Overheating at idle

M

MikeH

New Member
Apr 25, 2011
27
Chesapeake Bay
Boat Info
1989 200 CC
Engines
4.3l Mercruiser w/Alpha One Outdrive
I am having an issue with my boat overheating at idle or in drive at the lowest speed (650-800rpm). It is a 1989 4.3l w/Alpha One gen one raw water pick up. Sometimes it holds at 145 degrees, but most times it will continue to climb all the way to 210 degrees. I can immediately cool it down by revving the motor up to 1200rpm+. It operates just fine from 850rpm up to WOT. I have replaced the sea water pickup pump, the engine mounted water pump, the thermostat (cleaned the housing) and the manifolds. I cleaned out the passage ways on the risers as well. It is a 1989 and I have put a lot of hours on it over the last 5 years and it stays in brackish water most of the year, so all of those items were in the queue anyway (I have done the sea water pickup pump a couple of times in that span, I have never had one look bad, but replace it when servicing the outdrive). I ran it the other day and was feeling the risers and manifolds and it didn't appear they were getting really hot (I let the temp gauge get to 190-200 before revving). The manifolds were definitely warmer than the risers, with the port side riser being a little cooler that the starboard. I am wondering how to test the electrical portion of the system (temp sending unit, gauge, wire)? And if it is mechanical, if anyone has experienced something similar?

Thanks,

Mike
 
MikeH said:
I am having an issue with my boat overheating at idle or in drive at the lowest speed (650-800rpm). It is a 1989 4.3l w/Alpha One gen one raw water pick up. Sometimes it holds at 145 degrees, but most times it will continue to climb all the way to 210 degrees. I can immediately cool it down by revving the motor up to 1200rpm+. It operates just fine from 850rpm up to WOT. I have replaced the sea water pickup pump, the engine mounted water pump, the thermostat (cleaned the housing) and the manifolds. I cleaned out the passage ways on the risers as well. It is a 1989 and I have put a lot of hours on it over the last 5 years and it stays in brackish water most of the year, so all of those items were in the queue anyway (I have done the sea water pickup pump a couple of times in that span, I have never had one look bad, but replace it when servicing the outdrive). I ran it the other day and was feeling the risers and manifolds and it didn't appear they were getting really hot (I let the temp gauge get to 190-200 before revving). The manifolds were definitely warmer than the risers, with the port side riser being a little cooler that the starboard. I am wondering how to test the electrical portion of the system (temp sending unit, gauge, wire)? And if it is mechanical, if anyone has experienced something similar?

Thanks,

Mike
Click to expand...

if the risers are not hot to the touch when the gauge shows 190 - 200* i would suspect the sending unit is bad....they are cheap enough to replace...not sure if there is a sure fire way to test them....also check the ground on the back of the temp gauge....other than the things you have checked already a reduced flow of water through the transom can cause overheating...the supply hose can get pinched where it comes through the transom.....also an exhaust flapper can sometimes break off of its mount and clog or partially clog one side of the exhaust which can lead to overheating....

cliff
 
Do you have a "T" on top of your t-stat housing with two, extra hoses?
 
Check the water hose from the outdrive to the transom assembly for kinks. I had the same problem a few years ago on my V8 Alphas and after spending half a summer replacing risers, T-stats, thermostat housings and water pumps, I found that hose had a tight bend in it and was partially kinked. At low RPM it restricts water flow but at higher revs, the force of the water opens the hose enough to properly cool. I think Merc shipped the hoses about and inch too long, just trimming that off prevented the kink. Its a cheap thing to check and its obvious if the boat is out of the water. Just look at it with the drive all the way down and you will know if that is your problem or not.
 
another thing to check if this applies to you year boat is what kind of gasket did you use when you replaced the thermostat?....some (maybe all) mercruiser powered boats require a gasket that has a brass or some other metal rivet so that when the T'stat housing is reinstalled there is electrical continuity between the housing and the engine block...without this connection you can get errant temp readings on the dash gauge....

cliff
 
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Thanks for all the quick replies!

I will replace the Temperature sending unit as that is very cheap and check the ground on the gauge to start. When I did the manifolds in the fall, I inspected the flappers and they were in good condition and in good working order.

Regarding the water pump and the risers, I replaced both water pumps and I am curious, if the risers are not hot to the touch, could they still be the culprit?

The water hose from the outdrive to the transom is very interesting and I will check into that. It is in the water right now, so I can't check that immediately. Is it possible to remove the inlet hose from the motor water pump and check the flow rate there to check the outdrive impeller and outdrive hose or is this unsafe for the motor parts? And if that can be done, is there an amount of flow I should be looking for?

I definitely think an infrared temperature sensor is going to be purchased sooner than later.:smt001

As for the shape of the thermostat housing, here is a picture of the type I have:
images


It may be another week or two before I can do any more troubleshooting because it doesn't look like we're going to have favorable boat tinkering weather this weekend here in Maryland.

Again, thanks for all the replies!

Cheers,

Mike
 
Perfect. First thing to check (playing the averages here) is that ball/spring setup. Remove it and stretch the spring by hand - make it a good inch longer. When that spring gets compressed (less springy) over time, it leads to overheating at idle (only).

Yes, you could check water flow and this is the next place I would go (if a visual check of things reveals nothing). Remove the inlet hose from your t-stat housing. With a COLD engine, start the engine and direct the hose into your bilge or a bucket. Video tape it and post that. You're safe to run the engine for 30 seconds to a minute (probably longer), but play it safe. This is also why you start with a cold engine. There may be some 'specs' somewhere for GPM water flow for an Alpha pump, but I have never found anything definitive. Instead, I just go by experience and visually watching the rate of water flow.

Doing the water flow test will allow us to know if the problem is on the drive/supply water side or the engine side - narrowing down the variables. But the spring thing is where to start.
 
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Thanks Dennis,

I will start by stretching the spring and see what I get. It may be a bit before I get an opportunity to get under the motor cover again, not looking like a favorable weekend in Maryland.

Thanks again,

Mike
 
I had raw water pump impellers spin the brass hubs. I was using Sierra pump kits. I guess it happens because Sierra payed the engine repair.
 
I had a set of those impellers go bad a few years ago exactly the same way. Seems the bonding process that holds the hub to the impeller wasn't correct and they would slip over time. I found mine at end of season and engines were only running about 5-10 degrees above normal. I think it would have gotten worse in time.
 
Okay, sorry for the delay, but other projects forced the boat to the back burner, but I finally got somewhere yesterday. I tried stretching the springs, but that didn't resolve my problem...looked like they needed it though. I feel a little silly because it ended up being such a simple solution; I tightened all the hose clamps on the interior from the first hose connecting to the fitting that comes from the outdrive all the way through the cooling system. The two that were the loosest were the first two: the one from the outdrive fitting and then the other end of that hose where it connects to the power steering fluid cooler. None of them were actually leaking water, but I suspect it was pulling in some air and messing with the flow rate at idle. So, in conclusion, tightened hose clamps...problem solved. Thanks to every one who replied and hopefully this information will assist someone else in their diagnosis!

Thanks again,

Mike
 
That was exactly the same problem i had with an engine overheating. I watched the garden hose totally collapse when my wife tightened the hose clamp at the tansome.

Just saw this thread in the 48 hour search. Good catch.
 
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