My 2005 Gen 2 Prius has developed an strange intermittent issue over the past month (reproduced only once so far). While driving under load (especially climbing long grades), the engine suddenly misfires badly for a few seconds. It then either continues running "normally" with very low power or triggers P0A0F, forcing me to coast and power cycle the car. First occurrence: ~1 month ago on an interstate onramp; had to coast off with P0A0F. Unfortunately I just had to reset it on the side of the road to make it home and didn't think to screenshot the codes or save the freezeframe data. Second: Today, after ~1000 miles of normal driving since the first. Engine returned to running normally after ~5 seconds, and I was able to make it home, albeit on noticeably reduced power. Techstream scan today shows: P0171 (System Too Lean Bank 1) P0442 P0446 Live data at idle @~155F (heater on to force engine run)and partial throttle ~1600rpm shows: B1S1 STFT ≈ -21%, LTFT +44.5%. The lean code combined with the evap codes makes me think perhaps a cracked evap line and faulty valve? As I've only had it happen twice within the past month/1k miles, I find it difficult to believe a leaking gas cap would be the source of that issue. Not sure what to make of the fuel trims either in this case, but fuel economy is noticeably down over the past 1k miles since the issue first happened. Any ideas on where I should start looking? I'll clean the MAF tomorrow as a sanity check, but it seems very odd that it'd be intermittent like this.
I'd be wondering about the fuel pump capacity or fuel filter. Seems like fuel delivery isn't keeping up under sustained high demand.
Yep; over 20 years of pumping fuel into it, there's probably a bunch of crud that's made it into the fuel tank - clogging the filter sock too. You can verify by placing a fuel pressure gauge on the fuel rail. The EVAP system has NOTHING to do with this issue. You can run any car without a fuel cap; but you'll get a bunch or EVAP error codes. The engine isn't going to stall-out or die on you. You will fail a CARB state emissions test with those active codes.
On a normal car that wouldn't be so terrible, just pull it out and clean it. On this car though, is there any kind of fix other than replacing the whole tank? I mean, I suppose one could blow out the fuel line by unhooking it at the tank, and if there was some blockage there it could be cleared. But I agree the problem is more likely in the tank, so that wouldn't help at all. There are products which claim to be fuel tank cleaners, like this one (no personal experience with it) Star Tron Fuel Tank Cleaner - Concentrated Gas/Diesel Formula – Star brite Distributing, LLC I expect the most likely outcome is that it would finish off the fuel filter, once and for all.
I agree, however... There's really no way to check on a Gen 2 is there? I don't recall there being a live data spot for fuel pressure in Techstream. And the tank/pump assembly is a single unit, so there's not much I can do there either outside of replacing the entire thing and hoping it's my issue.
The repair manual (RM) has a procedure for checking fuel pressure. I believe you disconnect the fuel supply at the fuel rail and install a T-connector in line with the fuel rail and supply line, then connect a fuel pressure gauge to the other leg of the T-connector. Something like that, don't quote me, though. The best place to get the correct information in the RM. Toyota Service Information and Where To Find It.
I'll investigate this tomorrow, I believe my fuel pressure tester kit has a Quick Disconnect fitting T-adapter in the box. Not sure why the manual specifies to cut the hose, but I guess I'll find out. Looks like it's kind of behind and under the inverter coolant reservoir.
They mean for you to start with a new, or spare, fuel tube, one that you can cannibalize for the connector on the end, so you can make up a pressure-test adapter.
I was able to hook up my gauge, and unfortunately it didn't reveal anything enlightening. If it is the fuel pump, it's very intermittent rather than a constant low-pressure scenario. Although if it is the pump, not exactly sure what I'm going to do about it, since I don't want to put in another 20 year old pump/tank, and certainly not dropping the $1500 on the assembly from Toyota. I did take a short little Techstream data recording of a trip around the block with a few seconds of flooring it and didn't see anything weird in there either... I'll upload the file so you can look in case I'm not seeing something. I changed the file extension to .txt so I could upload it, just rename it to .TSE like usual. The lean code and evap codes haven't returned in the ~45 minutes of idle time and ~4-5 quick 2 mile trips up the road and back for testing purposes. Definitely feels... "flat" in the upper RPM range, and fuel economy certainly isn't nearly as good as it should be. Just as a 5 minute sanity check, I'm going to pull out the spark plugs tomorrow and see if there are any obvious signs of it running lean.
Did you already eliminate your first ideas? I would lean toward investigating that before I started going crazy on fuel tank replacement, etc.
I did reseat the gas cap, and like I said the P0442 and P0446 haven't returned since (although it may just need driven more). I did go into the Active Test portion of Techstream and manually activate the vent/purge/VSV valve and didn't notice a change in engine behavior (and it didn't set a code). I plan on checking the spark plugs tomorrow since it's a 5 minute job, and I guess I'll try and drive it some more for the rest of the week to see what behavior/codes return.
Go find a long hill and drive up it. That will put more strain on the fuel system. If the system is working at 80% capacity (for instance) you may not see anything until you try to push it to supply more than that.
A cheaper alternative is to hook up an inline electronic booster fuel pump and regulate it down to around 46-47 psi. This will ONLY work if it's a weak fuel pump in the tank. If it's a clogged filter sock (junk in your tank), your engine will still cut-out. That's the cheap <$100 fix and your time. I used to do this whenever a V8 would bog-down while towing or going over the mountains - fuel flow related issues; but I haven't tried this on a fuel injection car. The principals are the same and should get you further down the road. Good Luck
