No, I'm not saying that. But in this case that's not what's happening and in this engine it's unlikely to happen at all. Keep in mind I'm talking about a stock EFI system.
Many here mistakingly think the trim sensors (coolant, TPS, O2) on this engine are required to make it run properly. That's only true to a point. Like all trim sensors the coolant sensor is for emissions first and for tweaking mixture (along with idle speed) during "abnormal" conditions (such as when the engine is cold) second. In fact the entire EFI system is there mainly for emissions. Fact is the ECU is more than capable of keeping the mixture very close to what's needed when the engine is hot using nothing more than accurate airflow and rpm input. After all, carbs can do it. What a carb or an EFI system without trim can't do is keep it tight enough for maximum catalyst efficiency.
Many also forget the ECU will correct for false sensor input. After all, what good would the system be if it ran the engine very poorly or even stranded you every time a sensor went bad? From the TCCS manual:
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Fault in Coolant Sensor:
If an open or short occurs in the coolant sensor input the ECU will judge the temperature to be either below -50 C (open) or above 139 C (shorted). As a result the air fuel ratio will become too rich or too lean which could lead to engine roughness or stalling. Ignition timing will also be retarded if the coolant sensor input is deemed too hot.
Action taken by the TCCS: A standard value of 80 degrees C is substituted, the typical value for a hot engine.
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The fact he has a code means the ECU is ignoring the sensor and using the standard value because one action can't happen without the other. Use of the standard value will cause only a slight increase in richness when the engine is hot, which is what we're talking about here. This is true of all standard values because of their static nature. Not only do the books point all this out but I know it from testing my engine in just about every configuration you can imagine using a 5 gas analyzer and the factory TCCS simulator/checker.
Note the system is only capable of detecting shorts and opens. It can't detect shifted signals ie; signals that don't accurately represent the variable they're reporting yet still fall within a range the ECU is programmed to deem normal. That's the situation you're talking about so yes, a coolant sensor reporting way off could screw things up. However in that case no code would be set. It's the main weaknesses in this form of OBD which, like all OBD, is there not so much to help diagnosis engine problems as to inform the driver emissions may be exceeding 1.5 times the federal limit. That's why OBD was developed in the first place (by California) and it's what's codified in federal law.
Make sense? If you've understood everything I've written certain things will become obvious:
1) Codes are not all they're cracked up to be on this car and *not* having any set means squat.
2) Mixture problems (in a engine operating at normal temperatures) such as the one described can't be the fault of a trim sensor that has set a code.
3) Shifted trim sensors are an invisible evil lurking in the system and must be manually hunted down.
4) Excessive mixture imbalance or severe driveabilty issues are always the result of things the ECU can't compensate for, be they a shifted sensor or other things it can't detect. Bad plugs, wires, fuel pressure, induction leaks, injectors, on and on.
The bottom line is people need to stop being quick to blame trim sensors, all of which have limited authority in the system, for many if not most serious engine ills. Especially when codes for them have been set. The kinds of emissions/driveabilty problems like the one in this thread will always be the result of something else.
Whew. I hope somebody learned something from all that.
edit: Lol...what Jdub said.
