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Are you running an aftermarket header? If so, unless you are running 4 wire (heated) O2 sensors, the sensor may not get hot enough to accurately read resulting in a code and hesitation under tip in. I had to heat wrap my header when I was running one wire sensors to retain enough heat for the sensors to work correctly.
 

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Discussion Starter · #22 ·
At first I was using a wrapped header, but due to what you mentioned, swapped in a stock manifold with a different O2 sensor, and it still tripped the code. So I assumed the problem was not related to the header or manifold, or the O2 sensor.
 

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See if your getting rational O2 sensor voltages at the ECU. Back probe the ECU connector to pick up the O2 signal and a sensor ground. A straightened paper clip makes a good probe and use tape to stabilize it in place. Take the car for a drive and warm it up. Run for a few seconds at about 4K RPM then lift the throttle to coast down in gear. When coasting down the ECU cuts the injectors until you drop below 1500 RPM. When the injectors cut off it’s as lean as it will ever be and the O2 signal should drop well below 0.4V. At wide open throttle the ECU goes open loop and runs straight off the fuel map so should be rich indicated by greater then 0.5V.

Passing these test would show a healthy sensor, solid wiring, and good sensor ground. It should also show if the fuel system can deliver enough fuel to run rich under heavy load.

Here is a good link for details of the O2 sensor operation and testing.

The code 14 indicates an EACV issue so perhaps there is an interaction there. Does it have a high idle when cold and slow to a stable low idle when warm? The EACV will foul with carbon over time. The most common symptom of this is a surging idle but low cold idle or stalling when cold would also be indicators.

How confident are you with the ground junction at the thermostat? Since the engine is running well I doubt this is the problem but it is a more complex connection then it would seam. Did each of the ring terminals get a good cleaning? Any corrosion on the block or thermostat housing can interfere with most of the sensor signals. When replacing the thermostat I carefully remove all corrosion from the mounting surfaces of the block and housing then degrease before assembly.

Mike
 

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Discussion Starter · #24 ·
Thanks for the advice. I did check voltage from the O2 sensor but wasn't satisfied that I did it correctly. Here's what I found:

"Checked voltage at the 02 sensor, which should be between .1 and .9 volts, which it was. However at idle, it tended to stay at the upper end of the range, around .78 and then it would slowly count down to the high .6's. Which means rich."

I think what you're saying is that bad readings from the O2 sensor don't necessarily mean that it's bad. It could be a fuel pressure problem that's causing the bad reading.

Code 14 seems to have gone away for the time being.

I will try to run more tests this weekend. Determined to solve this.
 
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