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Mercruiser Mag 350 MPI Horizon overheat

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Mercruiser Mag 350 MPI Horizon overheat

Old 07-04-2020, 01:13 PM
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Default Mercruiser Mag 350 MPI Horizon overheat

Looking for some hints on overheating problem that started the first time out this spring. Boat is 2001 Albemarle 248XF with Mercruiser 350 MAG MPI Horizon (closed cooling system with raw water flowing through heat exchanger.) Motor has 650 original hours. Risers were replaced two seasons ago as part of routine preventive maintenance.

The motor starts and runs at 175 degrees while cruising at 3,600-3,800 RPM. One hour, two hours, whatever. No problem. Shut it down to fish for 5 minutes or 20 minutes, or whatever, start it back up and temperature steadily increases until the temp alarm goes off. Either idling, trolling speed, or cruising speed does not matter. After shutting motor off once it will overheat upon restart.

After the first time my mechanic checked the Bravo 3 intakes for blockage then traced back to the heat exchanger where fine sea grass was found blocking some of the passages. Cleaned it out. No change.

Anybody seen this before? Thanks in advance.
Old 07-04-2020, 01:20 PM
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Thermostat bypass hole blocked?
Old 07-04-2020, 06:51 PM
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After it overheats , how long does it need to cool until it can be restarted and run at consistent 175* again , or does something else have to be done for that to happen or you get towed in? . Is there loss of coolant from both engine(( heat exchanger) and reservoir that has to be refilled? - Wondering if entrapped air in engine coolant might not be getting purged fully and that issue did not cause problem until it migrated at shutdown or got sucked in then.

Even though the temp gauge says 175* ( which may be a little high if the Merc T-stat spec is a little less than that) the RW cooling passages may be just restricted enough ( or pump flow just weak enough)so any additional fouling from weed catching on barnacles in hoses or just on the intake can change flow from just sufficient to maintain 175* to insufficient and climbing rate of temp.

Is coolant level in the reservoir and H/exch. correct , not too high and not too low? Pressure cap on tightly and the correct-pressure cap?

If you have a crank oil and/or drive cooler I would suggest back flushing them into a bucket to check for debris.

And check every RW hose clamp for tightness to prevent suction air leak to the RW pump .... if... it is engine mounted. FW coolant hose clamps too

* But 1st I would run the engine when it is warmed up but still in the staying at 175* at any rpm mode, ... and at idle and at cruise rpm underway try the simple classic test of putting your palm on the top of the riser/elbows to see if you can comfortably keep it there indefinitely.
Even though the gauge is in safe temp zone , if temps are hot on your palm it means your RW side of the FWC engine's cooling system has degraded due to restriction anywhere from intake to exhaust outlet, suction air leak or weak pump output. Now is the time to fix it before gauge temps elevate.
But if the riser/elbows are still cool enough when it is overheating, the problem is strictly on the FW side.

That test should be done often even when all seems normal.

FWIW- my Crusader FWC 5.7L MPIs have 170* t-stats, the highest spec temp of any stock gas I/B t-sat I know of . And unless my RW system has a problem and elbow tops are too warm, the gauge ( and direct from engine ECM ) temps stay between 168* and <175* , within 5* of t-stat specs. .

How old are the risers/elbows? And RW impeller & housing? Belt tensioned correctly?
Old 07-07-2020, 09:00 AM
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Originally Posted by TTaxi View Post
After it overheats , how long does it need to cool until it can be restarted and run at consistent 175* again , or does something else have to be done for that to happen or you get towed in? . Is there loss of coolant from both engine(( heat exchanger) and reservoir that has to be refilled? - Wondering if entrapped air in engine coolant might not be getting purged fully and that issue did not cause problem until it migrated at shutdown or got sucked in then.

Even though the temp gauge says 175* ( which may be a little high if the Merc T-stat spec is a little less than that) the RW cooling passages may be just restricted enough ( or pump flow just weak enough)so any additional fouling from weed catching on barnacles in hoses or just on the intake can change flow from just sufficient to maintain 175* to insufficient and climbing rate of temp.

Is coolant level in the reservoir and H/exch. correct , not too high and not too low? Pressure cap on tightly and the correct-pressure cap?

If you have a crank oil and/or drive cooler I would suggest back flushing them into a bucket to check for debris.

And check every RW hose clamp for tightness to prevent suction air leak to the RW pump .... if... it is engine mounted. FW coolant hose clamps too

* But 1st I would run the engine when it is warmed up but still in the staying at 175* at any rpm mode, ... and at idle and at cruise rpm underway try the simple classic test of putting your palm on the top of the riser/elbows to see if you can comfortably keep it there indefinitely.
Even though the gauge is in safe temp zone , if temps are hot on your palm it means your RW side of the FWC engine's cooling system has degraded due to restriction anywhere from intake to exhaust outlet, suction air leak or weak pump output. Now is the time to fix it before gauge temps elevate.
But if the riser/elbows are still cool enough when it is overheating, the problem is strictly on the FW side.

That test should be done often even when all seems normal.

FWIW- my Crusader FWC 5.7L MPIs have 170* t-stats, the highest spec temp of any stock gas I/B t-sat I know of . And unless my RW system has a problem and elbow tops are too warm, the gauge ( and direct from engine ECM ) temps stay between 168* and <175* , within 5* of t-stat specs. .

How old are the risers/elbows? And RW impeller & housing? Belt tensioned correctly?
You should have authored the service manual! I learned more from this reply than spending an hour wading through service manual. Got towed in so don't really know how long to wait before problem goes away. Risers were barely warm and could easily lay a hand on them for any length of time. Also, risers were replaced two years ago. AFAIK, RW housing is original. Impeller shows up once on the maintenance records.

Coolant level is normal. Cap is not leaking. Nothing changed since last season except running antifreeze through the RW side and testing coolant with hydrometer.
Old 07-07-2020, 09:38 AM
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Consider getting the point and shoot laser temp gizmo. When you make your way through a running engine, it's helpful to see the temp of the water going into and out of the thermostat housing, heat exchanger, all areas of the risers/manifolds, etc. Thermostats are easy to check, also easy to fail & not open properly. More than anything, I'd check that the impeller on the water intake/circulation pump is in good shape. Then the Heat exchanger. If any of those holes that circulate water are clogged, it'll keep it from working properly.

Also, if you didn't let it get too hot, that's good, because it doesn't take long for the head gaskets and gaskets on the intake manifold to get cooked and once that happens, all variety of other problems will occur.
Old 07-07-2020, 09:55 AM
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Stuck thermostat? Easy enough test for a couple bucks plus new coolant. Same note as above, my Mercruiser always ran within 5 degrees of the thermostat when the system was working properly.
Old 07-07-2020, 04:09 PM
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Sorry, you did say the risers were only 2 yrs old.

Since your comfy palm on the risers test with engine running during the overheat indicates the problem is likely on the FW side, a sticking T-stat as suggested above is a possibility but I'd think it wouldn't run at normal temps other times. Can't hurt to check it though. You could put it suspended in a pot of warming water with a candy thermometer on the the stove and check at what temp it opens and closes a few times. Don't boil it though . The T-stat is installed arched-dome side up, right? What is its temp spec?

Still wondering if after the hot shutdown , an air pocket might somehow be forming around the t-stat. Does your t-stat have a small air purge hole in the flange yet? If not, you could drill one. Or something is odd with the circ pump but usually one would either see leaking from the weep hole and/or hear some screeching. Odd that it is overheating with no loss of A/F and normal A/F levels..

Is there any chance incompatible A/F types have been installed. to cause some gelling?
Do you know what A/F to water mix ratio you have in there? What did the hydrometer say the coolant mix was protected to ?

When the engine has completely cooled down to ambient temp , or overnight, will it next run at normal temps again until shut down? Or overheat quickly?

What is the gauge temp when the alarm goes off?

https://www.mercruiserparts.com/bam/...1597/11581/180 No way to know if that diagram is for your serial number though.

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