Mercury 25hp bigfoot overheating,

Lads this mercury 25 has had a new impellor fitted last year and worked fine, but since the start of this year it overheats,

It idles fine and there’s a good flow from the pisser, but when it gets a bit of throttle the flow slows down rather then speeds up needless to say but it then over heats,

Any ideas on this one gents,

In anticipation of your kind help I thank you,

Mike,

Does the engine seems very hot or are you getting an alarm. The Thermostat could be gone??

Kev

ha mike why dont you give derek in DDM marien a shout he is fantastic... pm me if you want his number.....

Not familiar with the engine (exchanger or direct cooled?) but from the symptoms I'd be suspecting a partial blockage thats being forced tighter when the revs push up the pressure. Can you try reverse flushing it with a hose? May not cure it but if there's any change in flow afterwards when you run the engine, its another clue.

If it is a blockage, maybe something got in, or it may even be a blade on your new impellor has cracked up. Did you fit yourself? If not, take it back to whoever did.

You say the problem started earlier this year. Was the engine left out in the Big Freeze? Does it possibly have a frost plug? If so have you checked it?

Hi lads,

Thanks for the info,
Sorry about the delay in getting back with more info on this problem, the chap who asked me to put this problem on here done so because I had told him how you all sorted out a few prob’s for me in the past,
Reason for the delay is he lives in Kilkee and I didn’t have a contact number for him but I met him today and have arranged to meet him tomorrow so he could give me more info on this,

Does the engine seems very hot or are you getting an alarm. The Thermostat could be gone??

Kev

Hi Mac,
I will ask him about the engine being hot, yes he is getting an alarm and has told me today that he checked the Thermostat and it seems fine but he has ordered another, I will know more tomorrow,

ha mike why dont you give derek in DDM marien a shout he is fantastic... pm me if you want his number.....

Hi Lucky,
I think that number will come in handy, if not for him then it might be for me, i seen a few post about the lads and they seem to know there stuff, so yes if you would be kind enough to send the number then please do, cheers.

Not familiar with the engine (exchanger or direct cooled?) but from the symptoms I'd be suspecting a partial blockage thats being forced tighter when the revs push up the pressure. Can you try reverse flushing it with a hose? May not cure it but if there's any change in flow afterwards when you run the engine, its another clue.

If it is a blockage, maybe something got in, or it may even be a blade on your new impellor has cracked up. Did you fit yourself? If not, take it back to whoever did.

You say the problem started earlier this year. Was the engine left out in the Big Freeze? Does it possibly have a frost plug? If so have you checked it?

Hi Hugo,
I will show him this tomorrow and let you know what he says, but he did tell me today that the impellor looked fine but as with the thermostat he has ordered a new one in case this one has got weak in some way,

Thanks again for your help with this prob lads, i will let you know more tomorrow afternoon, right now i have a fish to cook

I had the opportunity to speak to Cyril at Greene Marine Harolds Cross a few minutes ago about another matter, and in passing I mentioned this problem. Here is what he said:

If it is pumping at slow the impeller is OK though not necessarily 100%, and the thermostat is probably ok too.

I then mentioned this bit which was the part that intrigued me and the main reason I brought it up:

... It idles fine and there’s a good flow from the pisser, but when it gets a bit of throttle the flow slows down rather then speeds up needless to say but it then over heats,..

That sounds like it could be cavitation. Is he test running it in a tank or barrell? And does the water flow slow down both in neutral as well as in gear? If the problem disappears when in neutral with the prop stopped, cavitation is the cause.

There is also the possibility that the engine is still pumping under speed, but not obviously because the water is boiling and coming out as steam, having done it's job, and coming out as water when driving slowly. In other words the total flow is increasing as the engine revs up. (=no problem, or a weakened impeller, or a partially blocked water duct which is often dried weed fragments drawn in during last 45 seconds running at end of previous season ie they would have washed out the next day, but hardened over the winter. That is rare but he sees it about twice every year in customer's engines)

Hope this helps.

Lads this mercury 25 has had a new impellor fitted last year and worked fine, but since the start of this year it overheats,

The fact the problem started after a while would make me dubious about the cavitation idea which is generally related to the prop (unless of course the prop was damaged/bent recently). I'd still favour the partial blockage, and trying to back-flush it.

By the way, I'm sure your mate isnt putting a 25HP engine into gear in a barrel........

Well lads they just got it sorted,

I couldn’t come back with anymore info on this because I had to wait until I met the lads again on the slip as that’s the only place I seem to meet them,
I had already showed them your tips on what to check a few weeks ago, and they tried most things already but at the time they were waiting on more replacement parts,

Met one of them 2 days ago and he said they finally sorted it with the help from a chap that was repairing a fishing vessel’s engine, he found it to be a problem with the water supply pipe,
Apparently he came upon this problem before and reckons mercury’s are known to have this problem,

It was to do with where the water inlet pipe meets the engine, there’s a rubber that expands or contracts with heat and restricts the flow, hence the reason why it only happened while they were out long enough to heat the engine to the point where the rubber slowed the flow,
I told him to look on the bright side of it because now he has plenty of spare parts,

On behalf of the lads and myself I thank you all for the help with this problem, and who knows it may help other Mercury owners down the line,

Cheers,

Mike,

[quote="TubeNFish"]I had the opportunity to speak to Cyril at Greene Marine Harolds Cross a few minutes ago about another matter, and in passing I mentioned this problem. Here is what he said: quote]

Hi Norm,

I forgot to thank you for taking the time to help with this problem, I'm sure every bit of info on here will help me with my own problems as they arise,

It was nice to see one of the lads giving the engine a good workout in the Bay before i had to return home to earn more petrol money

Thanks again