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Old 05-20-2023, 07:21 PM   #1
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Unhappy Hesitation and backfire, start of season

Hi Everyone, Back with another engine issue.
This time was my first time out on the lake for this season.
Engine is a 1996 Mercruiser 4.3LH

Engine was performing fine for the first 10-15 mins. Then the hesitation started. Gradually got worse to the point where I couldn't even put it in gear. Same issue happens when I accelerate with just the throttle (without gear)

Tried the following:
  • Opened fuel cap to make sure it wasn't vapour lock. No change
  • Cleaned inside of distributor cap. Had a tiny about of lube/gunk around bottom. No real moisture, not concerned.
  • Starboard side manifold was pretty hot at the top. Port side was cold.
  • Engine temperature was reporting ~80c on gauge. Normal enough.
  • Waited 30 mins to see if heat was causing the issue. No real change in behaviour
  • Starts fine every time. Will idle around 830RPM. Occasionally dying on its own after 3-4 mins
  • Did get occasional backfire on lake when pushed too hard, to get it to go over 850RPM in gear was impossible
  • Coolant appears to be fine as its spitting out both sides when ran from cups/barrel in yard afterwards
  • Double checked spark plug cables and distrib cap

Couple of points to note..
  • It was in indoor storage for the winter..
  • Fuel was treated with stabil when winterised in 2022 and tank was ~75% full.
  • Oil dipstick is clear.
  • Only thing I changed was the starting solenoid but doubt that would cause it?
  • Someone mentioned it could be water in the tank. I doubtful as its been indoors in winter..

Any help or advice would be very much appreciated...

Thank you
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Old 05-21-2023, 12:25 AM   #2
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Will probably try pouring out whats in the fuel filter and check to see if there is any water tomorrow.. (Fingers crossed)

Any other suggestions welcome..
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Old 05-21-2023, 12:56 PM   #3
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Sounds like bad fuel. Is it Ethanol? If yes it can absorb moisture from the air.
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Old 05-21-2023, 10:18 PM   #4
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Thanks Mike,

Yeah its ethanol.. Probably 70% full of E5 in the tank from last year.. and added 20% of E10 just before we got out on the lake.

Kind annoyed because I added Red Stabil to the tank when I winterised it last year.. I didnt get round to checking the fuel filter today due to rain.. Will give it a try tomorrow.

If there is water in the filter, what do you recommend to solve the problem? Drain the 110L tank into other tanks or use something like Wynns dryfuel?

The country only brought E10 onto pumps in the last couple of months.. I may need to start adding some fuel treament (sigh)
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Old 05-22-2023, 01:19 AM   #5
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I prefer Stabil marine which is a very high concentrate and has additives for the marine environment. The red stuff doesnít treat much fuel so a lot of it is required. As long as phase separation has not occurred you can drain any water from the bottom of the tank since gas will be on top of it. If phase separation has occurred then the entire tank will need to be drained. You can try temporarily running on an external marine tank to see if the problem goes away.
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Old 05-22-2023, 10:29 PM   #6
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Tried a few more bits.... hopefully it will help to narrow it down....

Changed the fuel filter and poured into a glass container.. Looks normal, no water on the bottom after ~10 mins.

Tank is still 80-85% full of fuel. 60% of E5 and 20-25% of E10.. Added some Wynns E10 and headed to the lake for a test.

Started fine, ran for about 10-15 mins @ 4000-4100 RPM straight (no breaks) without any issue. I throttled back a little due to some wake on front of me and immediately got backfire, bogged down to a stall. Started each time but wouldnít go past idle RPM. Eventually it struggled to maintain its idle rpm and would die.

Brought it back home and immediately ran it on the barrel/cups. Got hesitation on acceleration exactly like the lake. I took the flame arrestor off and noticed some carbon on one of the jets. Sprayed it with powertune and the problem/hesitation went away immediately.. Put a good bit of powertune down both barrels in the carb and throttle response was excellent.. Once the powertune burned off it was back to hesitation especially at the first centimetre of acceleration. Then two big backfires when throttled.. (Nearly shat myself)

Could it still be the fuel? The powertune is essentially enriched fuel without ethanol?

Also thinking duration and possibly a heat issue? I did use a thermometer around the engine.. Seemed OK.. ~60c on starboard manifold, 38 on port and bottom block was 90-95c.

What do you think Mike?

Thank you again!
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Old 05-23-2023, 02:21 AM   #7
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Have you tried to slowly increase the throttle instead of a fast acceleration? This will tell you if the power valve is bad in the carburetor.
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Old 05-24-2023, 10:30 PM   #8
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Thanks Mike

Tried it out on the lake again tonight.. It works fine until it gets to a time period maybe 10-15 mins of use, where it just starts backfiring and the hesitation starts.. Once into this hesitation state, it just gets progressively worse.
Increasing the speed slowly at its peak worst, will result in a stall.

For easy wins, I replaced the distrib cap and rotor.. Minimal corosion on the inside of the cap bits.. Boat performed like dream for the first 10-15 mins and just as I thought it was fixed, it started backfiring at ~2000-2500rpm. Brought it back to the slipway, sprayed 30% of a can of powertune down the carb and got another 2-3 mins of perfect performance out on the lake before backfire/hesistation kicked in..

If you still think its fuel, Ill dump whats in the tank. Gutted its a complete waste of ~70 litres (my lawn is going to look great this season!).. I dont have a "boat safe" spare tank to try and its significant enough to buy one vs adding fresh to the current tank. So I might just siphon/store the fuel in a drum and refill it..

What do you think Mike?

Thanks again
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Old 05-25-2023, 01:17 AM   #9
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Based on this last post Iím thinking itís either the ignition coil or ignition sensor in the distributor. How long does it take before it recovers?
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Old 05-25-2023, 11:55 PM   #10
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Kinda difficult to answer how long it takes to recover... I left it for ~20 mins and it would get into gear but anything after that it would stall. I believe if I left it overnight it would get to a state where it would be fine for another 10-15 minutes the next day..

The coil is about 2-3 years old and I believe the sensor was replaced in 2018-2019 by previous owner...

What do you think? Try the coil first?
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Old 05-26-2023, 12:32 AM   #11
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Unless you have a way to see if the high voltage is breaking down then replacing the coil is what I would try.
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Old 05-27-2023, 06:49 PM   #12
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100 points to Mike.. It was the Coil. I swapped it out this morning and runs better than ever! Interestingly, I hooked the old one up to a multi-meter and it shows both points within standard OHM range.. So it must be faulty when it gets underload or hot.

Thank you again! Case closed
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Old 05-28-2023, 01:51 AM   #13
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Great news, as always glad to help. Enjoy the season.
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Old 06-05-2023, 08:48 PM   #14
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Hey Mike & Forum,

Just when I thought everything was fixed, my problem is back with a vengeance. I decided to keep updating this post as the symptoms were/are similar.

I got 2 days of full performance out of the boat when I replaced the coil.. No issues at all until today.. Was pulling a friend skiing all morning, no issues and out of nowhere, started getting hesitation on the first small split second of acceleration in gear.. Then over 5-10 minutes it just bogged down and refused to start.. After waiting 15-20 mins, it would start again and wouldnít go into gear resulting in stall.. 2 hours later back home, the engine wonít start at all on the cups. I did appear to get a miss fire and back puff through the carb on one occasion. Carb and flame arrestor have a bit of carbon build-up. Throttle advance makes no difference. Top Starboard manifold was hot, where I couldnít keep my hand on it for more than 5 seconds. Port side was lukewarm. Coil was lukewarm too. Drained the two manifolds using the plugs, to see if there was a blockage.. Little water came out starboard and alot from port side. Block was also full of water so no issue there..

We can rule out fuel as I've cycled about 75L in it since the issue has been fixed the last time.

Back of mind thought, the engine feels like it wobbles a little at low speeds? Could that be a missed spark plug symptom? Checked all ignition connections and they appeared fine? NGK Spark plugs were replaced in June of last year and honestly hasnít that many hours on them.

So what do you think is next to try?
My guess - Distributor sensor? Or spark plugs? Or something else?

Appreciate any further help and thank you again!
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Old 06-05-2023, 10:18 PM   #15
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Lets first determine if there is a cooling issue. Take temperature reading of the exhaust with an IR sensor. Check water flow to each exhaust by disconnecting the hoses from both and compare the flow with the engine at idle.
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Old 06-07-2023, 11:52 AM   #16
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Unfortunatley I now have an engine that will crank but not start.. It looks like it wants to start at the first split second... The engine is stone cold, it started once and struggled in one of my attempts.. I have the accelleration/throttle up a tiny bit.. I've never had trouble starting it before like this so it feels like a new problem.. I can hear the fuel pump in between starting and then the audible start alarm happens..

Any idea where I go next? My guess (not to sway you at all) could be spark plugs / timing sensor in dist / fuel pump / Oil pressure sensor?

Thanks :-)
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Old 06-07-2023, 04:32 PM   #17
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Hi Stephen I saw your PMs yesterday and have been chewing on it. Right now I would check base timing by setting the crank on TDC then ensure both valves are closed on cylinder #1, then verify the distributor is on cylinder #1. A bore scope through the spark plug will allow you to see if the valves are closed or you can remove the valve cover to see the both rockers are in the same position.
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Old 06-07-2023, 08:36 PM   #18
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Hey Mike, Thanks again for the reply and suggestions as always!

I took the valve cover off, got it nearest to TDC as I could for Cylinder #1. Both rockers were level. Snakescope showed what appeared to be two discs both closed.. Also did a compression test, nothing really changed since last tune up last year so happy with that. Changed the spark plugs and gap'd them to .045..

For kicks, I decided to try the old faulty coil which exhibited problems (after 30-40 mins of use).. and guess what?.......... to my amazement the engine fired right up first time!! without any issues at all.. Pumped throttle and no issues/hesistation at all..

Old faulty coil that worked was about 2-3 years old..
Coil that completely failed, was only 3 weeks old!

Any idea why it chewed threw two coils? I thought they rarely fail or am I just unlucky........ back to the shop I go..
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Old 06-07-2023, 09:36 PM   #19
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Provide a picture of how the coil is mounted. They do get hot and if not installed correctly or to close to a hot item they can prematurely fail but I’ve not seen one go this early. Also what is the engine serial number and part number of the coil you are buying, the wrong coil can also fail prematurely.
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