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BF225 - Confused about O2 sensor and engine-temp

brettmarl

brettmarl

Regular Contributor
For several months, I've been been watching the engine-temps on my Twin 225's (AK3) diverge. It appeared as if the Port engine (newer) was running 5-15 degrees hotter than the Starboard. Usually temps are bang-on. Tested and swapped T-stats, verified both engines have strong pee-stream, nothing helped, was planning to IR-gun the coils and look for anomalies, until...

Yesterday, I got a persistent O2 sensor code on Sarboard that would fire every 5m or so when underway. I've had one or two O2 code-trips in the last month, so kind of new it was on it's way out...

Swapped the O2 sensor for new - and boom - engine temps are perfectly matched again. So in hindsight, the Port engine wasn't overheating - it was the Starboard that was either a) running cooler, or b) under-reporting it's actual temperature.

Does this make sense? Does the 225 use the O2 input to report engine temperature somehow?
 
I'll reply in detail later. But off top of my head, with port sensor not working more fuel going into cylinders cooling them a bit.
 
OK. A little more detail. The HO2 sensor inputs data to the ECM. The ECM uses that information to control the air/fuel mixture by controlling the amount of fuel output by the injectors.

More specifically, the ECM effects correction controls to optimize the fuel injection quantity in accordance with output signals from the engine temperature sensor (i.e. block temperature sensor), intake air temperature sensor, barometric sensor, heated oxygen sensor and other sensors.

So, if the HO2 sensor is failing, one would suspect that the ECM is injecting more fuel than the engine needs, thus providing a bit more cooling.

A telltale for a failing HO2 sensor is black carbon buildup on the spark plugs, indicating more fuel is being injected than is needed.

Well, at least that is my theory of why you saw what you saw.

If you have the Helm shop manual, read the first few pages of Chapter 18 to get a better sense of how it all works.
 
chawk_man said:
OK. A little more detail. The HO2 sensor inputs data to the ECM. The ECM uses that information to control the air/fuel mixture by controlling the amount of fuel output by the injectors.

More specifically, the ECM effects correction controls to optimize the fuel injection quantity in accordance with output signals from the engine temperature sensor (i.e. block temperature sensor), intake air temperature sensor, barometric sensor, heated oxygen sensor and other sensors.

So, if the HO2 sensor is failing, one would suspect that the ECM is injecting more fuel than the engine needs, thus providing a bit more cooling.

A telltale for a failing HO2 sensor is black carbon buildup on the spark plugs, indicating more fuel is being injected than is needed.

Well, at least that is my theory of why you saw what you saw.

If you have the Helm shop manual, read the first few pages of Chapter 18 to get a better sense of how it all works.
Click to expand...

Interesting theory - I'll bear that in mind next time I see a temperature-diversion. I did just read the section you referenced - although your summarization of it was much clearer and easier to follow ;). Will check plugs next service.
 
Yes, it's a good tool for someone wanting to test their O2 sensors.

But I also liked it because it points out how a rich or lean condition can be generated
by other parts of the "system" not performing properly and to not to always blame the O2 for being the root cause.

Even when the O2 sensor is bad, there's always the possibility it may have been "poisoned" or destroyed due to other factors.
Something to keep in mind especially when you have repeated sensor failure.
 
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