94_Z28_guy

I've read in past threads that the MAF frequency should jump up to 7kHz when you snap the throttle response. I measured mine, at idle it was about 2.6kHz, when I revved it up, the frequency increased, but not up to 7kHz, I had to rev it up to almost 4.5k rpms to get close to 7kHz. Does this sound right or is the MAF messed up? Thanks in advance.

-Tom

Injuneer

You should be seeing about 2,800Hz at idle, and about 9,000Hz at max load/WOT. When you just goose the throttle, and its not under load, its hard to predict what the air flow would spike to. Here's the stock calibration curve.

HZ - GPS
0 - 0.00
1,488 - 2.45
1,616 - 2.88
1,744 - 3.34
1,872 - 3.87
2,000 - 4.45
2,128 - 5.12
2,256 - 5.85
2,384 - 6.64
2,512 - 7.49
2,640 - 8.41
2,768 - 9.40
2,896 - 10.46
3,024 - 11.60
3,152 - 12.82
3,280 - 14.11
3,408 - 15.48
3,536 - 16.98
3,664 - 18.60
3,792 - 20.34
3,920 - 22.22
4,048 - 24.23
4,176 - 26.35
4,304 - 28.6
4,432 - 30.97
4,560 - 33.43
4,688 - 35.99
4,816 - 38.69
4,944 - 41.54
5,072 - 44.52
5,200 - 47.66
5,328 - 50.97
5,456 - 54.42
5,584 - 58.04
5,712 - 61.81
5,840 - 65.77
5,968 - 69.92
6,096 - 74.27
6,224 - 78.82
6,352 - 83.59
6,480 - 88.58
6,608 - 93.8
6,736 - 99.25
6,864 - 104.93
6,992 - 110.88
7,120 - 117.04
7,248 - 123.42
7,376 - 130.03
7,504 - 136.88
7,632 - 143.92
7,760 - 151.14
7,888 - 158.63
8,016 - 166.31
8,144 - 174.03
8,272 - 182.36
8,400 - 191.01
8,528 - 200.07
8,656 - 209.43
8,784 - 219.12
8,912 - 229.15
9,040 - 239.52
9,168 - 250.23
9,296 - 261.30
9,424 - 272.71
9,552 - 284.48
9,680 - 296.61
9,808 - 309.10
9,936 - 321.96
10,064 - 335.20
10,192 - 348.80
10,320 - 362.77
10,448 - 377.14
10,576 - 391.88
10,704 - 407.02
10,832 - 422.55
10,960 - 438.46
11,088 - 454.78
11,216 - 471.51

cehan

A couple of questions about the MAF calibration curve:

Am I correct in thinking that the 2800 MHz at idle and the 9000 MHz at WOT are for a stock engine? I imagine that a modified engine with a faster idle and making more power at WOTwould have different readings.

Also, I guess you need an oscilloscope to check that? On my Datamaster I can see the AFGS, but I don't see anywhere that shows the frequency.

Chris

Injuneer

I gave him the approximate stock values because his signature indicates his car is "stock". If the idle speed increases, air flow increases. If you're making more power, it can only be because you are flowing more air.

Many cheap multi-meters now include a frequency meter. I've never seen frequency data in a scanner or scan software.

94_Z28_guy

Injuneer,

Thanks so much for the data. I was not able to find any freq data on the MAF in my service manual. I do have a scan tool, I assume I should read the gps from the scan tool while measuring MAF freq? Is that table for the engine under load?

Thanks again, I have a bad problem with the engine dieing under WOT, and afterwards the car won't immediately start until several hours later or I unplug the MAF. Just trying to eliminate the MAF as a cause.

-Tom

Injuneer

You don't need to monitor both. If your car is "stock" the table that I posted is what's in the PCM. When it reads the frequency out of the MAF, it translates that frequency to the grams/second value in the table. So you should only need to monitor either the frequency or the GPS.

That table is simply how the PCM translates frequency into GPS. It doesn't matter whether the engine is under load or not. The MAF is measuring the actual mass air flow into the engine. My point about "load" was that it you're out on the road, and have the pedal to the floor, and its cranking 5,800rpm, its going to be flowing 220-240GPS. If you are sitting in your driveway, and floor it under no load, the engine might reach 5,800rpm, but by that time the throttle is closing and the mass airflow is not going to be anywhere near 220GPS.

Air flow in "volume" (e.g. CFM) units is always related (roughly) to engine RPM, but the "mass" flow - which is what the MAF is measuring - takes into account the manifold absolute pressure (along with inlet air absolute temperature - but let's not complicate things), which is no where near full atmospheric pressure at idle, or when the throttle is partly closed. Its only at WOT that the manifold pressure approaches atmospheric pressure.

94_Z28_guy

I see now. Thanks so much for the info, I appreciate it.