5.7 Horizon Mercruiser getting a little too warm. Already replaced most things! GRRR

[newbienewb]

Hi Guys,

Merry xmas and happy new year!.

Cant seem to catch a break with this boat (grrrr).

Even though it hasnt gotten to the state where it is overheating, on the last few trips that ive had the boat out, the has been rising ever so slightly with each trip. My temp gauge always sits dead on half, and barely moves regardless of wether im idling or cruising. The last few trips it has slowly crept up to the mext mark on the gauge (which is the three quarter mark). I know there are a million threads out there on overheating mercruisers, and ive certainly had my fair share of overheating, but im a little perplexed, and hoping a mercruiser expert might be able to lend some suggestions on anything else that miht be a "common" issue to these motors. If i search for Mercruiser overheating issues, all i find is references to things ive already swapped out for brand new parts, less than a year ago.

It is a circa 2003 5.7 Horizon MPI 300HP. The boat also has a Bravo 3 outdrive.

I do all the mechnical work myself.

Last summer (when i bought the boat), it overheated badly, every trip, and within a number of minutes. I narrowed it down to the raw water pickup hose that runs through the transom from the leg. Bravoitis i think its called!. I swapped this hose out myself, and this immediately fixed thie issue. Each trip after that saw the temp rising no further than half way.

As part of me chasing that issue, i swapped out almost every other main cooling system part in an effort to find the problem!. So im not sure what it could now be causing my temp rise. Again - this time its not actually overheating. It will happily cruise at 3500 rpm all day and not actually overheat, but it sits higher than it normally does. If i come back down to idle speed, the temp will return to normal.

1) Thermostat changed this time last year.
2) Temp sensor changed this time last year.
3) Both manifolds and elbows changed this time last year.
4) Both exhuast flappers changed this time last year.
5) Exhaust Y pipe removed and all old flapper removed - again this time last year.
6) Raw water impellor and housing changed this time last year.
7) Entire cooling system taken off, flushed, and all old impellor pieces removed - this time last year.
8) Engine coolant drained and flushed this time last year (coolant level is fine).
9) Serpentine belt is tight, and not slipping)
10) Engine circulating pump has not been changed (i dont feel as though this is the issue)
11) Pickup hose running through transom changed this time last year.
12) Power steering and fuel coolers both removed and checked for blockage this time last year.
13) I have compression tested the engine and results are satisfactory.
14) Engine does not loose coolant.

Yesterday i pulled the raw water pump just to check that it hadnt deteriorated, and it was still in pefect condition, and has not hardened at all.

I run the boat in port philip bay only, and always flush fully for about 5-10 minutes are every use. I also start and run the boat (using earmuffs) each few weeks at home.

I am just about to buy myself a rinda scan tool, but unfortunately dont have it yet.

So - thinking about it

1) The guage could be wrong (very doubtful that it would be, but dont have a way of verifying)
2) Strangely, the charging system seem to be behaving oddly. When i start the boat it will sit at about 11.5 volts (reading at the guage), and will slowly get to about 13.9 volts over the course of about an hour). Something in the back of my mind tells me that last year when one of my batteries died, the temp gauge had a mind of its own. Anyone else experienced anything like this? Might just be a coincidence.
3) Ive chanfed out pretty much every common cooling system pain points with news parts in the last 12 months.
4) When i flish the boat, i generally get more flow out of the starboard side of the engine when flushing. This is inline with the port manifold being significantly warmer than the starboard. Reading a few posts about this last year, this didnt seem to be an issue.

SO - throwing it out there for thoughts and suggestions. Am i missing something obvious here?


Many thanks

Dave

[snatchy]

You have certainly done some work trying to fix the issue.

Here are a couple of my thoughts;

- The analogue gauges are not accurate. Hot to cold and somewhere in between is not ideal as a tool to diagnose temperature changes. A numerical reference is what you need so you know if it is really a few degrees hotter than last time, or if it it the gauge or just your memory. My analogue rpm gauge from mercruiser swings around and is unreliable, and when it is working is ~300rpm at cruise more than the digital smartcraft info.

- With reference to the differences between water flow when flushed from port to starboard, you could get a cheap infared temp gun (~$40) to check manifold or riser temps (or any other place such as heat exchanger) while underway or at idle to see if there are differences between each side. Another way to get actual temperatures is using a small aftermarket temperature sensors and digital displays. Have a look on ebay and search for "engine temperature sensor". They start at about $100

- If you plan on keeping the boat I would strongly recommend getting a smartcraft display. This can help you sort out such issues by giving you a lot of digital data from the computer. For example you would have numerical data (relevant to your current issue); raw water pressure, coolant temp, any faults, alternator voltage, voltage to engine, and some others I can't remember. Plus all the other engine parameters - fuel flow, rpm etc. You can get a 2nd hand navman or northstar gps display and the appropriate wiring loom.

- Is your raw water pickup through the leg only? That is what I gather from your post. It seems that later model mercruisers and particularly the FWC setups tended to have raw water pulled through hull by a serpentine belt driven pump. This setup would pull about twice the raw water volume of a leg driven impeller pump (compared to an Alpha drive at least). I'm just wondering if your raw water volume was always marginal with your setup. Have you tested water volume somehow? I know there is a test method available in a mercruiser bulletin type document.

Otherwise I don't have much to add to the cause of your overheating (assuming it is in fact getting too hot?).

All the best

[mal555]

- Is your raw water pickup through the leg only? That is what I gather from your post. It seems that later model mercruisers and particularly the FWC setups tended to have raw water pulled through hull by a serpentine belt driven pump. This setup would pull about twice the raw water volume of a leg driven impeller pump (compared to an Alpha drive at least). I'm just wondering if your raw water volume was always marginal with your setup. Have you tested water volume somehow? I know there is a test method available in a mercruiser bulletin type document.

Otherwise I don't have much to add to the cause of your overheating (assuming it is in fact getting too hot?).

All the best

I agree with your thoughts entirely, and the issue you explain in the above section was the first thing that came to my mind when the OP mentioned the Horizon, closed cooling option.

[The Woo]

Do yourself a favour and purchase a hand held infrared temp gun to clarify the real temp. No need to spend a million bucks decent ones sell via eBay for under $50.

[Fed]

I've used infrared guns a fair bit & found out they're not much good for measuring temperature.
Google emissivity.

They are good for comparison though when the surfaces are apples for apples.

You'd think if it's getting hotter the thermostat would open further & keep the temp at half as before.
Is the coolant going into the overflow bottle when it warms up and then getting sucked back into the motor when it cools down?