350 MAG MPI over heating

Black_Diamond

Had my 342 out for a run today. Ran maybe 5 min at cruise and the over temp alarm went off. Throttled down to idle, temp dropped from 180+ to 160F fairly quick. Stays at 160 unless I run at cruise then right back up. Impeller is new this spring. Ran fine all season. No sand injestion. Thinking thermostat?  No leaks. Fresh water cooled. Thoughts?  Got 1 week to fix it before I haul out. 

Black_Diamond

Thought I long ago answered this one. The tube going from the drive to the transom plate (new hose that year) got twisted during installation. Eventually it just collapsed or got blocked. The new hose kit has the hose end going to the transom as a solid piece so it cannot get twisted when installing. All happy now!  Plus new Jardin Marine SS water pump. 

MarkB

Yes check the thermostat. Might be stuck semi-closed. Did your old impeller break up or did you change it preventively. 

raybo3

Clean out the heat exchanger. Takes about 10 minutes to do. Take off each end cap and flush with hose. Trust me this works. I do mine 2 times a year.....

Black_Diamond

Mine are raw water cooled. No heat exchanger. 

raybo3

Oh I am sorry. Your post says fresh water cooled. 

Dream_Inn

Ray, I have yet to clean out my exchanger.  Do you just remove the nuts on the ends of the exchanger?  Then the entire cap comes off?

Black_Diamond

No problem.  Going to replace the thermostat and take a run and see what from there. 

LaRea

Sounds like you're pumping enough cooling water for no-wake speed, but not enough for cruising.  That's why the temps come back down quickly when you get out of the throttle.  

I forget -- does your boat have thru-hull water intakes?  Could this be the infamous Bravo 3 intake hose problem?

@Dream_Inn - yes, remove the nut and pry off the cap with a screwdriver.  There's a rubber gasket, but no gasket sealer.  Easy.

PartsMan

Could this be the infamous Bravo 3 intake hose problem? .... EXPAND please .. my boat does the same thing.  

raybo3

DI yeah just take the 2 end caps off. One bolt per side. You will be amazed at how little bit of debris it take to cause a problem. Use your finger to clean out the ends and then run a hose through the tubes. 10 min and done. Just reuse the old gasket. DB not so sure about the T-stat. I would think if its overheating because of the stat it would not cool down so fast.

Black_Diamond

My engines only pull cooling water through the drives.

212rowboat

How old is your engine circulation pump? 

rasbury

Well, Black, a subject I do have some experience with! First, the thermostat is a 165 degree part- it's not even going to open until it hits 165. My 350 MPI runs at about 170 at idle and then when I'm running I'm up a little over that mark but not sure exactly what the temp is but would guess 175/180. did yours always run at 165 all the time? I wonder if your gauge was reading correctly? You can always pull your therms and test them on your stove with a thermometer...unlikely you would have two bad ones in one boat I would think. If they are good then either like was mentioned above the pump itself or this issue with the bravo III and the pick up tube. My boat is fresh water cooled, it has a therm for only the coolant side of the system which is nice, I have two water pickups- a thru hull with a strainer and the pick up from the out drive. I have had serious issues with scouring in the raw water pump and I'm guessing that is where most of my cooking benefits are. Perhaps someone else will chime in on your temps but from my experience so far, and from what I have been told here in the forum, the temps would be withing the working range they operate in?

MarkB

Do you know what your water pressure is at say 3000 or 4000 rpm?

Handymans342

Whats wrong with 180?

rasbury

Handy- I don't think his gauge is or was working properly if it was showing running temp on plane of 160......unless he is at the north pole or something....

MarkB

My running temp at load (4000 RPM) is 160F.  At idle it is 155F. There's nothing wrong with gauge. I have higher flow going through my system (not sure why, but mercury marine says it's fine).

Black_Diamond

I think my gauges are correct. The port engine was hitting close to 200F. Remember I'm running in water about 65F right now. I'll have to get a scanner on it to pick up water flow. Can't have a trouble free season!  lol

LaRea

@PartsMan  - The Bravo 3 has an internal hose that brings cooling water into the engine.  At the point where the hose connects to the transom, there's a metal fitting that is notorious for corroding and reducing water flow.  You can visually inspect the fitting from inside the engine room, but to repair it, you have to haul the boat and remove the outdrive.

The typical symptom is that there's enough water flow to run at low to moderate speeds, but the engine will overheat above a certain speed.  When it overheats, if you slow down to idle, it will cool down quickly.  

The speed at which it overheats will depend on how badly the fitting is corroded.  Mine would run all day at 3200 RPM, but overheat within minutes at 3400 RPM.

Some boats are plumbed with a "dual pickup" system:  the Bravo intake hose plus a through-hull.  Those boats are less likely to have this problem. 

LaRea

Well, actually, ALL Bravo 3 boats (especially those operated in salt water) can have this problem.  But if the boat has dual pickups, the engine would just pull water from the thru-hull and you might never notice.

Handymans342

What if the boat has been in fresh water the whole time like BDs?

LaRea

Who knows ... I'm sure it would take longer, but corrosion happens in fresh water too.

Michael T

BD, I have seen exactly this symptom caused by water supply hoses that have lost their integrity on the supply side of the engine so that there is enough water flow at low rpms but the hose gets sucked in at higher rpms. Also have seen obstructions in the water exit lines (think burned flapper or impeller parts)(that have obstructed flow at higher rpms again causing rising temps. If your sender, thermostat and gauge are ok I'd look at the hoses.

Black_Diamond

MT. Good thought on collapsing hoses. My one other oddity I noted while changing my oil a week ago was the rubber boots connecting the riser to the exhaust tube going to the outdrive has a 'reddish' almost rusty color on the outside. Just the port engine which is the same one over heating. 

Michael T

@BD, that's interesting. If that boot was obstructed or collapsing and restricting water egress I'd expect some staining. Hopefully, that's it, as that's not too hard to fix. I have been there.

Black_Diamond

Pump housing looked good.  Impellor was not missing pieces.  Ran fine 2 weeks ago, was out running for an hour at cruise, at least.  No problems.

PartsMan

I replaced my impeller this spring with a new impeller plate and it would over heat on plane. put the old one back in and all is good. Now it seems to over heat every once in a while and the engine just dies when on plane.

Black_Diamond

Had the engine checked for any codes, nothing.  Pulled the water pump, impellor looked fine, they did find the blue plastic screw plug cracked and leaking.  Installed new impellor since it was out anyways and a new plug.  Betting I was pulling in air or losing pressure at higher speeds.  Could not get it to over temp in the slip, thermostat is cycling fine.  I'll run it this weekend and see if this solved the problem.

All of those blue drain plugs are original, should they be replaced at some point due to age?

rasbury

was that on the water pump? I have them as well for some fancy drain program that I never use.....just something else to keep an eye on!

rasbury

and I'd bet that was it - if your pulling any air, that's easier to pull than water...

Dream_Inn

I have bought those plugs in 5 packs and replace them when they start getting brittle or tough to put in.  On my prior boat, I had to drill one of them out because it got too brittle to screw out.  what a PITA that was!