overheat buzzer at idle

I recently acquired a 2002 Honda BF50 with about 200 hours on it. I had the impeller and thermostat replaced along with other routine maintenance items so that I'd be starting with a "clean slate".

After about 2 hours running time I am having no problems running at partial or at full throttle. However, after a few minutes idling, the engine will sputter slightly, followed by the red light coming on and the overheat buzzer sounding. The water continues to stream out of the telltale (quite warm) but once the alarm sounds I can only cancel it by shutting the engine off and waiting a minute or so (rather than by just increasing rpm's to pump more water through). If I go back up to speed after waiting with the engine shut off there is no problem but if I then begin idling again, the overheat will resume after a couple of minutes.

Any ideas? I finally just got this boat into the water and the nearest Honda dealer is a long trailer ride away....so I'd prefer to try what I can on my own before I load it all up again!

Telltale warm then you got a temp problem....probably lack of water circulation...

Wrong pump impellar...

Plugged cooling passages...

If there were plugged cooling passages and/or a wrong thermostat, wouldn't the problem show up even more at speeds above idle?

In the first two hours I used the motor I had two instances where I had to thread a wire into the telltale because the stream had been plugged. When the overheat at idle problem started, the telltale was working fine.

Would it be possible to have a tiny piece of dislodged debris lodge within the thermostat and prevent it from opening correctly? If so, would it be then a matter of just taking it off and having a look to make sure it's clear?

davidm said:

If there were plugged cooling passages and/or a wrong thermostat, wouldn't the problem show up even more at speeds above idle?

In the first two hours I used the motor I had two instances where I had to thread a wire into the telltale because the stream had been plugged. When the overheat at idle problem started, the telltale was working fine.

Would it be possible to have a tiny piece of dislodged debris lodge within the thermostat and prevent it from opening correctly? If so, would it be then a matter of just taking it off and having a look to make sure it's clear?

Click to expand...

Not neccessarily, if the pump impellar is too small then it will not act as a PD pump at idle, but maybe fine at higher speeds when the blades fold in and act has a centrif pump...

Same thing with the plugged passages, the extra head may cause the impellar blades to fold in rather than stay out straight at low speeds but at higher speeds the cetrif head may be okay....

However one would expect the tell tale to run at least a little hot at speed.

Of course inspect all the easy accesses ports to the cooling circuit...look for engine debris, sand, gravel, algae.....salt build-up (even if used in fresh water, some fresh water can be quite hard).

Ditto on advise from Shushwap. Often when an old impeller is changed out, pieces have broken off and lodge in the cooling system. You already indicated that could be the problem when you had to open the pisser with a wire.

Not that familiar with the 50, but there is a typical protocol for diagnosing overheat problems - from easy to harder...

1. Pull the water intake screens on the bottom of the lower unit, inspect, and clean. Look for any blockages inside. (It's amazing how often this simple item is overlooked.)

2. Pull off the cover and while the engine is running, thoroughly inspect all water hoses and connections for leaks.

3. Pull the t-stat(s) and test in a pot of water on the stove. Your owners manual should tell you what the initial opening temp and full open temps should be. Also, make sure the t-stats open wide enough, per the manual. (For the 225 beginning opening is approx 140F, full open is approx 160F, and open distance is 3 mm.)

4. While the t-stats are removed flush the water passages using a water hose with as much pressure as you can get. Typically I use a nozzle fully open, tightly wrap a rag around it, seal it into the t-stat opening by hand pressure, and turn the water on full force.

5. If there is a water relief port (usually a recessed hex head bolt), remove and install a water pressure gauge and take the reading at idle and check against spec's. If it is too low, you likely have an impeller or blockage problem before the port. It it is too high, you likely have a blockage after the port.

6. If all that fails, then drop the lower unit (as shown in the owners manual) and inspect impeller. To be safe, replace the entire water pump. It's not that much more than the impeller kit, and fairly easy to do.

You can download a copy of the owners manual free from hondamarine.com.

Thanks for all the tips, guys. Will investigate all as per your suggestions.

Now I'm really stymied! Opened up thermostat unit and found it to contain a 60 degree Celsius unit. Bought the Seloc manual which specifies a 52 degree unit for the BF50A....but the Honda dealer's computer specifies a 70 degree Celsius unit.

Removed the "old" thermostat, flushed cooling passages with a garden hose even though they appeared clean. With hose flow directed down one side of the thermostat housing, the flush water poured out around the propeller and from the other side a strong stream came out the telltale. Put the housing back together with no thermostat and tested the motor. It ran at speed and idled perfectly without overheating.

Purchased a 52 degree thermostat and tested it on the stove before installing. Installed with new gasket and ran at speed for 15 minutes and then idled in gear.....after about 5 minutes, the overheat light and alarm came on. Same old story!

Does anyone have a different brand of manual that might specify the required thermostat? To my way of thinking, the 70 degree thermostat would make things worse as it would close sooner at idle and speed up the overheating problem....but I hope I'm wrong. Any further ideas would be greatly appreciated!!

The one direction of flow on backflushing that you did not mention is how it flowed back out the water pickup tube that goes from the water pump to the engine. If there is any debris in it, the flow will not be good.

Before you take the lower unit back off. Remove the thermostat and cover and run the engine for a few seconds to see if there is good flow coming out of the thermostat housing. It should shoot up pretty good. If you only have a little coming out or a small stream, there is a blockage in the pickup tube or there about.

You can also connect the hose directly to the water tube (you may have to get a little creative)...with the thermostat cover off, there should be a really big stream coming out the top of the motor.

This also assumes that the correct impeller was installed and installed properly...and there are no restrictions to going into the water pump as was noted by a previous post.

My guess is...even if you run it without a thermostat, you will have the same problem. You could try it for test purposes, but the engine does have to get to proper operating temperature to run and idle properly.

The thermostat has superceded to the higher temperature. If you have the
19300-ZV5-043, then you have the most current temperature thermostat. It allows the engine to run cleaner.

The worst case is...if the motor has been extremely overheated at one time, the rubber grommet at the top of the water pickup tube (where it connects inside the powerhead) could have melted, hardened and/or partially closed.

If you get good flow back through the water tube and then everywhere else, the most likely problem is the impeller, cup, housing, and/or gaskets installed improperly. If they did not clean the surfaces completely, it could loose the pumping power.

One last thing...if you are trying to do any running of this engine on a hose, be sure that you rabbit ears are very very tight around the water intakes. Also, and probably more important, be sure to cover the intake that is on the cavitation plate just above the propeller. Duct tapek works good if you put if on when the unit is dry. If you do not cover this up, the impeller will suck air and the engine will overheat and the impeller may also get damaged.

That was a lot of repetition from before but there are a lot of things to check.

Hope that helps.

Mike

Mike

After all this, it is possible that the temperature sensor is bad...although I have never had to change one on any 50's that I have worked on.

Thanks, Mike. All my "testing" has been done in the lake (i.e. no rabbit ears required) Everything runs fine without overheating when I have no thermostat installed. One thing I neglected to mention is that when the overheat happens it is only when idling in gear, when out of gear and idling it seems fine.
I'm a little unclear as to how I test the water uptake from the pump without having the leg off and the pump exposed.... I sure appreciate anything further you can suggest before I haul everything out of the water and trailer back to the folks who charged me $500 to go over "everything" when I bought the boat!

Just a concluding message to this thread. Took it in to the dealer....they replaced the impeller and thermostat again using original Honda parts and the problem is gone. (Expensive) lesson learned!