Installed a K&N Air filter on my 99 4runner 3.4 yesterday along with changing the radiator and now I'm throwing a P0171 and P1133 codes. I did unhook the battery for 15 mintutes after installing the air filter and I also erased the codes this morning but they keep coming back. Was the filter dirty and I need to clean the MAF now? I don't see any vaccum leaks either. Any input appreciated.

 

P0171 is Lean Bank 1 and P1133 is a lean condition, IIRC. Sometimes putting a K$N air filter on an engine will cause a lean condition. Usually the PCM makes up for it but I'm not familiar with '99 air/fuel management.

Have you tried installing an OEM or aftermarket air filter like WIX or FRAHM ? See if the codes still set ?

 

Not sure if your intake hoses are old and brittle but sometimes they'll be hiding a little air leak underneath which can throw a code. if you have a stock airbox it may not be the case but that happened on my 3.4 during install and on my other car when moving the air box lid. If the above doesn't work you may want to inspect all the areas where air could be leaking in past the MAF.
I'm personally not a fan of the K&N since it messed up my MAF sensors on 2 of my cars and the AF meter on my old 3.0(this was over oiled but the others were right out of the box). I now just change my filters more often and I don't notice a difference

 

Not sure if this helps or not but installing the K&N air filters with to much oil can throw a code. Gets into the sensors. I think the say some about it in the cleaning instructions about only putting oil on the outside. Wipe off excess oil. Then try some throttle body cleaner and clean the sensors. You can also spray the hose while running and if idle changes you found a leak

^^like he said gets into the MAF

 

The K&N will cause you to loose power anyways, just go back to stock and leave the elbow off. It will make more power and run better.

I have posted this before but for those that have not seen it. Here are some back to back to back to back tests (yes, i ran the test twice since i could not believe the results but it came back with the same results everytime).

The stock intake is in blue and the cold air is in red:



The K&N is a waste of money. Or any intake for that matter.

 

+1 on the above post.

Installing a different air filter won't give you more horsepower. In fact the oil in the K&N filter can sometimes cause problems with the MAF and that's bad news.

The 3.4 is one of the best engines in the world. Toyota engineers designed it to run one way within very tight parameters. Instead of tossing money at it to "improve it" with aftermarket parts, you'de be better off putting that money into routine maintenance with factory parts.

'99 4Runner 318,000 miles.
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