Mercruiser 350 over heating at hign RPM

PartsMan

I’ve been dealing with a sporadic over heating issue for the past 3 years. The engine only over heats when on plane at higher RPMs. This year it’s consistent, which might be a good thing. Everything I have read it’s due to lack of water flow. This year I replaced the drive water pick up hose and the heat exchanger. Thought for you I had the problem beat, NO! New Mercmonitor shows coolant temp at 159* no wake zone on the river. Get up on plane and the temp quickly climbs to 170* putting the engine into limp mode. I’m in fresh water, but the boats first 5 years was in salt water (dock in a blackish river). What do I do next??

Engine:

2003 Mercruiser 350 MPI w/Bravo III (closed cooling)

What I have done:

Impeller (4 times)

New stainless plate for impeller

Belt

Thermostat

Removed risers and cleaned (not too bad)

Water pick up hose

Heat Exchanger

Removed all hoses looking for blockage

212rowboat

pull the boots off the exhaust and check the flappers atop the Y pipes...

raybo3

Hey PartsMan did you take the end caps off the heat exchanger and clean out the tubes? It only takes a very small amount of debris to cause overheating at high RPM.

PartsMan

@rowboat212 - I checked that a couple years ago, but an going to do it again.
@raybo3 - Heat exchanger brand new

What I've been reading it points to the exhaust manifolds. Is there any way to check them without pulling them off?  

PartsMan

@rowboat212 - I checked that a couple years ago, but an going to do it again.
@raybo3 - Heat exchanger brand new

What I've been reading it points to the exhaust manifolds. Is there any way to check them without pulling them off?  

Dream_Inn

You've mentioned hoses, I'm assuming you checked in the power steering cooler?

As far as checking manifolds/risers.  On my prior boat I took them apart at the manifold/riser connection to check them.  Just have a new gasket & spare bolts.  Mine were clean and easy to check.

212rowboat

riser and/or manifolds doesn't ring true... 

those things aren't supplied with a constant velocity/volume of water- it is dependent on RPM- either pump... the raw water or the circulation... if you're moving enough water at low RPMs to maintain temperature but are losing the ability at high RPMs, it's either obstructed or it's not pumping enough... 

these things don't have a heavy clutch fan hanging off of them like a car does, and that water pump bearing doesn't take abuse like a bad impeller because it's lacking that fan... the circulation pumps impeller could be worn to a nub and how would you know it?  most likely because it loses it's ability to move necessary volume of water at high RPMs.... 

check your engines circulation pump.... I wager it's worn... 

69fastback

170* puts it in limp mode?  I admittingly don't know much about the closed cooling engines, but I guess I assumed they'd run a temperature similar to a car. 

MarkB

Yeah I agree 170F shoud not be putting it in limp mode.  That's still well withing operating range. I would expect overheating to be more like 185 to 190F.

PartsMan

@rowboat212 - just replaced the impeller. I checked the engine water pump and there was to play

@69fastback - 170* goes into limp mode. I'm sure 170* wont hurt the engine but the temp is going in the wrong direction.

What if the exhaust manifolds are blocked by corrosion?

LaRea

Are you sure it's the engine coolant temp (ECT), and not the exhaust manifold coolant temp (EMCT)?  If the EMCT sensor goes above 212 deg F, it will put the engine in Guardian mode even if the ECT reads okay. 

Faulty EMCT sensor?  

reneechris14

2003 Smart craft you can read water pressure on computer.I chase mine for months till I found out .mine gets to 30psi now was 8psi.changed pump housing.but my guarding mode is 200 on temp gauge.which did cook of my shutters (flappers)

F1100

170 is not very hot to put an engine into limp mode.

i would start to get worried over 200

reneechris14

Pull hoses at the bottom of manifolds.should be like a garden hose even with your thumb on it.

rasbury

for sure on the temps, you have something else going on...mine is at 175 at idle and runs a little over that while on plane. 350MPI Bravo III. I also have closed cooling.

PartsMan

@reneechris14 - Mercmonitor is readings 0.0 on water flow/pressure
@F1100 - Thats what I'm thinking
@rasbury - maybe you are on to something

Maybe my water flow sensor is bad??

PartsMan

@rasbury - 175 on your gauge or smartcraft? My gauge reads 180* but the smartcraft reads 159*

rasbury

on the gauge....180 should still not be at over heat. I'm sure there are some good suggestions above to track down your problem- service manual could not hurt. No mechanic here, but if you have a warning alarm/limp mode, something electrical is making that happen and when you solve that-

Liberty44140

On my 06 350's with bravo 3's I read 180F on on both gauges. They come up to that while at idle and stay there on plane. The may come down a little on plane but not much and not always. I suspect that if I had smart craft I would see that are really 10 degrees or more cooler. Something i will look in to as I own the boat but with them matching now I am not concerned. 

Michael T

My 2013 350s and 2014 502s all ran at 170. Something else is causing the engine guardian IMO.

reneechris14

Should read about 8 at idle.up to 30 when you tac it up

Dream_Inn

Renee is correct.  I watch my water pressure on my VV and initial startup I get around 3-5 psi...then it climbs a few more for idle and around 20psi for me on plane.  If your at zero then that gives you somewhere to start.  This would be the time for them to bring out that Bluetooth smartcraft connection for you to use!!

PartsMan

Pump is good. My water pressure reads 0.0 on my monitor. Could it be a bad sensor?

raybo3

Bad sensor or bad wire from the sensor

rasbury

bad sensors does not cause overheat- if indeed it is overheating.

212rowboat

sensors are the only way it would know if it's out of parameter. 

LaRea

If the engine computer was seeing zero water pressure, it would issue a "Sea Pump PSI lo" alarm (constant horn) and put the engine in Guardian mode.

MarkB

If you truly had 0 psi out of the pump, you have no flow. With no flow, your temperature would be through the roof. You get a temperature of 170F.  My guess is that you have flow, but the pressure sensor is gone, so the system thinks there's no coolant, and that's what's putting it into guardian mode.

rasbury

I would agree Mark assuming the motor is not actually getting hot- can you take an hand held sensor and shoot it at some spot to know for sure? We are going back and forth on gauges and sensors and don't know what the real temperature is!

MarkB

He can put his hand on the risers, you'll know for sure!

Pam

Don´t know if this is applicable to your problem but have you had a look at this thread? In my case the water flow were restricted by a pipe that collapses as the speed of the incoming seawater flow increased in higher rpms. 
https://rinkerboats.vanillaforums.com/discussion/4039/are-my-raisers-running-to-hot-350-mag-mpi-2003-solved#latest