2000 Toyota Tundra 3.4 P0171

I arrived to a 2000 Toyota Tundra 3.4 running rough with a P0171 that was cleared by the shop. They had in hand fuel fouled spark plugs and no explanation as to why.

I connected my scan tool and observed the fuel control and feedback PIDs. I found fuel trim to be positive (indicating a lean condition) at idle and at 2500 RPM. This ruled out a vacuum leak.

The engine air flow was 7.56 grams per second (GPS) at idle. With an engine size of 3.4 lites I expected to see a GPS number close to that. A general rule of thumb: An engine flows about 1 GPS at idle per liter. For example a 2.0 liter engine would flow about 2 GPS at idle.

With a reading twice what it should be normally I wanted a close look at the sensor. Gaining access was easy, the mass air flow sensor is mounted in the air intake after the air filter near air box (arrow).

Once removed the MAF hot wire was clearly covered in debris. This can lead to a skewed reading. When the hot wire is covered in dirt at idle (low air flow) the dirt acts like a radiator, cooling the wire and reporting a higher than actual reading. Off idle, the dirt acts like an insulator, protecting the hot wire element from the passing air and it’s cooling effect. This leads to a lower than actual air flow calculation.

The dirt was cleaned and the MAF sensor was installed.

The reading dropped a few tenths of a volt but not quite enough.

After confirming the reference voltage and ground was good to the MAF sensor I suggested replacing it. The MAF sensor itself was faulty.

 

2003 Mercedes-Benz CLK430 Power Seat Motor Fault

I was called into a shop to perform an adaptation on a power seat module. They replaced a fore / aft motor as the old one was thought to be defective.
Once there I read and cleared fault codes, then attempted to adapt the new power seat motor. The motor would not move nor would it adapt. I read fault codes a second time and a fault for the fore / aft motor hall sensor was stored.

Monitoring the data stream would not prove to be useful due to the stalled motor.

I checked hall sensor voltage at PIN 6. It was normal. I then manually activated the motor and monitored the hall signal. The signal was normal.

Lets take a step back. Why would the seat module set this fault code? It expects to see the digital signal from the hall sensor, if it doesn’t it sets a fault. The module does not have the ability to see if the seat has moved. Was the original diagnosis way off when they chose to replace the motor? They said when the seat switch was depressed you could hear the relays in the seat module click, and the motor did not move.

I looked over the wiring and started from the beginning. While depressing the seat switch the voltage sent to the motor was low. I expected to see battery volts and had 6.8. I disconnected the motor and checked the voltage, it was now 10.8 (open circuit).

I added an incandescent bulb test light to the circuit and re-checked it. Voltage again dropped. We had a bad feed from the module. I checked power and ground to the module (loaded) they were all good. The internal circuit in the module had failed. It may be a faulty motor damaged it, it may have been the only problem this entire time.