Scientific (Almost) Data Of Before And After a Tune Up

I recently bought an OBDII to Bluetooth adapter so I could do some troubleshooting on my car, which I recently fixed by rebuilding the head and replacing all the belts, hoses, and gaskets I had to take off to rebuild the head.  It’s been getting really bad gas mileage, and I had a few theories about why.  I bought this device: BAFX Products Bluetooth OBD2 Scan Tool because it was recommended on a few car forums, and it has over 1000 reviews on amazon with an average of 4.5 stars.  I was amazed at the amount of data this combined with the Android Torque application I could get from the computer and display as a graph, dial, or number, and log…

I had three thoeries about why the car’s MPG was sucky.

  1. The temperature sensor was bad, this is because even after replacing the thermostat the temp guage was about 20 degrees below middle.
  2. The oxygen sensor needed to be replaced (always a possibility).
  3. Old ignition components were reducing efficiency (spark plugs, wires, cap, and rotor).

So, I plugged in the OBDII scanner, and looked at coolant temperature, it was right on the money at 185.0, that ruled out the temp sensor.  Then, I pulled up the Air/Fuel mixture status widget, it showed me that the O2 sensor was working fine, and the ECU was operating in Closed Loop mode. So, while it could still be the O2 sensor it wasn’t likely.  Just in case, I ordered a new O2 Sensor, Plugs, Wires, Cap, and Rotor from Rock Auto (best prices anywhere) and (it was cheaper after shipping to order the wires from here)

I wanted to see if I could get some good data showing I had solved the problem, so I first did some data logging during a trip around the block after the car had warmed up but before I did anything.  Then, I did another similar run after changing the ignition components, and then a final run after changing the O2 sensor.

After gathering the logs on my computer, I used the Throttle Position sensor data to split each set of data into Idle, and Not Idle.

Then I computed the mean average, and average deviation of both Fuel Trim Percentage (this is the percent of fuel the cpu is adding or taking away from it’s (fuel map + long term trim value) to get the O2 sensor to be right, and O2 sensor voltage.

The results I got were

O2 Voltage at Idle Before Changes:  0.46 / 0.22
O2 Voltage at Idle With New Ign  :  0.45 / 0.24
O2 Voltage at Idle New O2 Sensor :  0.44 / 0.31

Temp Fuel Trim at Idle Before Changes: 2.04 / 2.198
Temp Fuel Trim at Idle Before Changes: 0.2  / 1.35
Temp Fuel Trim at Idle New O2 Sensor : -0.3 / 1.78

The only noticable difference is that the average Temp Fuel Trim went down after the new ignition components.

O2 Voltage While Operating Before Changes:  0.62 / 0.24
O2 Voltage While Operating With New Ign  :  0.73 / 0.15
O2 Voltage While Operating New O2 Sensor :  0.69 / 0.18
Temp Fuel Trim While Operating Before Changes: -0.65 / 2.47
Temp Fuel Trim While Operating Before Changes: -0.75  / 2.56
Temp Fuel Trim While Operating New O2 Sensor : -0.7 / 2.66

So what does this tell me? Not a whole lot. Other then that the mixture seems to have gotten more accurate while driving after the new ignition components, and a little better still after the new sensor.  And, I don’t know what the long term fuel trim was, it’s possible my car adjusted somewhere in there.  Also, the test wasn’t that great, all 3 logs were done a little differently.  Which may account for the differences.

So, the ultimate question…  Did this fix my gas mileage?  I don’t know yet, we’ll see after a few tanks of gas, that is the litmus test.

One other component that may be the problem is a clogged catalytic converter, so that’s next, and if it’s not clogged, then I’ll do a smoke test and see if I have any vacuum leaks, and if that doesn’t fix it, then I’ll put in a cold air intake, instead of the current setup I have where the average air intake temp is 150 degrees f