BajaRunner

Hey guys, I did some searching here but haven't figured what could be going on. I am trying to diagnose my buddies 99 3.4L 4runner.

Code pulled was P0171 - Lean Fuel Trim.
Looks like most people clean the MAF and its OK. Well, I cleaned the MAF, and the thing still idles horribly and when you press the gas, the car almost dies. Fuel filter is less than 6 months old. New plugs and wires.

What else should I check? I have OBDII scanner to computer and can check fuel trim and O2 sensor readout, as well as MAF. Everything looks to be functioning correctly.


Any help would be great! Thanks!

logsurfer

Trouble areas:

Air induction system
PCV hose connection
PCV hose
Injector blockage
Mass air flow meter
Engine coolant temp. sensor
Fuel pressure
Gas leak on exhaust system
Open or short in A/F sensor (bank 1 sensor 1) circuit
A/F sensor (bank 1 sensor 1)
ECM

DTC No. P0171
When the DTC P0171 is recorded, the actual air–fuel ratio is on the lean side. When DTC P0172 is
recorded, the actual air–fuel ratio is on the rich side.
If the vehicle runs out of fuel, the air–fuel ratio is lean and
If the total of the short–term fuel trim value and long–term fuel trim value is within ± 35 % (80°C (176°F)
or more), the system is functioning normally.
The A/F sensor (bank 1 sensor 1) output voltage and the short–term fuel trim value can be read using
the hand–held tester or OBD II scan tool.

TheDurk

While it could be anything on the list above, my P0171 and the chugging at idle was resolved by cleaning the throttle body, which had a varnish like coating on it. That was about 2002. I started using Techron before every oil change and clean the TB every Spring. I haven't seen that code again.

BajaRunner

The TPS test via FSM revealed the ohm's were not correct, but I am not real conviced that is the problem. O2 sensor appeared to be functioning as well via ODBII scanner.

The MAF, after being cleaned, appears to be registering the correct temperature and registers increase in air flow, via ODBII program.

It almost sounds like a fuel pump or fuel filter. I would like to test the fuel pressure but have no idea how. Can anyone direct me>

logsurfer

did you check the gas cap? might just be a bad gas cap..

BajaRunner

Mmmmmmmmmmm...I could probably check. I don't have the truck with me right now.

Ok so I found the procedure and specs for the fuel pressure. I think I'll do that next. I don't remember hearing that its common for the pump to go out so soon. The filter is a NAPA gold filter... might go back to OEM.

Roadtripr

a couple of things. I got the code and while the MAF seemed to read fine it finally got replaced and it runs much better. Another thing I have had issue with is dying at idle. Turns out I need to clean the intake control valve by the throttle body (both were quite dirty). Much better idle after that

prorunner

I'm throwing this code every other day. Except my truck seems to run and rev up just fine. And it idles very smooth.

BajaRunner

Prorunner : Most likely a dirty MAF. Clean it with Mass Air Flow Sensor Cleaner or Electrical contact cleaner.

This particular truck just needed to replace the MAF