I ended up replacing intake plenum gaskets, all vacuum lines, AF sensor, MAF sensor. I had already changed PCV valve and grommet as well as valve cover gaskets. I'm now getting the best MPG I have ever seen in my 99, and more so than my 04 tacoma I had. 320 miles to a tank highway on 15.2 gallon fill up of 87 running royal purple oil

'99 Limited V6 4x4, 193k miles. P0171 came up a few weeks ago, cleared. Came back again. Truck running fine, but decided to get this code taken care of for good. Removed the MAF, cleaned with CRC MAF cleaner twice, let it dry overnight, replaced. Idled fine, blipped the throttle a few times, seemed alright. Never took it for a test run and didn't reset the computer.

Came back out the next day, noticed a little hesitation pulling out of driveway. Got on the freeway, hesitation got worse, until it finally stalled a mile down the road. Tried disconnecting battery for 30 minutes, turned over, hit 1500 rpm, then sputtered out. Tried starting a few more times, but wouldn't stay running. No CEL.

Towed it home and checked all the vac lines. None are misplaced or cracked. Tried cleaning MAF again, replaced it, still same issue. Tends to run for a minute or so if it sits awhile, but sputters out again. If I try to restart again immediately, hits 1500 rpm, then stalls out after struggling to stay running. ODB scanner shows nothing.

Fuel filter replaced at 120k, original MAF was cleaned at 130k. New MAF is reman from NAPA.

Maybe the filter went bad already? Just purchased this truck a few weeks ago and have had no problems.

I would say that the reman maf from Napa might be throwing it off. From what I've seen and heard, only dealer or denso on the maf

You don't say if you cleaned the throttle body. After MAF cleaning, that's probably the #2 thing to do and what cured mine.

Ugh, that's going to be another $130. Weird thing is it was running just fine, except for the CEL, but as soon as I removed the MAF, cleaned it, and re-installed, it can barely idle for more than a couple seconds, or not even that long.

http://www.rockauto.com/m/mobilecatalog.php look here for it if you are going to replace it. $72

It should stay running even if the MAF is bad, it'll just run like hell. Sounds to me like a bad fuel pump, that is what was wrong with mine with a P0171. It had symptoms of a bad MAF,drove to the junk yard to grab a new MAF for $20. as soon as I got block away from the yard it fell on its face and didnt even want to limp there. Got it there and changed the MAF with two others and still no bueno. It ran but as soon as i gave it gas, it wouldnt move. towed it home and the next day it wouldnt start, Fuel pump went bad.

That's what I'm thinking now. I checked the plugs and they were fouled badly. Changed them, still nothing. Opened the fuel line in the engine bay near fuse box, no spray and bone dry. Just got done swapping the fuel filter hoping that was the issue, still no change. Fuel pump should be here tomorrow from Rock Auto. Will be dropping the tank Saturday. Sprayed down as many bolts as I could get to with PB Blaster the other night. I just love living in MN with all the salt they put out. And I've never done a fuel pump before, let alone dropping the tank. Not looking forward to it.

Its pretty simple I never did either. I suggest getting to the access panel under the rear seat and disconnecting it and testing it first. I used jack stands to suppport the tank until I put the jack under it to lower it down. It intimidating until you start doing it and realize there isnt much to the system.

Couldn't get the rusty bolts loose, so took it to my friend's shop. He got the fuel pump swapped out. It's a Denso from rock auto parts. Driving home and about 15 miles down the road on the freeway, it starts lurching again. Was not liking it when I gave it more gas. Now I can't get it started. Just keeps cranking. This is driving me nuts.

To summarize, P0171 was the code, cleaned then replaced the MAF. Then changed the fuel filter. Now the pump. All vac lines are hooked up. No CEL right now. What else can I look at?

Hesitated and stalled on the way home from the shop at around 20 miles. Towed it home. Decided to order another fuel pump, thinking I got a defect, and planned to remove the tank again in a few days. But in the meantime, I decided to try a couple bottles of Heet to remove any water from the gas. Put that in, alt with a few gallons of gas and let it sit overnight. Next day, it fired right up. Put a full tank of gas in and ran about 50 miles of city driving. It sputtered a few times, but didn't stall. It must have been a bad tank of gas. The shop that dropped the tank and installed the fuel pump should have emptied the tank and put fresh gas in, but oh well. I'm just glad my truck is running again!

Still running rough occasionally on this tank of gas, and I'm halfway through it. Forgot to mention I used the mid-grade octane (89) hoping it would help. Also randomly throwing up a P303, cyl. 3 miss-fire, when it does run rough. And the CEL will either flash, or stay on steadily for a little while, then go away. Going to check the plug wire for cylinder 3 as I did change the plugs when I was chasing the original issue. Also going to put some seafoam in and top it off with 87 octane.

So it looks like it comes down to a bad MAF and I'm in need of replacing it. What's the suggestion for a new MAF? I have seen a lot of names thrown out there and there is a wide range of the prices.

Problem is I'm having same code pop up but problem is I replaced upstream and downstream o2 sensors at same time replaced maf sensor both upper and lower intake plenums both value covers the idle air control valve the spark plugs all the spark plug wires and I still have check engin lite and system to lean code and when I step on breaks softly doing 60 the rpms Guage drops rapid from 1400 to 900 then bounces back to 1350 within seconds I have had this lean code for over a year and I stopped driving it until now fixing all this I checked there no leaks on vacume system or plenums mine is a federal emissions 2000 toyota 4runner millennium silver sport edition 4x4 3.4l v6

I used this thread as a reference dealing with my recent dealing with the P0171 code.

After checking everything I could I stumbled across a small vacuum leak at my catch can. After bypassing the can I have been code free for 2000+ miles.