Throttle position reported by OBDII port

I sometimes take my car to the track. I use a data logging program to time my laps and monitor certain engine parameters including maximum throttle position over the course of a lap. I have noticed that the reported throttle position is always exactly 84% or on occasion, 83%. I can assure you that over the course of a lap, I am hammering it. Why isn’t the max throttle position 100%? The throttle body is clear and operating as it should be. The car doesn’t seem down on power and there are no check engine lights. The air filter is not brand new but it looks clean. The program records all the other data channels exactly as I would expect it to. For example, speed, coolant temperature, etc. all look as they should.

For the record, the software I used is Race Chrono Pro installed on a Moto G (something) with a cheap generic Bluetooth OBDII reader.

To see if it was a car problem or a software problem, I also tried to used the OBDII reader on another car and with a different program (Torque Pro) on the street. I floored it a bunch of times and held it there for a few seconds each time. That resulted in a throttle position no higher than 88%. Torque Pro was recording throttle position once per second. The air filter on that car is filthy.

What gives? How does the OBDII determine throttle position? Why isn’t it reporting 100% throttle opening?

Drive By Wire or a cable?

If cable, try adjusting or replacing the throttle cable.

ECU map may also be due to this and some vehicles the throttle will close slightly at higher RPM. If the air filter was dirty this could also be part of the cause, but is really ECU and car dependant.

Track car is a cable but the throttle opens fully when I floor it. I don’t think the cable needs adjustment but I’ll check it again.

The street car is drive by wire.


Based upon my recently gained “expertise” :rolleyes: on the subject, my WAG:
The OBDII may be reporting throttle position based upon feedback from the mass airflow (MAF) and/or manifold absolute pressure (MAP) sensors, thereby reporting throttle position by inference. If so, the discrepancy you note may be due to inaccuracy of the sensor itself or, error in the program you are using for interpretation.

I’m unaware of any make/model that has a dedicated rheostat directly attached to the throttle positioner/control for actual throttle position. (Again, my WAG!)

What type of car is it? My Chrysler has a procedure to calibrate the throttle position sensor by stepping on the throttle fully and releasing it slowly with the ignition on, but car not running. I found the procedure Googling forums on my specific car.

KMS94’s post above piqued my curiosity…And a simple search: “throttle position sensor” leading to a Wiki article confirmed that I was TOTALLY wrong concerning my prior statement about the lack of such.

I should have started brain before engaging mouth… Sorry.

You tried it on another car, so this is very unlikely to be the case, but before I got to that part, I was going to suggest a problem with your throttle position sensor. Now, if the TPS is throwing odd data, your engine will/can act equally odd, so it’s very likely that that’s not the case.
You can test this by, leaving the car off, but the key turned to on (KOEO), pull up that screen and watch the data as you push on the gas pedal. That’s how you typically test the TPS for bad spots.
But, like I said, we know that’s probably not the case or you’d have other problems, and likely a CEL on since the TPS and MAF/MAP/RPMs etv not jiving.

However, you said you’ve used combinations of other devices, different software and other cars and always get the same number. The only thing that hasn’t changed is the OBDII scanner. That’s what I’d focus on. Get another scanner and see if that clears up the issue.

Also, FWIW we can be fairly certain that it’s just the scanner misrepresenting the data. If there was a physical problem with the throttle body, throttle cable, air intake system,or anything else, it would manifest itself in other ways. Plus, it wouldn’t happen on other cars.
Why, yes, I have been spending too much time with my scanner (BlueDriver).

If I were you, I’d first google the name of the scanner (or let us know what it is) along with the symptoms and see if that turns anything up. Then I’d probably bite the bullet and get a new one.
If you can borrow one from some at the track, even for 2 minutes to see if it reports data that makes sense with the KOEO test I mentioned earlier, I think that will answer all your question (ie it’s a problem with the scanner).

If you’re absolutely certain that the throttle is opening all the way, then I’d say the TPS is either misadjusted or faulty. The air filter will not affect TPS readings.