16 Feb

A heavy-duty relay was added near the fuel pump, along with a direct connection to the alternator through #10 wire – there won’t be much voltage drop on this setup.

A short tube was welded across the GM fuel composition sensor – pictures when I get around to it. Curious if I’d hurt the sensor by welding to it, it was temporarily wired up to see if it was working. Somewhat surprisingly, all the online wiring diagrams for the sensor were wrong, but since the +12V supply feeds the center pin on the 3-pin connector, it didn’t blow it up. Connected correctly, it output a 55-Hz square wave, indicating that the fuel contained 5% ethanol – cool.

Because the fancy capacitive fuel level sensor won’t work with ethanol, it was replaced by an old-school float type sensor. Not wanting to mess with converting the sensor current to a voltage for the dash, a corresponding old-school gas gauge will take the place of the water/methanol flow meter, which is coming out in anticipation of the ethanol setup.

A month ago Midlana ran Pomona speedway without a hitch. At the end of the day it was driven onto the trailer, then backed off the trailer and into the garage, where it sat for a month while the new fuel system was fabricated. With the fuel system finally done, 6 gallons of gas was added (no leaks!) and the car taken out for a test drive…. and it ran like crap!

As the car limped home, it would run fine at idle and up to about 2000 rpm, but then die back to idle before speeding up again. Back home, fuel pump voltage was confirmed good, but the worry was that there might be something wrong with the new pump, a hose, or the filter, meaning that the tank had to come out. The “good news” was that the problem was easily producible; if the gas pedal was floored in neutral, engine rpm would shoot up to about 2000 rpm, then immediately die, coasting down to maybe 500 rpm, then repeat. If it was a fuel flow problem I’d expect the issue to be load-sensitive – that is, in neutral when it consumes very little fuel, it should rev to as high as desired, yet the cut-off speed didn’t seem to care whether the engine was loaded or not.

Fuel pressure was monitored during the oscillations and was fine, which made things even more mysterious. Connected the laptop to check for ECU errors and there were none – but suddenly, there was the issue. The throttle position sensor (TPS) output should increment smoothly between 0 – 100% as the pedal moves to the floor. It worked fine – up to about 25%, then jumped around erratically. This failure wasn’t unexpected, but it happening now was – what did a gas tank replacement have to do with the TPS failing? Apparently nothing.

The Honda K-series TPS isn’t very reliable; what makes it worse is that it’s part of the throttle body assembly – you can’t buy just the sensor from Honda. Out of this arose a business opportunity, and K-Tuned sells a Hall Effect TPS sensor. The cool thing is that there are no sliding contacts like inside the OEM sensor, so theoretically it should last forever. Because of numerous Honda forum posts about bad throttle sensors, it seemed like it was only a matter of time before mine failed as well, and so it had. Fortunately I’d ordered the K-Tuned part last Fall – and today it started its new job.

The next step is a biggie, switching to the new ECU.