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4.3 Mercruiser - randomly dies

R

RBN_TUL

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Need Help Diagnosing Stalling Issue – Tahoe Q5 with MerCruiser 4.3


I have a Tahoe Q5 with a MerCruiser 4.3L engine, and I’ve been fighting an intermittent stalling problem for over a year. I’ve had the boat at three different mechanics, and nobody has been able to permanently fix it.

  • Boat starts and runs normal most of the time.
  • Sometimes, not always, , the engine will randomly shut off after driving for 20 minutes to 1 hr.
  • After it dies, it will always restart immediately.
  • Once restarted, it will run for about 5-10 seconds and then die again.
  • Sometimes, after restarting dozens of times it will run normal again.
  • Some days, it will not have this issue at all.
  • The problem is intermittent and does not seem to happen under a specific set of conditions or after a specific amount of time.
  • I have taken it to three different mechanics that cannot figure it out.
  • New fuel pump install installed.
  • No ethanol in the fuel and fuel stabilizer used.
Video
Picture

Any suggestions or recommendations are greatly appreciated.
 
Loosen the fuel filler cap- if it starts and remains running, it could be caused by the vent being clogged.

Don't start an engine dozens of times, especially in a short time- you'll kill your starter.

How long did those mechanics run it?
 
jimn said:
Loosen the fuel filler cap- if it starts and remains running, it could be caused by the vent being clogged.

Don't start an engine dozens of times, especially in a short time- you'll kill your starter.

How long did those mechanics run it?
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They all took it out on the lake “for hours” and could not get it to replicate.
 
Can we see a steady picture of your 2 gauges?
The main wiring harness cannon plug with the zip tie sort of doesn't appear to be fully plugged in. with just the key on, engine doesn't have to be running wiggle and push in the wire harness and see if the gauges power on and off looking for a bad connection.

Your RPM gauge kind of looks like it surges as it's dying out... Coud be a failing tachometer gauge causing an issue...might try running with the gray wire disconnected at the back of the gauge to see if it helps any...Also look really close for spark leak at the high-tension coil wire to the distributor cap.

May also have an internal issue with the oil pressure switch that is shutting down the fuel pump 12-volt power before the engine dies out.
Item number 10, https://www.marineengine.com/parts/mercury_marine_parts/sn/1A080381/4211025US/13798_40

 
Start with the basics. Make sure both battery cables are tight & terminals are clean. If you have wingnuts on the terminals get rid of them & use self locking nuts. Make sure the battery ground terminal on the engine is clean & tight. Same thing with the positive lead from the battery to the starter. As mentioned check the big cannon plug connector make sure the terminals inside are clean & not oxidized.
Check the coil lead that runs from the distributor to the ignition coil, made sure that the terminals lock into the sockets tightly. A tight fitting boot can mislead you into thinking the terminals are tight when they aren’t. As mentioned you can try disconnecting the tach to see if a defective tach is grounding out the ignition.
I would carry a timing light on board. Next time it dies you can hook it up to the battery & #1 plug wire & crank the engine while holding the trigger & see if it flashes meaning you have spark or not. If no spark you have to further troubleshoot the ignition system if you have spark then check to see if you have fuel when you pump the throttle. If no fuel check connections for electric fuel pump.
 
louc said:
Start with the basics. Make sure both battery cables are tight & terminals are clean. If you have wingnuts on the terminals get rid of them & use self locking nuts. Make sure the battery ground terminal on the engine is clean & tight. Same thing with the positive lead from the battery to the starter. As mentioned check the big cannon plug connector make sure the terminals inside are clean & not oxidized.
Check the coil lead that runs from the distributor to the ignition coil, made sure that the terminals lock into the sockets tightly. A tight fitting boot can mislead you into thinking the terminals are tight when they aren’t. As mentioned you can try disconnecting the tach to see if a defective tach is grounding out the ignition.
I would carry a timing light on board. Next time it dies you can hook it up to the battery & #1 plug wire & crank the engine while holding the trigger & see if it flashes meaning you have spark or not. If no spark you have to further troubleshoot the ignition system if you have spark then check to see if you have fuel when you pump the throttle. If no fuel check connections for electric fuel pump.
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Just an FYI- the Coast Guard doesn't like wing nuts and even NyLock nuts can be a problem if they become excessively hot- the plastic ring can melt and stop gripping the stud. Nuts with a star washer or serrated head work best.
 
louc said:
That's a good option too. I use custom made clamps with bolted on battery clamps..
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I think someone should tell the battery manufacturers- I still see new marine batteries with wing nuts. Wing nuts are OK for low current but I have seen a lot loosen form vibration, so I usually tighten them with a small slip joint pliers.
 
Its not the battery manufacturers, is usually the individuals rigging the boat...wings nuts are 'cheap & easy'.

How many marine batteries have you seen, with the wing nut studs, that did not have an alternative connection method ???
 
makomark said:
Its not the battery manufacturers, is usually the individuals rigging the boat...wings nuts are 'cheap & easy'.

How many marine batteries have you seen, with the wing nut studs, that did not have an alternative connection method ???
Click to expand...

True, but the battery manufacturers could save money by omiotting the nuts. OTOH, that will give dealers something else to biatch about (as if they need something else).

They often have the larger posts posts too, but the threaded studs are great for adding low current accessories if the boat has clamps.
 
Enough on the Wing nuts on batteries..................They work perfectly when you have the right tool.

20260617_025028.png
 
louc said:
Why not market it? For sure that would make getting at the drain plugs a lot easier!
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Hahaha
Ya then Everyone would have one...

I may give it some thought. In the mean time I am able to breeze through winged plug removal/installation and tightening wing nuts on batteries properly!
 
Wasn't there a similar tool for the old-style radiator petcocks? They were kind of the same size/shape as the wingnut or drain plug.....
 
No, I made a small T handle about 2" long with the same end as the one in the picture. No need for additional attaching tools. Small and easy to use. Added a wrist strap incase It falls out of the hand
 
kghost said:
Hahaha
Ya then Everyone would have one...

I may give it some thought. In the mean time I am able to breeze through winged plug removal/installation and tightening wing nuts on batteries properly!
Click to expand...
As I mentioned, my small Craftsman slip joint pliers make it easy and I can go in from many angles in cramped compartments.

If I had a dollar for every boat I worked on for a no start problem with loose wing nuts......
 
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