2009 CR-V

A couple weeks ago, after idling in a parking lot for 20 minutes, randomly stalled out while trying to get going again. Few days later, again after idling awhile, got a CEL for P0171 (fuel too lean).

After hooking up a scanner, I see that pressing the brake pedal reliably causes a surge in short term fuel trim, especially at idle. Like, I can get it to 30-40% by pumping a few times. Seems worse after the engine heats up. While highway driving with minimal braking, fuel trim generally stays reasonable, but consistently biased positive (up to 5-10%).

Now I'm trying to figure out if the next step is replacing the booster and/or master cylinder, or if it's possible the check valve could be all it is. What I don't understand is that braking seems completely normal and not to require any more force than usual, even though my vehicle does fail the traditional "press pedal, turn off engine" test (brake pedal immediately pushes back up).

Unfortunately, I'm just a regular non mechanically inclined dude without special tools, and I'm finding it impossible to get the clamp off the hose to remove it from the booster. If I could get it off, I'm guessing all I'd need to do to rule out the valve is cover up the hose and see whether I get good fuel trim at idle.

Does anyone know where the booster check valve is located inline along the convoluted series of hoses going from the booster to the intake? Also, just any relevant experience with this kind of thing would be great.

I'll take it to a shop this week, but the first one I took it to last week was going to replace the freaking O2 sensor for $400, so I've lost all faith and want to minimize my chances of throwing money down the ******* for a vehicle with 200k on it.

 

For P0171, the issue can be due to a dirty mass air flow (MAF) sensor. Get some CRC MAF sensor cleaner to clean it and see if it resolves the issue. Also, check your air filter and the housing.

This is from an older Honda Service News Article.




This is the location of the MAF/IAT sensor on your car.

 

Brake booster has been replaced, which may have helped some, but high fuel trim remains after idling for extended periods.

However, I found that when I clamp off the vacuum hose from the EVAP purge valve to the intake manifold, STFT always drops from high to 0-3% within seconds. Knowing this, I tried swapping in a new purge valve, but that did nothing. Also replaced PCV valve for the hell of it - no luck.

Now, I'm trying to figure out whether it makes any sense to continue looking for a culprit farther back in the EVAP system. Even if the EGR valve or something else EVAP-related were bad, would it make sense for a known good purge valve to allow a continuous vacuum leak? Clamping off the hose even behind the purge valve still drops fuel trims immediately to good levels. I'm just trying to understand under exactly what conditions the purge valve remains open, and whether it may do so simply because the EGR valve is malfunctioning.

The only other thing I can think of is that there's a leak elsewhere, and cutting off the EVAP vacuum just happens to compensate for it almost perfectly. Still, the fact that clamping off that EVAP hose reliably changes STFT abruptly every single time seems like something's amiss.