30 Dec 2017

I had this week off but for many it was a partial work week, making otherwise-enjoyable test drives less so, so it was a good time* to deal with the inconsistent throttle issue.

Turns out I had two spare throttle bodies but one was a mirror image of the correct part, probably coming off the original Honda CRV donor engine. Anyway, I removed the one from the engine and stripped it down, leaving only the throttle angle position sensor (TAP) in-place. Playing with it by hand showed no stickiness or inconsistencies, so I was betting one of the removed parts had caused it – I would have lost the bet! The TAP sensor was temporarily reconnected to the ECU and the throttle shaft moved by hand. Sure enough, closing the throttle 10 times resulted in 10 slightly different readings, some differing by more than a full percent. Strange; the throttle plate, shaft, and bearings looked fine and felt fine, no slop, no stickiness, yet it just wasn’t repeatable, and cleaning didn’t help either.

The first thing transferred to the spare throttle body was the K-Tuned Hall Effect TAP sensor, and the readings returned to within 0.1% of zero every single time. Close it slow, close it fast, didn’t matter, it was very repeatable, proving that the TAP sensor was good and that the throttle body wasn’t.

Bolted everything back together and while in there, also cleaned the idle air valve – a known issue with Honda K-series engines when dirty. Next, my tuner buddy, JR, recommended setting the mechanical throttle stop at just under the target 1000 rpm idle, like around 950 rpm. The idea is that regardless what the ECU tries to do, idle cannot drop much bel0w the target. This does not rule out a decelerating engine with “momentum” blowing through this limit as well, but that’s a separate situation.

After much flailing about, I learned out that the Honda idle air valve is partially open at all times when controlling the idle of an OEM engine, no surprise I guess. The way around that was to remove its mounting screws and physically rotate it to close off the valve ports, which lowered idle speed to around 800 rpm, then used the set screw to push idle back up to 950-ish rpm.

After that, the TAP sensor was recalibrated for full open and closed, plus setting the idle air valve’s operating range. I’ve seen several recommended ranges but after again flailing around, settled for “-3%” at target idle.

After all this, went for a few test drives and it’s now close to an OEM-quality idle. There’s still a bit more tweaking to do but it’s refined enough that I can work on it when I want to rather than needing to.

*In the back of my mind was a concern that someday, this “slightly-sticky” throttle body might just decide to stick open at, oh, 50%. Having that much power unexpectedly pushing the car could end very badly. One time at Autoclub Speedway, I got a hint of what that could be like when braking for a turn and catching the edge of the gas pedal on my shoe without realizing it. Nothing bad happened but only because there was no wall or cars ahead of me as I “drove” straight off. It was a lesson I fortunately didn’t have to pay for (kind of like what I learned by overhearing a woman ask another woman “when are you due?”, and hearing her say “I’m not pregnant”). Anyway… a stuck throttle is mildly terrifying, especially in a light car with this engine. The problem was solved by trimming back the gas pedal to move it further from the brake, but still close enough to do heel-and-toe downshifting.