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2011 BF75 shuts down on overheat when idling

S

shepley

Member
Hey folks, hope somebody can help me troubleshoot this motor. I have flushed with Rydlyme, replaced both t-stats and temp sending unit. Tell tale is strong, the stats appear to be working ( hot water comes out of prop) but the alarm goes off and motor shuts down after about 5 to 10 minutes on the hose. This seems to be getting worse, as occasionally it used to NOT alarm or shut down. Yesterday I pulled the lead from the sending unit and plugged in the old one, letting it just dangle from the harness. I assumed this setup would bypass the issue, but in fact nothing changed--alarm and shut down in about 7 minutes. The motor does not seem hot (I can touch the head next to the sending unit and, while uncomfortable, can keep it there) or run poorly. I am thinking the ECM is bad. Any suggestions? Thanks
 
We both know ECM's are expensive. To avoid that, or with that in mind, it might be worth your trouble to inspect the entire harness carefully, pulling apart and reconnecting everything, then do the same for all the ground attachments where they attach to the block. If the engine is used near salt water, do not discount the potential for wires that have deteriorated internally. Generally that leaves a visible bulge, but not always.....

Another thought, just to assure yourself it's not actually getting hot, would be to use an infrared thermometer on it. If you don't have access to one, they're very inexpensive now, and not hard to source. You don't need anything real fancy. 20-25 bucks should get the job done....

Best of luck!
 
Thanks Alan. I just spent the morning going over the connections as you suggested. The motor is pretty darned clean, and I found absolutely no sign of corrosion or a bad wire. I pulled just about every connection apart and re-seated them, including the fuses. The maximum temperature I found was 160F, right at the head stat. Another suggestion I got from a local mechanic was possible blown head gasket. He advised me to check the compression hot, which i just did, and got 180 - 190. I have photos of a cold compression test from a couple years ago. They range from 210 to 225. I will test again cold in the morning. It seems like a pretty large drop from cold, but I have never checked a motor hot before so I have no idea about that. Regardless, I don't think the gasket is the issue.
The weird part is that it shuts down even when the temp sensor is out of the circuit. Is it possible that there is another sensor somewhere the ECM is using? It pretty much runs 6 minutes from cold to shutdown consistently. The motor is well warmed up before then. When I try to restart it shortly after shutdown, the alarm is still buzzing, but letting it sit for a bit starts the cycle again. That sounds like a temperature thing to me. Also, I dis-mounted the ECM from the block and thermally isolated it, thinking maybe the board gets hot and a marginal component temporarily fails, but no difference. It would be pretty flukey if there was a temperature related issue that results in a false overtemperature alarm..but I'm starting to think that is the case-- a marginal connection somewhere in the temp logic circuit that results in that (I think) false alarm.
 
Agree the chances of a head gasket causing this are not real good.

I think there are 2 temps sensors.

see #24 here
https://www.boats.net/catalog/honda...in-bbaj-1200001-to-bbaj-1299999/cylinder-head

and #16 here (likely a PIA to get at)
https://www.boats.net/catalog/honda...n-bbaj-1200001-to-bbaj-1299999/cylinder-block


The ECM is going to be looking for a range of values from the sensor, so they have to be plugged in.

From my notes when I had one that was absolutely giving me fits-
"All the EFI motors use a 5v reference, ambient temp is usually around 1 volt increasing with temp to 5 V which will activate alarm."

I don't remember where I found that, but it led to the repair of the motor I was working on.....
 
Well, it was certainly worth a shot. I've had that port leg cowling off a few times now working on the fuel system, so that was easy enough. The block sending unit is the same part as the head unit, so ohms should be the same at the same temp. I assumed the entire block was at the same (ambient) temp ( sat over night and is in the shade) but the block unit read 1835 ohms while the new head sensor was reading 1795. That's only a couple percent difference, but maybe it's that finnicky? I installed my old head sensor in the block (which happens to be a blind hole - no water immersion) and it stabilized to within 1 ohm of the head sensor. Then started the motor and hoped..
The stat(s) (head and/or block I have no idea) opened at 1:15 min. Dang alarm went off at 5:00 min and shut down at 5:26. Pretty consistent anyway..sure looking like the ECM, but I am going to try a few more things while I prepare myself mentally for dropping too much money on this hole in the water
 
Would you be able to measure the actual voltage at the sending unit while the motor is warming? Just thinking that if they are in range while running (under 5v), and you have it setting off the alarm/quitting your problem is elsewhere.

Have you tried getting a trouble code from it?
 
I have a set of better pin probes for my Fluke coming today or tomorrow, so I will be able to better access the connector. Th local guy has a Honda diagnostics package on the way so I will get a reading when that shows up in a week or two.
Today I let the block Temperature Sending Unit hang from the connector, so it was reading (mostly) ambient temp, hoping to see some difference, but it acted the same. Did the same drill with the head TSU and same deal-it's almost as if the TSU's aren't even part of the equation. If that's the case, then why does it run at all? Why run 5 min then alarm (about 2-3 when warmed up) and shut down? The engine is up to max temp in just a minute or so. What is causing any delay that seems to be tied to engine temp? Sampling rate? I read somewhere, working on one of the Subarus, that some data is averaged over time by the ECM, so parameter changes do not cause too jagged a response, and, I suppose, to reduce an unstable feedback loop. I would think the ECM clears all temp data when turned off, otherwise it would be averaging the previous running temp with the now-current cold engine temp on the next start up, resulting in an incorrect starting mixture. If it is not getting accurate temp data from the now-disconnected TSUs, why does it always result in a consistent overtemp shutdown in 5 min? Very puzzling. You can tell I am trying real hard to avoid that $800 butt whipping!
 
So here is the data from this morning's run:

Time Block TSU Head TSU (DC volts)
0:0 2.72 2.72 (key on, motor not running and cold)
0:30 --- 2.20
1:00 2.60 -----
1:30 ----
 
So here is the data from this morning's run:
I had to move the probes back and forth so each TSU reading is staggered every 30 sec

Time; Block TSU; Head TSU (DC volts)
0:0> 2.72 2.72 (key on, motor not running and cold)
0:30> --- 2.20
1:00> 2.60 -----
1:30> ---- 1.29
2:00 >2.36 ----
2:30>2.16 .90----motor has shut down on overheat at this point

I cranked the motor and ran it to shut-down a few more times and the voltages stabilized at 1.62 on the block TSU and 2.09 on the head. All this adds more confusion to me. Why did the head TSU voltage drop so precipitously at first but then come back and stabilize on subsequent runs? What should the initial "key on" voltage be? And if the voltage range is 1 to 4, both the block and head voltages seem to be in a reasonable range at 1.62 and 2.09, so why shut down then? Miller time......
 
The question I have is regarding the fact the head and block temps seem fine, with the temp sending units seeming to confirm, what are you looking at/seeing that makes you believe the motor is overheating?

This is REALLY reaching, but wondering when's the last time you checked out the water separator?
 
What's that saying?? One's reach should always exceed one's grasp? Well, the alarm at the console goes off and the red LED at the console temp icon turns on. I checked the separator when I overhauled the fuel system. But I will check that again right now. Now would be a handy time for a code reader.....
 
"Overheat Indicator/BuzzerThe overheat indicator turns on andthe buzzer sounds when the enginecooling circuit is faulty. The enginespeed slows down this time"

From the Owner's Manual
 
So I just ran it at 3000 revs, prop engaged, and in less than a minute the buzzer went off and the revs dropped to 1500. It didn't shut off, though it immediately started running very rough. I just switched it off.
 
Alan: i really appreciate your help on this motor. I went ahead and ordered an ECM ( from PartsPak..about $765 inc shipping) and I will post the results here once it is installed
 
Have you checked the exhaust temp, it also has a sensor , there are 3 temp sensors.. Ihate to say it but Ithink you've just bought an ecm for nothing. From your description the motor is definitely overheating. You need to run DrH to see which sensor is indicating an overheat. Low waterflow usually triggers exhaust temp first. This can be caused bt a cortoded oil pan where the water tube connects of a passage blockage in yhe head or block
 
Have not checked exhaust temp. Where is that sensor located? I can't locate it on the parts diagram. But the ECM will be here today or tomorrow so we will know then. Seems like it would have to be a hot pocket right at that sensor because the rest of the motor is running pretty normal
 
Well.... I am now much less confident that it is the ECM. As iang6766 indicated, there is that third sensor above the fuse block kinda behind the ECM. First thing I did was remove that sensor and stick a screwdriver in the hole to dig around a bit. It was pretty muddy in there, so I thought it possible that it was a partially plugged pocket, maybe causing a local overheating right at the sensor. I then poured about 2 cups of Rydlyme into the opening and let it sit for half hour. Then I re-connected the sensor to the harness, letting it dangle, and, leaving the sensor hole open, cranked the motor on the hose. So: it ran fine for 10 minutes, until I shut it down. Once the motor was running, water blasted out of the hole at close to what an open garden does. A ton of scale and corrosion debris came out, and the stream was warm to the touch. I then restricted the until it was a stream about the volume of the tell tale, and the temp quickly become too hot to touch.
I then re-installed the sensor and checked voltage with the key on, which was 1.80 VDC. Then I started the motor and watched the voltage drop to 0.98, about 3 min in, at which point the alarm sounded, and then shut down. And I had used the replaced sensor from the block, which was good.
So the next question is: is the starting 1.8 volts right? The other two sensors had 2.72 volts cold with the key on when I checked the other day. Are all three sensors on the same circuit (easy enough to check that ) ? Could the ECM output voltage for the exhaust sensor be to low to start with, indicating a bad ECM? Tomorrow when everything has cooled and stabilized I will check all three sensor voltages at cold, key on, and see what I have.
If it is a blocked water passageway, I am not sure how to address that except more flushing with Rydlyme or maybe something more aggressive. Water certainly gets to that sensor location great, but possibly the escape route is blocked.
More tomorrow...
 
Maybe posting this passage from my notes again will help. No idea where I got it -

All the EFI motors use a 5v reference, ambient temp is usually around 1 volt increasing with temp to 5 V which will activate alarm.

Thought here would be there are 3 wires at the sender.

1 is ground
2 is 5v
3 is output and will depend on temp. Should see 1v here around ambient temp, and if it gets up to 5v it will activate the alarm.


Best of luck! These things can test the patience of a saint.... -Al
 
That's weird..there are definitely only two wires to the sensors, and at ambient temp the "key on, engine not running" voltage at all three sensors (this morning) was 2.82. The voltage drops with a temp increase. You sure that's for Honda motors? Yesterday the alarm sounded when the voltage on the ECM area sensor dropped to 0.98. I was hoping that the voltage to that problem sensor was lower initially, indicating a bad connection or bad ECM, but all three were the same. So I have swung from 95% certainty the ECM was bad to 95% certainty the ECM is NOT bad. As iang6766 suggested, there likely is an actual overheat occurring at that location behind the ECM. Next I am going to run the motor without the thermostats installed. I cheaped out and bought aftermarket ones, so there may be an issue there. It's happened to me before. If the block stat is malfunctioning, it could absolutely cause an issue at that ECM area sensor, but still have ok temps at the other two sensors
 
Guys, you're way overthinking this issue. Best pull the head off and clean out the block and head, you will not get the crap out of an 8mm hole, believe me,been there many times
 
It has been obvious to me for some time now that ian knows what he's talking about so I'm not disputing what he's telling you.

I just wanted to clarify that the temperature sensors are 5 volt sensors but don't need three wires. They are simply 2 wire ntc (negative temperature coefficient) thermister types. Lots of info on the web about how they work and how to test them.

Good luck.
 
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