2006 Monterey Boat with a 4.3L Mercruiser TKS Engine S/N 0w354897 Mysterious Loss of Power

[Rosedeal]

Regarding replacing parts before determining what the problem is; we did do test and thought we had determined the issue because the boat ran so much better than before the part was replaced. In all cases of parts that were replaced, they needed to anyway because either they had not been maintained/updated in several years, or they seemed to be the logical source of the issue; fuel flow or spark.
As mentioned, the boat runs much smoother and stronger as a result of the part replacements therefore I don't believe anything unusual was introduced to the issue.
You presented a good example of what I was referring to as to items that can be eliminated because either they work or they don't. The example of the TKS temp switch starting to short out causing a fuse to blow. In my mind if a fuse blows that an obvious cause and effect. By the way, where is the TKS temp switch? I'll put that on the list of things to check out.
Regarding the clogged fuel tank vent; that was an early on good suggestion and the test was to loosen the gas cap to allow more air to the vent hose. I have the vent system type wherein the gas fill component has both the fuel fill hose as well as the vent hose connected to it. When the most recent problem occurred loosening the gas cap was tried and made no difference.
Thinking about it, I go back to my comment that I tried spraying starter fluid into the carb when the engine quit running which should have helped start the engine. It didn't, so does that give anyone an idea of what may have prevented the engine from starting after it cut off and why it would not start with the introduction of the starter fluid? Does that lean toward a lack of spark?

 

[alldodge]

Temp switch is on top the intake, has 2 wires

 

[Rosedeal]

I searched several other forums to see what the function of the TKS temp switch was and I didn't see where its malfunction would cause the issue I'm having. Can you expand on why you think this could be involved? I realize the TKS temp switch reacts to heat and is in play during engine cold start but I'm not sure I understand how it has anything to do with fuel flow or spark.
Thanks for any education you can provide.

 

[alldodge]

I said

If TKS Temp switch starts to short out it might kill the power but when that happens it normally blows a fuse

Temp switch is connected directly to ignition power and if it starts shorting out it normally blows the fuse. If it just draws enough power to low the 12V supply below 10V and not blow the fuse this can cause spark issues

Again need to test

 

[Pigpenffx]

Had the same problem earlier this year on a 2007 Monterey 194 4.3tks, would run great for about 25 minutes and then start to die, backfire, and stall out. Had to motor back to the dock at idle speed and could bump up the throttle to for a bit before symptoms resumed.

What fixed it for me was a carburetor rebuild...turns out the gasket in the carb had dried out and was pulling in more air combined with a stuck throttle plate (the motor would diesel when turning off as well which was my big hint), in addition some bad fuel. Replaced the F/W Separator.

No idea if that is helpful or not. I started at the fuel pump as well, replaced it but it wasn't the problem, verified with a fuel flow gauge. They tend to be a working/not working thing and don't gradually start to go.

 

[Rosedeal]

Pigpenffx response: Thanks for your input. It's uncanny, your symptoms sound exactly like mine. On a side note, did your engine, after a short rest, always start back up and run fine for some period of time before it started to die again?
To recap FYI, here are the repairs I've made so far. New carb, new fuel pump, new plugs & wires, new distributor cap & rotor, new hose between fuel tank and F/W filter, new F/W filter and finally cleaned the gas cap and added a new O-ring to it. BTW, all these repairs/replacements were coming due anyway and were not done all at once, so it wasn't a case where no testing on each component was performed.
My question on the repair that fixed your problem; could there have been something else in the carb rebuild that fixed the problem rather than a dried gasket? Just trying to understand the mechanics of why a bad gasket pulling in more air would cause a problem after an elapsed time rather than all the time. And what caused the throttle plate to stick? What would be the variable impacting the gasket; heat, contact with the gasoline, etc.? As mentioned above, I installed a new carb so I would hope any internal gaskets would have been new too. And do you remember which gasket in the carb was bad? My throttle doesn't seem to be sticking but I noticed if the idle is set too high giving too much RPMs, it will diesel at times.
At this point I'm willing to check everything.
Lastly, there was an earlier suggestion for me to check the TKS temp sensor switch since the theory was that as the engine heats up that switch, if failing during high heat due to the load which in gear, it might somehow be causing the same symptoms you and I are/were experiencing.
I had some time the other day and wanted to test the TKS temp sensor heat theory assuming, does it matter if the engine is in neutral or in gear to generate the heat needed to cause the switch to act negatively. I ran the engine in neutral at between 1000 and 1500 rpm for 35 minutes and the problem never surfaced. So my question there is, is there a range of heat that the switch needs to lead into a fault? And what would that fault impact?
I looked at the diagram in the previous posting and I'm not sure where the TKS temp sensor comes into play affecting voltage to the ignition system. Could someone address that for me? Thanks for everyone's input.

 

[Pigpenffx]

Time is what caused my carb gaskets to crumble, I attributed it mostly to the gasket at the base of the carb where it connects to the intake manifold, atleast that’s what the smoke test showed but they were probably all bad to some degree that one just was the worse. Throttle plate was stuck with varnish from old gas the last owner left in over winter untreated. It needed to be rebuilt anyways so I didn’t bother trying to clean it attached to the motor.

It did tend to run okay back on the hose, yes I could start it up again and it ran fine. I did have an issue with the throttle being stuck as well, specifically the throttle cable not moving all the way or properly due to a loose remote. I don’t attribute that to causing that particular engine issue, but it’s likely on some occasion that the cable wasn’t moving all the way and would engage the shift interrupt switch and not disengage it fully or hold it a little longer than necessary, this was more noticeable if I was running for a while and throttled back to neutral, the engine would shutter and sometimes die, but would restart quickly and idle well. This did also cause a high idle (1200 rpm) since the throttle cable never fully returned to neutral. May want to give that a once over if you think the throttle may not be adjusted correctly.

I’m having a hard time thinking of anything else, given you’ve replaced the fuel pump, carb, plugs, wires, distributor, presumably you’ve checked the spark and ignition coil, maybe check the timing if all else fails? Easy enough to do and the only other motor related thing I can think of though I’m sure more experienced members have suggestions

I did have to pump the old gas out of the tank as well, was about a quarter that overwintered from the last owner, check the anti siphon valve?

The TKS module/diode is a piece IMO, I’m not sure what problem it solves. Mine hisses and has gotten stuck sometimes, probably need to clean or replace it. I don’t understand the system fully it doesn’t get noticeably warmer for me touching it with my hand on the back left of the carb and the metal part it funnels air through remains cool so I assume it’s not disengaging all the time, I’m not sure how to check it fully, but it doesn’t seem to affect the running currently. Honestly had I really thought about it I’d probably have gotten a TKS-less carb, capped off the connector and not worried about it…it’s not like pumping carb is hard to remember to do.