I'm working on a 4 cyl. 4Runner. The truck has low power, surges while accelerating, and if I put my foot to the floor, the engine bogs down and I slow down. I'm getting a P0171 code (meaning I'm running lean). The engine idle doesn't seem to vary much. I've also changed out the intake manifold gasket.

Ideas? I'm a little stuck.

 

First, go for the cheap fix: clean MAF and throttle body, and check carefully for vacuum hose and intake air leaks.

 

My P0171 was my MAF. I agree with the post above.


Clean the MAF, see if you still have problems.
Clean the throttle body, see if you still have problems.


Good luck, and let us know what you find.

 

Forgot to mention. I have cleaned the MAF and checked the engine for vacuum leaks by spraying MAF cleaner on all (I think) the vacuum hoses. Neither appeared to make a difference.

The problem came on about 200 miles into a 1000 mile drive mid-summer on a hot day. It took about 50 miles between when the ECL came on and when I noticed the problem. At the end of the 1000 mile drive, the engine was low on power and had really poor acceleration. Was working away from home for a few months, now its surges pretty badly accelerating and needs miles to pass anyone, but made it home 1000 miles just fine by modifying my driving (ie. no passing, build up speed before hills, rely on momentum).

How can i test the MAF to see if it is toast?

 

Im not even sure how to clean a MAF sensor! is there a write up or something? going to search now..

 

Take off the intake duct behind the air filter box, there will be a thin metal wire in there that may or may not be dirty. Take "air intake cleaner" (not carb cleaner, needs to be for EFI) and spray at the wire. Do not touch the wire or it will break potentially, just spray til clean.

 

I have a link for that...

http://www.top4runners.com/ja/cleanmaf.html

OP: You don't mention cleaning the throttle body. A dirty TB can cause all things you are having happen, don't ignore the suggestion.

Before somebody asks....

http://www.top4runners.com/ja/cleantb.html

 

The repair manual only suggests checking resistance and voltage fluctuation during operation, so if you've got a multimeter lying around, it shouldn't be too hard to at least test those parameters.


If it turns out your MAF is clean and electrically sound, I have no clue where to go from there besides assuming your problem lies elsewhere.

 

Ended up being a very very dirty MAF sensor that wasn't cleaned sufficiently the first few sprays. Used half a can of cleaner on it before it started working. Cleaned it a bunch, put it back in the car, the car still ran rough and surged. Was disappointed, frustrated etc. Came out the next morning, and it ran fine. I guess the sensor still had cleaner on it and that affected it.

In the future, if I get a code P0171, or have surging on acceleration and low power, I will more thoroughly clean the MAF sensor with intake cleaner (NOT carb cleaner, must be for EFI).

 

A few links that helped me out in my quest to figure out how to test the MAF:

http://troubleshootmyvehicle.com/toy...e-maf-sensor-1

http://www.mr2.com/files/mr2/techinf...Runner-ECU.pdf (page 2)

So apparently for 3rd gens, the MAF sensor has 5 wires numbered on it. #1 and #5 are power and ground respectively. I tested with a multimeter, on DC volts, putting positive lead to #1 and negative to the neg batt terminal with the ignition key on, but engine off to determine appropriate voltage...should be about 12v. Then test the ground #5 with the negative lead and the red lead to the positive batt terminal in the same manner. Should be the same voltage. If that checks out ok, then the next step is to crank the engine and put the positive lead to #3 VG wire and the negative to #5. While revving the throttle, there should be a voltage increase, and a corresponding decrease when its released. I couldn't find a site that listed values for the voltage changes, but they all stated that if there was no voltage change or the readings jumped everywhere unrelated to throttle changes, then the sensor is faulty/dirty.

I recently dealt with that common code and cleaned my MAF a couple times and tested it too, but the code would not go away. So despite the advice NOT to touch the wires, I used a pick tool to manually remove some debris on one of them.

After resetting the EFI relay and fuse, it turns out the code finally disappeared. I even passed state emissions. Saved me from buying a replacement. So for me, the sensor cleaner wasn't enough.

Careful with the sensor mounting screws...mine were stripped or missing from the previous owner. Avoid stripping them by taking the intake box/duct together with the sensor. And it's been said to let the wires dry up from all the spray before running the engine. That may explain the hesitation after cleaning.

If the code still comes back, there's a whole plethora of info online on various sites regarding possible causes, as MAF is only one of several possibilities for that annoying p0171.