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

[Rosedeal]

I have a 2006 Monterey boat with a Mercruiser 4.3L TKS engine that after operating under a load for 35 to 40 minutes consistently experiences a gradual loss of power to the point where the engine appears to be starving for gasoline. When this happens, I back the throttle down to just in gear keeping the engine and propeller running. The engine never totally cuts off and appears to still be running smoothly. I am then limited to a 5mph crawl back to my dock. If I attempt to advance the throttle to give more gas, it will stall the engine. This has been occurring
I have had the boat into a couple of different marine repair shops over the last several years for this particular issue in which they have replaced multiple parts; anti-syphon valve, fuel tank hose, fuel pump, distributor, spark plugs, rotor, etc. and made several adjustments; throttle linkage, carb fuel mix, thinking they were solving the problem. But the problem keeps coming back.
I recently replaced the 2bbl carb with its same OEM remanufactured model, replaced the plugs, distributor and rotor. The engine, when cold, takes multiple full thrusts of the throttle before starting and then once warm purrs like a kitten and runs strong during sea trial up until the issue occurs. Then again, the issue starts with a gradual loss of engine power to the point of a noise from the engine that sounds like it's starving for fuel until I back off the throttle. The next day in the boat lift the engine cranks up OK and runs/accelerates smoothly.
After much research and reading many iBoats forum posts, I'm leaning towards a heat/extended usage related failing oil pressure switch since that controls voltage to the fuel pump. My question however is, can the oil pressure switch partially fail reducing the voltage to the fuel pump limiting the amount of fuel pumped to the carb or is it an all or nothing behavior if the oil pressure switch fails? Any differing ideas or advice would be greatly appreciated.

 

[alldodge]

Pumping the throttle on a TKS does things but if motor has sat long enough then carb is dry and pumping is doing nothing, but pumping air

Crank motor a few seconds before pumping the first time if it has sat for a while

Look at fuel pressure to the carb. Need a Tee fitting to do this but it may be loosing pressure

To loose pressure it can be from a antisiphone valve restriction, fuel line restriction, fuel tank vent being clogged and pump starts to draw a vacuum on the tank

 

[Cdn20valve]

I have a 2006 Monterey boat with a Mercruiser 4.3L TKS engine that after operating under a load for 35 to 40 minutes consistently experiences a gradual loss of power to the point where the engine appears to be starving for gasoline. When this happens, I back the throttle down to just in gear keeping the engine and propeller running. The engine never totally cuts off and appears to still be running smoothly. I am then limited to a 5mph crawl back to my dock. If I attempt to advance the throttle to give more gas, it will stall the engine. This has been occurring
I have had the boat into a couple of different marine repair shops over the last several years for this particular issue in which they have replaced multiple parts; anti-syphon valve, fuel tank hose, fuel pump, distributor, spark plugs, rotor, etc. and made several adjustments; throttle linkage, carb fuel mix, thinking they were solving the problem. But the problem keeps coming back.
I recently replaced the 2bbl carb with its same OEM remanufactured model, replaced the plugs, distributor and rotor. The engine, when cold, takes multiple full thrusts of the throttle before starting and then once warm purrs like a kitten and runs strong during sea trial up until the issue occurs. Then again, the issue starts with a gradual loss of engine power to the point of a noise from the engine that sounds like it's starving for fuel until I back off the throttle. The next day in the boat lift the engine cranks up OK and runs/accelerates smoothly.
After much research and reading many iBoats forum posts, I'm leaning towards a heat/extended usage related failing oil pressure switch since that controls voltage to the fuel pump. My question however is, can the oil pressure switch partially fail reducing the voltage to the fuel pump limiting the amount of fuel pumped to the carb or is it an all or nothing behavior if the oil pressure switch fails? Any differing ideas or advice would be greatly appreciated.

When the oil switch detects low/no oil pressure. it cuts power to the electric fuel pump. It is not gradual.

You mention that the engine is carburated. so it will not be guardian/limp mode.

If your fuel pump is electric. it could be getting tired. I would recommend installing a fuel gauge inline between pump and the carb. see what your fuel pressure is when the engine is running well. once the power cuts off, see if the pressure dropped.

 

[Fun Times]

Has the fuel tank vent system been checked? Confirmed air flow without blockage, vent hose routed correctly, etc.?

If you haven’t already.. you could try opening the boats fuel filler cap to see if you get a vent and the boat runs normally again.

Also next time out water testing, have a portable fuel tank with hose ready to connect to the engine to switch from the boat tank to the portable once you notice the issue getting near severe to see is symptoms change / improve.

 

[Rosedeal]

Cyclops222, thanks for the clarification on the oil pressure switch function.; Yes, the engine is carburated and has an electric fuel pump which was replaced in 2019 by a marine repair shop. Please clarify what you mean by the fuel pump "getting tired". Regarding suggestion for testing the fuel pump, it never cuts off. When the problem occurs, the engine just loses power gradually and when I back off the throttle it prevents the engine from totally stalling out. So some fuel is still making it to the carb providing the 5mph running capability. So my question here is, and here's the mystery, why is it consistently 35 to 40 minutes of running when the failure occurs?

Fun Times,
Where would the fuel tank vent system typically be located? I haven't seen any videos or discussions showing the location. I have heard about the fuel filler cap opening solution but never think about it when the issue occurs. That and the portable fuel tank idea are both good ideas. My question there would be, so if it does seem to solve the problem what exactly would that be pointing to?

Thank you both for the ideas and suggestions.

 

[Fun Times]

With temporarily installing a portable fuel tank you're separating the boat from the engine to help narrow down if you should be working on the boats fuel system from say the hoes connection fitting at the fuel separating fuel filter base assembly back to the fuel tank or work further upstream towards (or on) the engine to the Carburetor/other engine components... The fitting of mention would be in the general area of item number 13 of #12, https://www.mercruiserparts.com/bam/subassembly/31492/11812/60

If the boat (Engine) runs good with the portable tank then you have a boat side fuel issue that needs fixing...If the boat/engine continues to not run right, then you know you're working on the engine itself.

Same idea would apply if you believed there to be an electrical type issue.... Need to separate the boat system from the engine system...It's easier to do in a boat vs a car. Just be safely prepared to switch the 2 hoses back and forth before heading out.

Sometimes it's possible for the fuel pump wear down internally due to various reasons... The following link are some helpful ideas regarding checking the fuel pumps that MerCruiser suggests...

https://manualzz.com/doc/6390802/electric-fuel-pump-troubleshooting?p=2

For the fuel tank venting system, this varies per boat model as far as vent location.... Some are vented at the fuel fill cap assembly and others may have a vent sticking out the side of, 'or rear of the boat, etc..

If the vent hose happens to be at the filler cap, then if you're able to see the backside of the filler neck from inside the boat, then you may see 2 hoses running up to the filling cap.... One bigger and one smaller., https://shopwms.com/black-combination-gas-fill-and-fuel-tank-vent-for-boats/

Image search online your boat model, size, etc. boat fuel vent system types for a better understanding if needed... Just need to get some eyes on the fuel tank and looks for hoses and their direction and make sure things seem clear even if it turns out not to be the issue...You need to know for sure to move on.

https://www.mercruiserparts.com/4-3l-carb-alpha-bravo-0w310000-thru-0w

 

[Rosedeal]

All good information. However, my dilemma still is, which of the mentioned possible causes would affect the operation/performance from the get-go, meaning engine won't start, engine idles rough, engine will not accelerate, engine starving for fuel, etc. if there was a real issue with them. Can we narrow down to what elements would affect the operation based on the fact that the engine runs fine from startup until the problem presents itself 30 to 40 minutes later after running the boat under load? This would eliminate elements like the oil pressure switch which, as was said, is either providing power or not to the fuel pump therefore not influencing its functionality in my scenario. Question here; does an electric fuel pump vary in pumping flow as the throttle is moved forward and if so where does it get that additional voltage? Or is it just using 12volts to provide the fuel pressure at a constant psi and the carb solely provides the necessary fuel on advance of the throttle linkage?
I'd like to sort out, if we could, those possible causes that exist from the start and immediately impact performance against those that can surface at a later time based on equipment conditions changing. For example, if the fuel tank vent system is clogged that should impact operations from the start and/or during the first 20 to 30 minutes of running. But the engine/boat runs strong during that period of time.
So far I've tried the process of elimination by replacing the carburetor, spark plugs, plug wires, distributor cap, rotor, and ignition coil which greatly improved the performance prior to those repairs. That said, I'm not opposed to replacing elements like the fuel pump if someone says that's the most likely culprit for the issue because its performance could diminish due to run time and/or heat impact.
I will take a look at the fuel tank venting to see if there is an obvious issue there just to eliminate that from the mix. That may be a simpler start from modifying the fuel system to accommodate an external tank.
Thanks again.

 

[alldodge]

Fuel pump gets 12V full time when motor runs. There is nothing to give it more or less voltage

If the tank vent is clogged, as the pump sucks fuel from the tank a vacuum will start increasing until a point where the pump can no longer suck enough to move any fuel. The vacuum will start sooner the fuller the tank is. If tank is half full then it will tank longer to create the same amount of vacuum. Next time motor has issue, loosen the gas cap and listen closely for air being sucked in

 

[Rosedeal]

More good information. Period of time it takes to form a fuel tank vacuum based on fuel level makes sense. I just finished running the boat through its paces on the river for about 15 minutes and it ran flawlessly. Fuel level, best I can tell from the gasoline level console gauge, is between 1/2 and 3/4 full.
I was also able to photograph the top of the fuel tank showing the large fuel fill hose, the smaller fuel pickup hose, the 5/8-3/4in vent hose and the tank access cap where it looks like the fuel level sensor is located. I also took a picture of where the gas fill hose and the vent hose terminate at the deck fuel fill port coming into the boat (where the gas cap is located). Looks like I have a Perko or similar make of gas fill/vent and I assume the gas cap provides the venting. Am I correct? Not sure how I would check the gas cap venting efficiency. Any ideas?

 

[alldodge]

As mentioned before, run until the problem starts or at least long enough to be able to start and create a vacuum (if this is the issue).

Stop boat, get your ear close to gas cap and open it, do you hear air being sucked in?
That's about it

 

[Fun Times]

?, Did you check over the boat and open the fill cap, etc. before or after running the boat today? As it seems thus far, ‘The engine might be running correctly vs all the symptoms mentioned throughout this topic…. If it seems to hold running good past the time period, then maybe something was blocking the vent at the fill cap area on down to the tank and you changed the issue with disturbances… even spilled “gas” when filling sometimes could get into the vent hole/hose causing a type of blockage.

Thinking back, did this seem to appear say after last fill up?

Just talking out loud to see if anything rings a bell before heading further into more diagnostics..

 

[cyclops222]

You said a good 15 minutes? Original run times were 45 minutes to reach the problem.

 

[Rosedeal]

I found a good iboats forum thread here that has a clear explanation with pictures on how the combination fill/vent necks work. This might help someone, like me, understand what to look for on this possible issue cause.
My concern with the suggested manor of recreating the problem is I could get caught out in the middle of the river on a bad weather day or if the problem is due to another failure I may have to get towed.
I think so far we've narrowed the possible/probable causes to be fuel tank venting or gradual weakness in the fuel pump surfacing
Anyone want to pipe in on the odds of the fuel pump being the culprit? Is there a condition that would cause the fuel pump to work OK to a point, then lose adequate flow, then recover after a time only to repeat the same issue during another outing?

 

[cyclops222]

I had 1 die like that.

 

[Pmt133]

Anyone want to pipe in on the odds of the fuel pump being the culprit? Is there a condition that would cause the fuel pump to work OK to a point, then lose adequate flow, then recover after a time only to repeat the same issue during another outing?

Possible yes, how likely? Depends. Looking over post 1, you said the shop already replaced the fuel pump. It would seem awful odd to have 2 fail in the same manner in a very short time period, no? Or in this case, never solve the issue.

If you want to test the venting theory... remove the fuel line from the carb. Get a couple empty 5 gallon gas cans and put a rubber hose from the fuel pump to them and pump down the tank using the rest of the fuel system as is. This will both draw fuel much faster than a running engine would, and can be done on the trailer. It would show rather quickly if there is some restriction in the vent and you need not be on the river to test.... Just my 2 cents.

 

[Rosedeal]

FunTimes post reply; I did happen to open the fill cap as part of the vent system discovery. I wasn't sure how it vents until I read the aforementioned thread (see previous link) describing the cap structure. I am going to give the cap a good cleaning. Regarding the last fill up; since my boat is in a lift I usually put gas in from my 14 gal gravity feed wheeled gas cart. I don't often completely fill the tank unless I make multiple trips to the gas station to refill the portable gas cart; and haven't lately due to feeling the boat performance was unreliable and afraid to make any extended runs.

cyclops222 post reply; I plan to make an extended run once I clean the gas cap to see if that solves the issue.

pmt133 post reply; I agree that since the fuel pump was replaced in 2019 it should be unlikely that is the culprit. Based on my initial description of the lead up to the problem; gradual loss of power before noise depicting the engine was starving for fuel and backing off the throttle so the engine didn't fully cutoff, I included the electric fuel pump as a logical culprit. Regarding the fuel pump down test am I to turn the ignition key to on, not starting the engine, so that it gives initial power to the fuel pump thus generating the flow of fuel to the gas cans? In this scenario you are thinking if there is a venting issue a vacuum will be caused in the boat's gas tank affecting the amount of fuel flow into the gas cans, right? BTW, my boat is in a lift on our dock. And I like the idea of not having to be out on the lake to perform this test.

 

[Pmt133]

You could do it with the ignition on, you would have to jump the oil pressure switch in order to power the fuel pump that way, or just supply 12v directly to the fuel pump. Whatever is easier for you. I keep a jumper on hand to bypass the switch incase my oil pressure switch fails.

My thought is pump 5-6 gallons out then just crack open the fuel fill and listen for a rush of air while still pumping. Yes you should see the flow drop off as well but the sound of releasing a vacuum is going to be easier to hear than visually watching the flow from the pump changing in my opinion. That 5-6 gallons would be equal to 45 minutes of running around 3500 rpms in my 4.3. And would only take a couple minutes and you have fuel to return to the tank. Since you have a large fuel caddy just use that when empty to pump into.

 

[Rosedeal]

Well the mysterious loss of power is still a mystery. I was finally able to take the boat out for another sea trial since the river and weather here were calmer. Prior to that I was going down the suggested road that the issue may be related to a fuel tank vacuum issue. Since the boat has the gas fill/vent opening type of fill cap I checked the gas cap to see if that could be the problem. I cleaned it thoroughly and replaced an O-ring that was missing.
So today I took the boat out, it ran strong and then after about 15 or 20 minutes it starting losing power as in the past. Unfortunately, I went out about 3 miles instead of just running circles just out from my dock.
When the boat started losing power I backed the throttle down to just in gear and went back and loosened the gas cap. No swoosh sound like there was a vacuum and the boat didn't change behavior. This time the boat would not go any faster than 3mph so it took us a while to get back to the boat lift.
When I tried to advance the throttle the engine acted like it was going to shut down or stall. I did take the boat out of gear once and advanced the throttle. The engine revved up as normal so I didn't think there was a vacuum or fuel pump problem. But when I would put it back in gear the boat still acted like it wanted to stall out.
I was really hoping that the fuel tank venting was the issue. Sounded logical. But now I really don't know what to try next. Any ideas would be greatly appreciated.

 

[nola mike]

No mention about you checking your water/fuel filter?

 

[Scott06]

No mention about you checking your water/fuel filter?

Agreed - if the oring on the cap was sealing it what are the changes there is water in there ?