overheating while trolling

bashr52

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Aug 2, 2009
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Issue is on a 1994 2 barrel 4.3L.

I was out on the water last weekend and on the way back in decided to troll an umbrella rig just for kicks. After about 5 minutes at 2.8-3 mph and engine just off idle, I noticed the temp gauge was creeping toward the hot end, about 3/4 up the gauge. Pushing in the shift bypass knob and running the RPM's up slightly dropped the temp back to normal range within a minute or so. I just replaced the thermostat with a new 140 temp version this season, temp gauge usually sits between 1/4 and 1/2 during normal operation. Impeller was new last season. I have noticed the engine has the same tendency to run hotter than normal at idle on the muffs as well, but I thought that was just due to the last of good flow on the hose compared to being submerged in the water.

Is that normal operation at low RPM? Am I trying to troll to slow at low engine speeds? The only other I can do is get some trolling bags to slow me down and run at a higher RPM, or maybe re-prop?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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what was your RPM?

generally low speed over-heat is from a weak impeller or other partial blockage in the system.
 

garbageguy

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It's a '94, and the impeller had been replaced recently. IFF the impeller is working well, I'd be looking for partial blockage in the cooling system as SD mentioned. Maybe a hose is compromised, maybe collapsed or crimped? But crud in block passages and/or exhaust riser might be the culprit.
 

dingbat

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Do you have the correct thermostat? Some have a "bypass" hole. Some don't
 

Rick Stephens

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Definitely not normal. Generally speaking, boats have an unlimited heat sink in the water they move through. You should be able to idle basically forever without temps rising. I would agree that either you have a blockage or an impeller that is failing or faulty. Tough to know which without picking one to address. I'd probably get a new impeller kit and at least inspect the impeller to make sure it isn't damaged. Really doesn't matter how new it is. If you have a blockage, I would think you would have overheat problems under higher loads.
 

bashr52

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It should be the correct impeller, ordered for a '94 and it was a drop in and looked the same as far as I could tell. Hoses all look ok from a visual inspection, but the impeller housing was pretty crusty and nasty. I took some time to clean the whole thing out well before reinstall. Any main areas of concern that can be accessed and cleaned out?

RPM was around 850.

Funny just sitting at idle in the water temps remain normal, it was just idling in gear under load that caused the problem to occur.
 

achris

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May 19, 2004
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There may be something up with your impeller housing. You need to ensure its properly closed up and not leaking air in anywhere... I recently had a 1995 4bbl 4.3 doing exactly the same thing. There was nothing wrong with the impeller or the hoses, turned out to be a leaking elbow gasket. When were your elbows last checked?
 

TyeeMan

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Feb 27, 2006
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X's 2 on what Scott said. You might have still had enough crud in the impeller housing to eat up the new impeller. New Mercruiser impeller with pump kit and you should be good.
Double check the water supply hose between the drive and the transom shield, it is possible for them to kink in the down position if someone prior installed a new hose that was too long. Had a personal experience with that.
 

QBhoy

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When my impeller was on the way out..exactly what you have described was the big give away and tell tale. It would never really overheat, but went to around 165-170 odd at slow displacement speeds and would cool down to the normal 152 roughly when on plane.
As mentioned there are other variants on T Stats but that was my particular case.
My impeller on the Alpha takes a bit of a hammering with the boat running so high out the water at full chat. Thankfully I have the side exit exhausts which also let me keep an eye on water flow and volume.
 

bashr52

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I had an issue when I first bought the boat 3 seasons ago with a bad seal in the outdrive which was leaking gear oil. That winter I removed the outdrive and rebuilt it with all new seals and gaskets, including brand new impeller kit. Before the drive went back on I did the bellows, gimbal bearing, and water hose from drive to transom. If I can assume the water flow getting to the circulation pump is correct, could there be some buildup in the circulation pump that effects the low speed flow?
 

TyeeMan

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Any and all things being possible I would say maybe. If you were running salt water through the block the possibility could be greater.
Do you have power steering? If so pull the water lines off both side of it and make sure there's no sea weed or whatever else stuck in there blocking flow.
 

Rick Stephens

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I had an issue when I first bought the boat 3 seasons ago with a bad seal in the outdrive which was leaking gear oil. That winter I removed the outdrive and rebuilt it with all new seals and gaskets, including brand new impeller kit. Before the drive went back on I did the bellows, gimbal bearing, and water hose from drive to transom. If I can assume the water flow getting to the circulation pump is correct, could there be some buildup in the circulation pump that effects the low speed flow?

Apparently, you REALLY don't want to consider the impeller to be the problem.
 

dingbat

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Over heating while trolling in cold water is a pretty common problem.

Its caused by the large temperature differential at the thermostat that influences the temperature which the stat opens and closes. The higher the delta T the longer the delay.

Back in in my I/O days, going with a 140 stat and drilling a small by-pass hole helped the problem.

The stats for my outboard have factory by-pass. Still watch my water pressure gauge pretty closely to make sure the stats are opening on a regular basis once water temps drop below the mid 40’s.
 

QBhoy

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I think you really need to go back to the impeller, as advised. Honest. You’ve been quite lucky in that some of the real experts on the forum have chipped in with advice above. These guys know their stuff and widely considered amongst the best placed to advise.
Its text book impeller fault symptoms
 

bashr52

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Aug 2, 2009
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Just wanted to update this thread with my solution. I pulled the thermostat and there were not bypass holes. I drilled a set of 1/8 holes 180 degrees from each other and was able to troll as long as I wanted on Sunday with no movement of the temp gauge. Issue seems to have been resolved.
 

bashr52

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Aug 2, 2009
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How is it now at other rpms ?

Once the engine is up to temp it never moves. From what I have read the bleed holes not only let any air trapped in the system to escape, but allows some hot water to dribble onto the thermostat element and open a little quicker than having to wait for the whole thermostat to warm up and open.
 

Tom Maslar

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Jul 29, 2011
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You know, I had the same problem with my mercruiser 3.0--the cooling system was scaled out! I had to replace the manifold and elbow, which where defective..so I had to do it twice! I think your cooling system could be scaled out my friend.
 
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