P0171 P0441 P0455 | System too lean AND Evap leak detected

Check engine light came on; had the codes read at auto parts store:


They suggest it's a bad MAF sensor, but how would that also cause a large leak to be detected in the EVAP system?? I've already cleaned the MAF sensor (which didn't appear to be very dirty) by rinsing it with isopropyl alcohol. And yes, the gas cap was checked first, and last time we had an issue that seemed to be solved by tightening the cap, it did not include P0171.

Based on the limited informated I found about this combination, I suspect it's a bad EVAP purge valve or a bad PCV valve.
I cannot find any information about replacing the EVAP purge valve in a Prius, and couldn't find where it's located. But there's plenty of information about replacing the PCV valve, so maybe that is more likely to be the problem?

What do you think the problem is?

 

No

Not that I'm aware of, other than a mouse living behind the driver-side hood hinge (i saw it (or its tail at least) pop out and back in from under the hood, after starting a travel, which was intriguing)... and some rodent making noises somewhere in the underside near the rear... but there are rodents crawling throughout it all the time... of course I first suspected that it was caused by rodent gnawing, but I haven't ever find gnawing damage anywhere, even with all these years of rodent traffic, so it seems they just don't gnaw... (they do, however, frequently tear up soft stuff to make nests, most recently someone tore up that soft pad that goes on top of the spare tire, along with an envelop with a spare headlight bulb, to make a nest there ) still might be caused by that... would probably need a smoke test if that's the case... but it must be somewhere that would cause both problems

Occasionally the engine behaves strangely, running when it shouldn't be and/or reving up and down repeatedly; just like it did the last time there was an EVAP leak.
A few times it was stuttering on acceleration, making a pulsating vibration...
But most of the time it has been working fine.

Looks fine; smooth and rubbery, seat is also smooth

 

I think I found the problem!

So I was trying to improvise a smoke test using incense sticks and whatever I had but then I thought I'd try something simpler first. I taped a vacuum cleaner hose to the evap hose (the one that connects to the purge valve, which I discovered was under the air box) and plugged the other end into the air output. First thing I looked at was the gas cap, and guess what, air was coming out!

It is weird, but apparently the rubber seal leaks, even though it looks perfectly fine. The only reason I could think of is that the seal has shrunk or softened and isn't providing enough pressure.
Guess we'll be getting a new cap, but in the meantime I might have fixed the issue; I stretched an o-ring around the cap's ring, into the groove, to thicken the seal. Now it tightens up and clicks before it bottoms out; before it did the click at the bottom of the thread. I did that after putting things back so I didn't test it.

 

Not sure if that problem has been solved or there is a separate problem, but, as I'm told, this morning, black smoke was coming out of the tail pipe. That means too much fuel is being burned, which probably relates to the lean condition.

Considering that last time we had an EVAP leak caused by the gas cap, there was only those two codes, if it's also the gas cap this time, then why is there also P0171? Looks like there are two separate problems...

I think we'll be getting an ODB tool to aid in troubleshooting.

 

Will need to find out what the codes are now (if that's the only code left then indeed they are separate problems).
So what happened was, a motorcyclist notified the driver (not me) that black smoke was coming from the vehicle, especially during accelerations. After that the driver noticed the smell but couldn't see it. Later today nothing was noticed. Then I tested it myself, by reving up the engine while in park, but didn't see anything coming out of the tail pipe.

 

Haven't noticed any black smoke since.

I got a basic ODB reader which can view freeze frame data; here is what it said:

1. DTCFRZF : P0171
2. FUELSYS1 : CL
3. FUELSYS2 : N/A
4. LOAD_PCT (%) : 28.6
5. ETC (˚F) : 189
6. SHRTFT1 (%) : 20.3
7. LONGFT1 (%) : 28.9
8. MAP (inHg) : 11.5
9. RPM (/min) : 1002
10. VSS (mph) : 5
11. SPARKADV (˚) : 6
12. IAT (˚F) : 81
13. MAF (lb/min) : 0.217
14. TP (%) : 14.9
15. 02B1S2 (V) : 0.015
    
16. RUNTM (sec) : 1293
17. EQ_RATB1S1 : 1.232
18. O2B1S1 (V) : 4.474
19. EGR_PTC (%) : 0.0
20. EVAP_PCT (%) : 0.0
21. WARM_UPS : 255
22. CLR_DIST (mile) : 19081
23. BARO (inHg) : 29.2
24. EQ_RAT11 : 1.232
25. O2S11 (mA) : 0.438
26. CATEMP11 (˚F) : 997.52
27. CATEMP12 (˚F) : 848.66
28. VPWR (V) : 13.535
29. LOAD_ABS (%) : 10.2
30. EQ_RAT : 0.993
31. TP_R (%) : 0.0
32. TP_B (%) : 46.3
33. TAC_PCT (%) : 14.9
34. CLR_TIME : 599h:45m

The barometic pressure appears to be correct so it might not be the MAF. I see fuel trims are up but I already know that. Does anything else appear to be wrong?

 

I had cleared the codes. After a day full of driving I checked it again and found P0441 & P0455 pending! I did a quick EVAP test again, and gas cap is still leaking. So my fix didn't work—need to buy new gas cap.
P0171 isn't pending, yet.

 

Well more than a week and thousand miles after clearing the codes, the light did not come back and there are no pending codes. My fix worked.

Also I can't edit my own posts on this forum??

 

Update Jan/Feb 2019 for anyone finding this thread via search: I believe this issue was caused by a vacuum leak caused by the EVAP purge valve randomly binding and being stuck open, as I diagnosed in this thread. The solution is to replace the EVAP purge valve.