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What should the feeback from the MAF look like

I have a 2001 Corolla (recently purchased).

The P171 code came on and I searched the forums and decided to clean the MAF.

I used car chip to plot the feedback from the intake air temperature and intake air flow rate. I took the car on the highway, set the cruise control for 55 and drove it for five minutes. Then I took it home, downloaded the data to my computer and plotted it. The temperature is reasonably steady for the duration of the highway time but the intake air flow rate is all over the place. I thought, aha, the sensor is dirty. So I took it off, cleaned it with CRC MAF cleaner and I could notice a clear change in color. I put the sensor back on, took it back on the highway and plotted the results.

I got the exact same results, the intake air flow reading is all over the place despite a constant speed on relatively flat highway.

So my question is, is the MAF readings still bad, or is this the type of feedback you would expect to see from the MAF. I could understand the readings from the O2 sensor changing but you would think that the air flow would be relatively constant. I don’t see how any control system can react to what appears to be an almost random event generator. I can email jpg images of the plots (speed, o2, etc) to anyone interested.

Any thoughts would be greatly appreciated.



First, I presume that your air filter is in good shape & clean. Is it an oiled-type filter such as a K&N?

If the throttle position and rpms remain relatively flat then the MAF reading should also be relatively flat. If your TP and RMPS are stable while the MAF jumps up and down and cleaning it didn’t help, first check the wiring and harness for physical damage. Then, there should be a 5V ref signal and you should check to see that it is good.

You can also set your chip to recording, start the car and let it sit at idle. Give yourself some basic time reference marks or something (rough ones would do), then do a wiggle check on the wires. After that just rap on the MAF a bit to get it to vibrate a little. Then check out the results. If the wiggle test messes with the signal then assume a wiring problem. If tapping the MAF sets the signal wacky then you probably need to replace the MAF.

You can use “Attach File” to post .jpg files if you wish.

Your point about the rpms remaining constant is a good one. I can’t believe I didn’t consider logging the rpm instead of the velocity. The amount of air taken in will be a function of the speed of the engine and not the car isn’t it.

I will take your advice and try the wiggle and tap test.

Thanks for your help. I’ll update with results but it will probably be a day or two.

Thanks again.

Airflow is not related only to RPM, so just logging RPM is not enough. You need to at least log throttle position too.

The closest correlation will be between the TPS & MAF. But…the airflow will rise and fall independently of TPS when the engine shifts - the RPMs will rise/fall and will draw more/less airflow.

I’m just onto this lately b/c I am currently having a TCC lockup problem. I can map my MAF/TPS readings and the TPS can stay rock solid while the MAF reading shoots up and down. Its going with the rise & fall of RPMs when the TCC locks/unlocks. If you watch TPS rise during acceleration, you also get MAF dips whenever you get a shift - which drops the RPMs and thus the draw of air.

So…on the off chance that the OP is having a transmission issue, it will play with the MAF reading regardless of TP.

In any case, Tom - best to plot the TPS & MAF. I don’t know the device you’re using but throw the RPMs in there too if you can plot more than 2 things.

Thanks for all the feedback and suggestions. I followed the advice in the replies. I plotted engine speed, TPS, and MAF. I turned the car on, let it idle for 1 minute, then tapped on the MAF sensor and surrounding area including the cable for 1 minute. Then I gradually increased the engine rpm in one minute intervals. The results are attached and from them I would have to conclude that it seems that the MAF is good and I need to look elsewhere.

I know this is a long way to go, but I always feel better about diagnosing my problems if I can perform a controlled test that proves or disproves the diagnosis.

Also, just in case you wanted to know, the chip I am using is here:

I bought it at Autozone quite a few years ago but haven’t had many opportunities to use it. I paid $99 for it and this is only my second opportunity.