. I think with a little packing it should go back fine.

So next up? Might try getting the distribution cap and coil to cover both.. Any bets on which one to replace first? .. Would like to try narrow it down bit by bit :)

Also took the flame arrest off the carb to see how it works.. Interestingly the flap at the top remains open.. is that normal? I didn't put it in gear.. only throttle..

Thanks again!
 
So who doesn’t love a happy ending to a sad story? I replaced the distribution cap & coil. The engine now runs without a hiccup! Thank you so much to everyone on here for all their help.

The finer details.. The coil looked pretty old and could have been the original with the boat? I tested it with a multimeter and it appeared to be within limits however doesn't indicated how well it performs under load or heat.. Second, the distribution cap and to my surprise (!?!?) had moisture around the edge of the rotor cap bottom. The replacement had a gauze layer to be installed to prevent recurrence and was missing from the original installation.

For anyone considering undertaking the same.. its pretty easy to change the two components. I took my time to learn ignition systems on youtube and how they work.. fascinating stuff!

I’m delighted.. Thank you all again! :cool:
 
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