2004 ST THERMOSTAT, Maybe I found something interesting not mentioned before.

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Hey guys and gals. I usually don't post, but I am along time lurker. I have been searching previous posts for sometime and have not found anything mentioned about what I found concerning a failing thermostat and the affect it has on gas mileage. So if I am posting a repeat or in the wrong forum, let me know.

I have found many posts that refer to the bad gas mileage being a precursor to replacing a bad thermostat. I also found many questions about gas mileage and the posters were refered to checking for bad sensors.

What made me start digging deeper into the mystery of bad gas mileage was that it was coming and going. Bad thermostat? Coming and going? I always held 3 bars no matter the ambient temperature. Sometimes gas mileage in the 50's traveling 70 mph down to 35mpg traveling under 50mph What in the world?

My problem was not necesarily the thermostat, it was the ECT sensor. The ECT sensor does more than send a signal to the dash and ECM. It also controls the power to the MAP sensor, the TP sensor, IAT sensor, and O2 sensors on both banks. In essence, the signal comes from the ECM to the ECT sensor and the the ECT sensor turns on the other sensors that I mentioned. If the engine coolant fluid is not hot enough the ECT will not turn on the other sensors, essentially never letting the ECM get in to closed loop.

To test this, I jumpered the signal wire to the ECT (yellow/blue) to the power wire going to the other sensors (green/orange). Sure enough, that was the answer! My bike now runs in closed loop with my gas mileage back to normal. Before the jumper, all my sensors read as dead or not working. Now, all the sensors are reading correctly.

AT least on my bike, I found out what was going on.

Like I said, if this is old news, sorry for the repeat. I was not able to find it anywhere else in the searches.

Pop
 
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Glad it worked out for you. Fortunately our 11's don't have all that head scratching "stuff", but I still easily get 50+mpg.
 

Mellow

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Great info, I don't think I've ever heard of anyone trying this....
 

Scooter

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My problem was not necesarily the thermostat, it was the ECT sensor. The ECT sensor does more than send a signal to the dash and ECM. It also controls the power to the MAP sensor, the TP sensor, IAT sensor, and O2 sensors on both banks. In essence, the signal comes from the ECM to the ECT sensor and the the ECT sensor turns on the other sensors that I mentioned. If the engine coolant fluid is not hot enough the ECT will not turn on the other sensors, essentially never letting the ECM get in to closed loop.

To test this, I jumpered the signal wire to the ECT (yellow/blue) to the power wire going to the other sensors (green/orange). Sure enough, that was the answer! My bike now runs in closed loop with my gas mileage back to normal. Before the jumper, all my sensors read as dead or not working. Now, all the sensors are reading correctly.

AT least on my bike, I found out what was going on.
I believe the only bad sensor that you have is the ECT sensor. What you did by shorting the yellow/blue wire from the ECT sensor to the green/orange wire was to essentially bypass it; i.e. mimic a hot engine signal. The ECT sensor is simply a thermistor and has no bearing on the operation of those other sensors that you mention. I think you are getting confused because they share what appears to be a common reference line, the green/orange wire.

Look at the good news though, it appears that you solved your problem...
 

Blrfl

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(EDIT: D'oh... Scooter beat me to it and in fewer words. C'est la vie.)

I hate to throw [-]cold water[/-] coolant on an interesting find but...

My problem was not necesarily the thermostat, it was the ECT sensor. The ECT sensor does more than send a signal to the dash and ECM. It also controls the power to the MAP sensor, the TP sensor, IAT sensor, and O2 sensors on both banks. In essence, the signal comes from the ECM to the ECT sensor and the the ECT sensor turns on the other sensors that I mentioned.
I presume you're looking at the green/orange wire. That wire is ground and connects the sensors back to the ground pin on the ECM. The ECM has another ground pin and a second green/orange wire that's tied to the central ground under the seat, which is where it gets its electrical ground.

If the engine coolant fluid is not hot enough the ECT will not turn on the other sensors, essentially never letting the ECM get in to closed loop.
Each sensor has its own circuit back to the ECM, and the ECT plays no role in whether the ECM uses the other sensors. The temperature sensors are all resistive, and the ECM puts a small voltage across the sensor and measures the amount of current that flows through it. Since current varies with resistance and the sensor's resistance varies with temperature, the ECM can calculate the temperature based on the current. (The ECT is really two sensors in one; the other circuit feeds the instrument panel.)

To test this, I jumpered the signal wire to the ECT (yellow/blue) to the power wire going to the other sensors (green/orange). Sure enough, that was the answer! My bike now runs in closed loop with my gas mileage back to normal.
What you did was show the ECM no resistance across the sensor, which it will see as a short. I'd be surprised if there isn't a code 7 stored in the ECM.

Based on your improved mileage, I would also not be surprised to find that your ECT has failed and was providing plausible-but-wrong data to the ECM, which ran the engine richer than it should because it thought the coolant was too cold. You can test the ECT using the procedure outlined in the service manual. My money is on your ECT showing more resistance than it should or there having been some oxidation in the connector that added resistance.

Shorting the sensor will cause the ECM to ignore it and never enrich the mixture to get the coolant temperature up. You may run into problems with the bike warming up or being hard to start in the winter if the temperatures where you are fall below about 60?F.

Before the jumper, all my sensors read as dead or not working. Now, all the sensors are reading correctly.
When you say your sensors read as dead, had the ECM stored trouble codes? How did you determine that the ECM is running open vs. closed loop?


Fortunately our 11's don't have all that head scratching "stuff", but I still easily get 50+mpg.
Let's not have the why-EFI-is-better-than-carburetion discussion in this thread.

--Mark
 
OP
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Replying from my phone, so please forgive brevity. Can't argue with what you guys point out. Apparently misread wiring diagram. My o2 sensors showed 20 mv before bypass and average 1.000v when ect bypassed. No code 7. No other codes stored in ECM. You have to admit I at least started an interesting topic.
 

Blrfl

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My o2 sensors showed 20 mv before bypass and average 1.000v when ect bypassed.
I'll have to ponder that a bit, but it might actually make sense.

No code 7. No other codes stored in ECM.
Don't rule it out yet, because the description for code 7 includes a shorted sensor. The ECM may not throw it until the engine's had a certain amount of run time with it that way.

You have to admit I at least started an interesting topic.
Absolutely. I'd be very interested to know how your ECT tests out if you get a chance to measure it.

--Mark
 
OP
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Thanks Blrfl. I have the thermostat in my hand ready to change. I just can't convince myself to go out in the hot garage to change it and get to the ECT. 102 in the shade today.
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OP
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UPDATE:
I thought I would put an ending to this thread. At the ECM, I tested for continuity between the y/b and the g/o. Nothing. So, broken wire or bad ect? I finally got around to taking the thermostat housing out so I could continue testing. No continuity from ECM y/b to ECT y/b. (ECT shows good resistance reading.) Ah, Hah!

I pulled the wire connection on the right side of the mc apart to test that portion of wire. Continuity. Checked from connection to ECM. Continuity. Put connection back together. Continuity all the way through. Went back to the beginning. Tested continuity from ECM y/b to ECM g/o. Continuity + ECT resistance reading! Bad connection, I guess.

While I had it apart, I changed the thermostat and fluid.

All added together, it has made a big difference in my gas mileage. I have now run a tank and a half of fuel and it still is running like it did before I started having mileage problems.

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