Honda 20 reports overheating and stops after 30 sec or so

I have 2 2007 BF20D3 with extra long shafts on my catamaran. I am having trouble with one engine alarming(overheating) and consequently stopping, when it can be used but not at load. The engine has had salt remover through it and has had the cooling manifold cleaned and the thermostat and the temperature sensor changed despite no fault found. It has a new water pump and good flow through the telltale but still alarms. When it alarms the block and sensor temperatures are not discrepant to the other engine running normally. Does anyone have any ideas???

Hi Cicala,
Sorry you're having trouble.
This sounds as if it's a probable electrical concern. I mean, you seem to have established that the engine is NOT actually overheating. Correct?
If so, then this may simply be a result of a poor connection between the thermo sensor and the CDI unit.

The temp sensor (thermo sensor in "Hondaspeak") is a two wire sensor connected to the CDI via a red wire with blue stripe and a green wire with a black stripe.
The CDI will illuminate the red indicator lamp and sound the alarm buzzer when a particular resistance threshold is reached by the temp sensor.

If there is a bad connection between the sensor and CDI the resultant resistance could signal the CDI to warn the operator.

The marine environment being harsh for most metals, bad connections at component terminals is quite common. Sometimes, simply unplugging a connector and plugging it back in is enough to knock off oxidation and restore the contact.
But, for a totally reliable connection, the terminals should be cleaned of as much oxidation as is reasonably possible. The use of a quality electrical contact cleaner such as De-Oxit can help restore pin to socket contact for a long term solution.

Another reason for poor pin to socket contact is vibration or damage to the connections. Vibration causes the female components to open up slightly and contact is worsened or lost altogether. The same might occur if the connector has been struck accidentally or pried on in some way. So, close inspection of pins and their socket mates should be done while the two are separated.

Another possibility is a damaged or corroded wire. This happens more than most of us realize. Corrosion can attack any wire in the salt water environment and, where connectors are crimped on to the wire ends this is particularly true because of the dissimilarities of the copper wire and the connector's metal.

If you suspect a bad wire, you can test it by simply using a jumper wire to overlay it and see if the problem resolves. If so, it's easy enough to replace the wire with a permanent overlay without having to replace or open up the entire harness.

What I've laid out here may not be causing your problem but it's where I would start looking.

It is a possibility the wiring and connections are ok and the problem is internal to the CDI. But I would go over the wiring and connections carefully before considering replacement.

Good luck.

In my experience with these motors, you will have a blockage in the small water passages in the cylinder head, very common with saltwater use.The only fix is to remove the head and physically clean it out. Often the use of a small drill is necessary to drill the salt out of the passages, there is no chemical that will clean these out. To prove it's not electrical, simply run the motor with the temp sensor disconnected, you'll find she'll run all day. What happens is with blocked small passages hot spots develop which are very difficult to detect with and infra red thermo.⁶

Dear JGMO
Thanks. I was thinking in that direction. Evan a bad earth perhaps. If I exclude that I may try swapping the CDI unit between engines and see if the fault moves too. Regards Nettuno/Cicala

Thanks iang6766
The temperatures that i measured were the block at the thermostat housing and the head at the temperature sensor using a probe in my multimeter. Its the reporting of the sensor that stops the engine so I would have thought the temp of the sensor should be elevated to stop it. I have put quite a lot of saltex through the engine and much flushing but I guess if I can't identify an electrical issue I may eventually need to remove the head. Regards Netuno /Cicala

Just sharing my thoughts, I work on these almost every day and your problem is fairly common. Like you, the first time I came across this I spent a lot of time messing around trying to fix it.

Cicala said:

Thanks iang6766
The temperatures that i measured were the block at the thermostat housing and the head at the temperature sensor using a probe in my multimeter. Its the reporting of the sensor that stops the engine so I would have thought the temp of the sensor should be elevated to stop it. I have put quite a lot of saltex through the engine and much flushing but I guess if I can't identify an electrical issue I may eventually need to remove the head. Regards Netuno /Cicala

Click to expand...

Just rereading your post, I mentioned a very quick way to test your electrics, disconnect the temp sensor and you will find it runs fine confirming no fault with your ECU

iang6766 said:

In my experience with these motors, you will have a blockage in the small water passages in the cylinder head, very common with saltwater use.The only fix is to remove the head and physically clean it out. Often the use of a small drill is necessary to drill the salt out of the passages, there is no chemical that will clean these out. To prove it's not electrical, simply run the motor with the temp sensor disconnected, you'll find she'll run all day. What happens is with blocked small passages hot spots develop which are very difficult to detect with and infra red thermo.⁶

Click to expand...

To remove the head, do I have to disassemble the whole engine or is there an easier way?

Firstly, have you tried running it with the temp sensor disconnected? If it fixes it, then try swopping the temp sensor with the other engine to rule out sensor fault. You can also try running a 50 deg thermostat part 19300 ZY6 003. Once this has been tried then remove the cylinder head. This is a simple procedure, cowls off, carb off, cam pulley and timing belt off. Remove wiring harness from cyl head only, tappet cover off, head bolts out, that's it.