2008 Mercury 25 HP EFI 4 Stroke Flyway

[Woodcock]

Hello,
I have a 16' Lund with a 2008 Mercury 25 HP 4 stroke that began sounding an alarm usually when trolling at its lowest engaged speed over the last three years. To solve this on the water I turn the motor off and then back on and that normally "solves" it for a while. Sometimes it simply doesn't happen at all. My neighbor suggested reviewing the issue on this forum prior to bringing it in to the local marina for service ($150/hr labor). Any help would be greatly appreciated. Here are the details:

1. This happens usually at very low speed, trolling for salmon/trout, however it will also sound occasionally while under way at higher speeds. The alarm is unbroken and the red warning light is illuminated.

2. When the alarm goes off, the water being pumped out of the motor is pretty hot. Hotter than I would imagine is "normal" and certainly hotter than when the alarm is not sounding.

3. The last professional maintenance performed on the motor was in 2013 as I don't use the boat a lot (approximately 3 weeks a year). From what I understand it's not particularly good to not use the motor this infrequently but that's the way it is at the moment (young kids, etc.). Additionally, this is only used in fresh water, not sure if that's a consideration. I had the motor tuned up and the impeller changed (suggested every 3 years). The mechanic mentioned there was an obstruction in the water line that he snaked to address the alarm at that time (I was a little skeptical).

4. When putting the motor away this fall, I ran it "dry" with treated fuel. There was not a lot of water was being pumped out of the motor (spitting, inconsistent stream) and the alarm sounded a few times.

5. Based on research, I'm seeing the following: bad impeller, deposits in the coolant system, heat sensor in head going bad, alarm module going bad, loose wires, and intermittent thermostat. I imagine there is a valve or switch that releases the coolant water when the motor gets to temperature that might be the problem.

Has anyone had issues like this with their Mercury and can provide some suggestions on how to approach diagnosing the root cause of the alarm and perhaps an approach to fixing it? I have the original owners manual which doesn't really address fixing things and a SELOC Repair Manual at hand. I am not a mechanic but am not completely motor ignorant.

Much appreciated!

C

 

[Mohawkmtrs]

When you "ran it dry", do you mean running it out of fuel or running it without having water circulating through the engine?

Either way, it sounds like you need a water pump kit and possibly new thermostat.

 

[Texasmark]

In 2 stroke Mercs the tstat has a pellet stamped with the opening temp on the stat which is usually 143F. Running at slow speeds for long periods, the engine could be maintained at or around that temp. The temperature of a US domestic hot water heater in the mid (Normal) position is 140F for a reference number as to how hot the water would be. If the pee is plumbed off the exhaust manifold it doesn't tell you how hot the engine water is. Feeling the top rear of the powerhead is the best solution there.

Personally I would do as Mohawk said but ensure that a new tstat is part of your maintenance activity. It will be under a cap with 2 screws more than likely, either on the rear of the engine block, or on the top rear, possibly to one side. The water jacket surrounds the combustion chamber and the end of the stat sticks in that water jacket.

 

[bruceb58]

Go get it scanned and see if there are any codes set. Scanning it will also tell you how warm its running on the muffs. I detected a stuck thermostat even before I took it out and looked at it. I own my own Rinda scanner.

 

[ondarvr]

# 4 on your list indicates a bad/leaking head gasket, exhaust gasses are released into the water jacket and it over powers the water pump and reduces circulation intermittently. Check the compression.

 

[Woodcock]

Folks,
I completely forgot I posted this and just remembered while working from home. Here's the story, and it's expensive but might help those with a similar issue. I will respond to the follow up comment and then let you know what the problem is.

Running it dry - I run treated fuel (stabil) through the engine and then disconnect the gas line until the motor consumes the fuel and it stops. This is done while the lower unit is submerged in a tub with circulating water.

Here's what the issue is/was:

Maintenance #1 - Had the certified Merc mechanic do a complete tune up including combustion check etc. The motor showed use but nothing irregular. The impeller was missing 4 fins. This was PART of the problem and is due to the boat sitting idle for extended periods of time. Evidently, they dry out. So they did the complete tune up, associated checks, replaced the impeller, flushed the system of fin bits, etc. They confirmed the motor came to temperature and all was good.

FISHING TRIP 1 & 2 - I troll for salmon and trout primarily and the motor ran like a champ when TROLLING... then when I opened it up to run the water stream became weaker and intermittent and bingo the alarm would go off. This was really annoying as I just dumped a pile of money into it. This happened during two fishing trips and I considered it a safety concern as weather can sneak up on you at times and you need to get out Dodge in a hurry.

Maintenance #2 - I notified the Merc mechanic of this issue and returned the boat (last week). I asked them to "throttle only" and bring the motor up to a running speed to replicate the issue. After extensive research including them contacting their Mercury Product Contact they found that there's a grommet in the powerhead that has history of shaking loose and partially blocking the water hose to the power head in this Mercury model. In fact, they had a trade in motor that was experiencing the same issue that the previous owner could not solve. The mechanic mentioned they glue the grommet in place rather than just seat it as the manufacturer does so hopefully this never happens again.

Thank you for your follow ups!! I won't shock you with the charges.

Chris