99 Tacoma 3.4L hesitates at full throttle

Like the title says I have a 1999 Tacoma 3.4v6 automatic, 4WD and for some reason, the truck is unresponsive and hesitates at full throttle, but as soon as I let off to about 3/4 throttle, I feel it pulling like it should, just doesn't really accelerate at full throttle, but revs up just fine.

So far I've:

-Replaced Spark Plugs
-New Plug Wires
-Cleaned MAF with MAF cleaner
-Air Filter
-Seafoam through the tank
-Oil / Filter Change


Any Ideas? It's got 210,000 and runs like a champion, however It's got a check engine light on, (Last time I had it checked, it was PO171, engine too lean bank 1?) But it does it regardless if the light is on or not. I can reset the code, and it goes away for a day or so, then comes back.

 

Maybe this will help.

 

I changed both of mine at 177,000 miles after doing a tune up, no check light was on but they were dirty..

made all the difference in running and mileage

 

Thanks, sounds like I know what to do next. Hopefully that fixes the issue. Not that I pass, or floor it very often, but It would be nice to have the power be there when I need it. Thanks for the replies, and the Images!

 

When I bought my truck the light was on and just like yours it would fall on its face when I would floor it but I could let off a little and it would gain speed. The problem turned out to be the maf. I cleaned it and it didn't make a difference, so I replaced it and that fixed it. The maf was bad and it wasn't reading that the truck was sucking in more air at full throttle, thus not letting the engine get enough fuel.

 

How hard is it to change the front O2 sensor?

edit: nvm, seems simple enough if this write up is correct http://wiki.answers.com/Q/How_do_you_replace_an_O2_sensor_on_a_1998_Toyota_Tacoma

 

Its easy, but be careful because I broke one of the studs on accident. Luckily one stud is enough to keep it from leaking or anything.

 

its not too hard, just tight for adult or wide hands. i went by this
http://www.lieblweb.com/o2replacement.html

YES,,,, spend 5 days soaking it with power blaster, give it a shot in the morning.... to help loosen the nuts.

 

http://www.yotatech.com/f2/changing-oxygen-sensor-when-nuts-rusted-197311/

and if you need tips for bad rust read this.
not trying to sell you anything, but its been my experience that you should change both while you are at it. 2 fresh ones seem to work better than a new one and a old one.
and alot of soot that covered my old ones fell off before I could take a pic... these were the worst ones I had ever seen.

 

Two things to consider. First, how rusted are the studs/nuts? That can be a PITA and some people have already posted remedies for that.

Second, is it an automatic or a manual. The automatic is no problem. The connector for the O2 sensor is on the right side of the trans. Easy to get to...well easier than the manual.

With the manual, you have to remove the console around the shifter. Then remove the boot. You also have to remove the 4x4 selector boot, if you have 4x4. The connector is stuck on top of the trans and attached to a bracket. I removed the bolt from the bell housing to get the bracket to the opening by the shifters to get it apart. Then connect the new one and drop it down to put in the exhaust pipe.

I have done both and the auto is much easier...IMHO.

 

Little update on this.. I changed out the o2 sensors (both front and rear) and the problem is still there. Still throwing the code and still isn't happy at full throttle. Any other ideas?

 

Did you check the coil packs or ignition coils (may be same thing--it's what sits on top of the spark plugs). When I had my last '00 Tacoma, I was going down the road just fine one day and then it started missing and I noticed white smoke coming out the exhaust. Long story short, some how, one of the spark plugs had came out and was firing up into the coil pack. Damaged both. Ended having to put a helicoil down in the head to hold the new spark plug.

 

So far I've:

-Replaced Spark Plugs
-New Plug Wires
-Cleaned MAF with MAF cleaner
-Air Filter
-Seafoam through the tank
-Oil / Filter Change
-o2 sensor front and rear.

The truck runs beautifully besides full throttle, it's the weirdest thing. Idles correctly, sounds just fine. But for some reason, at full throttle, the truck downshifts, and the engine revs up, but it doesn't really make any power. However, if I just let off the throttle to about 3/4, it takes off. Strange... I've noticed on steep grades if the engine is under heavy load under about 3k rpm, there is a slight pinging.


EDIT: Was just doing some searching, and came across a YotaTech thread where a guy mentions when his MAF was going bad he had no full throttle power, maybe I'll take it out and try cleaning it again, or replace that next. Sound feasible?

http://www.yotatech.com/f16/problem-accelleration-98-4runner-60044/

 

as far as hesitation at full throttle...

I didn't see "fuel filter" on your check list

 

Quoted just in case you missed my post earlier in the thread. The maf cost me 75 bucks but had a $60 core charge.

 

i believe that it could be your maf ive been having alot of problems lateley had no power on my 2.7 changed everything and checked everything. I threw a maf code twice when i first had it cleaned it both times and the code never came back. But i started losing power gradually until here i am hOrrible power and doesnt rev. pulled my plugs and they are whitish brown very little black at all around the plug. it bogs everywhere and anything over 1/2 to 3/4 it doesnt really rev at all.

Just my 2 cents. It been bugging me for ever hope you fix your problem and report back. may have to hit the junk yard to test one out

 

Just to follow up on this, I fixed the issue. It was the MAF sensor. I went and picked one up from Kragen for $75 (Re-manufactued Toyota OEM by A1Cardone) and it runs beautifully now. For those of you doing this, be careful when removing the Air Box for MAF replacement. I snapped my airbox where the MAF housing tube meets the box. It's extremely weak right there and any pressure on it will most likely snap it.