Here are some readings from my 2001 Blazer at idle (in drive IIRC) that I had previously taken using Dash Command (different software displays different units). Maybe it will be of some use to you. I consider these to be quite normal. You can convert the units from kPa and g/s if necessary:

RPM = 578 rpm
MAP = 46 kPa
Baro = 98 kPa
ECT = 91°C
IAT = 63°C
IAC Position = 109 counts
EGR Position = 0%
CMP Retard = -0.8°
MAF = 6.46 g/s

Now I see that your vehicle is in your signature!

You can definitely see misfires with an oscilloscope, but you need to know what to look for and where to look in the traces. The leading part of each trace when displaying secondary traces in raster format is where I always used to look (when I had access to an oscilloscope).

In Car Gauge Pro, you will find two different PIDs named CMP Retard. Use the one with the single * for your Blazer. Somewhere on this site I have written some basic instructions for doing some things using Car Gauge Pro. You can search for that if you like. Tells you how to set up multiple gauges on same screen among other things.

 

Here's readings at 55mph steady, taken using Torque Pro

RPM = 1701 rpm
Speed = 56 mph
MAP = 10.3 psi
MAF = 35.0 g/s
Intake Temp = 100.4°F
Coolant Temp = 194.0°F
Throttle Position = 22.7%
Timing Advance = 27.5°

 

Let's say I'm an old school guy in these matters and I prefer oscilloscope or any other device that measures a real, physical value rather than the dongle gadgets, where You are not sure exactly what is measured and what is just calculated.

I have two channel DSO, but only one probe, so not much of a choice. I may:
- watch one trace at a time; quick math here: 500rpms/60seconds = 8 1/3 turn per second; 8.33/6 cylinders= 1,39 firing pulses per second or a pulse every 720msec
- quickly make 2nd probe from a transformer and a diode set of two Zener's (in antiparallel) to limit the voltage on the output. Then I could set the coil pulses as a base for the trigger and use 2nd channel to check out the pulses on the selected cylinder. I guess I'd be too lazy for that.

Kindly asking once more, has anyone have any specification to the MAF circuit please? Or at least remember the values?

 

And sometimes the only thing that is important is what the vehicle computer thinks. You don't get answer because factory manual specifies a scanner reading under certain conditions, not a simple resistance. I don't have my manual here at work to give you the spec, but I have given you scanner readings of MAF sensor at two different sets of conditions. You should be able to use Torque lite to read the MAF and tell if it is OK.

 

Don't get me wrong, I did not say the dongles aren't useful. They are. By a dongle I could do trim lookup and do accurate troubleshooting today. Personally (and that is just only my personal preference) I prefer to stick to the physical values. Lots of indicators, where many amongst them are just a result of math equation (that are not explicitly given) within ECU are somewhat confusing to me. As an example, if my ride is in an ambient temperature (so does intake) of 40F rather than 100F as in Your data, what changes (and where) should I expect? What if that is just only a wiring problem..? ECU cannot neither divide the problem between wirings and sensors.

I have reverse engineered the flow sensors used in an oil mist detectors on marine equipment. Don't remember the brand, but the idea was yet simple. And a same idea goes for a MAF sensors. There's no rocket science There were two PT100 temperature sensors located in a same air stream. One of them just measures the temperature of a flown gas, which is used as a reference (IAT). The other one does the same, except it is driven by a much higher current. The high current results in significant heating up of a sensor. Not to involve into thermodynamic rules, the air flow cools down the heated element proportionally to the air flow. By reading and comparing these two values into an equation formula You get the calculated flow amount.

Easy. Except, that if You lookup the PT100 conversion table, a 5 Ohms difference (that could easily be a result of a corroded contacts in connectors), for PT100 sensor would result in a 10K (or Celsius) temperature difference. That is pretty much, and if it is on a line to the heated element, that plays more significant factor in the equation You may have everything messed up.

Sorry if I went too much into theoretical digression. I am just trying to keep a common sense - the shipping from US takes time, and a parcel handling is a bit pricey. And I love troubleshooting. Thought somebody might have done it before me and could drop some feedback. Thank You for the data, I promise I'll do the MAF sensor tests tomorrow.

 

The MAF sensor outputs a changing frequency to the ECM. You check power and ground then watch the frequency of the output at different flow rates. Since you like direct measurements, break out your DVM and sharp probes. This may help:

Part 1 -How to Test the GM MAF Sensor (3.1L, 3.4L, 4.3L, 5.0L and 5.7L)

On fuel trims: you always add the short and long term together for a given bank. That tells you how much the mixture is different than the look up table for those conditions. A positive value is adding fuel. You usually look at idle and 2500 rpm. 10% absolute value is generally considered OK. I think 25% sets a code but many things can be wrong on your way to 25%. It is highly unlikely that you actually have zero fuel trim, especially if an engine is running that crappy and straightens completely out with the MAF unplugged.

George (Grzegorz)

 

Yesterday I drag my multimeter to a garage to check out the resistance at the MAF terminals. To be honest, I did not expect hundreds of KOhms or MOhms, but then I have realized, that probably the designer was smart enough to design a signal converter, as for a simple PT100 connectors resistance would play a role.
Today morning I have a really prompting reply from You George, some more fun on the way
I have read the article up to the pinout description and went for a kynar wire (30AWG in a teflon coating), removed the insulation, stick stripped end into a female connector of MAF sensor and put the plug in. I did not like the idea with punching probes. Here You may see how it looked like:


Then i start the ride and check the traces I got on my DSO:
Here's the one for idle:

And here for something about 2k rpms. Couldn't know exact value as I've had m oscilloscope under the hood and been operating throttle manually.


The units did not record (probably didn't set something while USB data storing, but it were 2V/div vertically and ... horizontally 100uSec )

If You wanna look how it looks live, here's a short video I made:

Now for the fuel trims. I haven't bought Torque Pro yet (waiting for BT dongle and Car Gauge) so my Torque Lite version gives me a chance to look at the STFT bank1 only and LTFT both banks. On idle, with around 500rpms I have readings in range of 0 up to -2%. No big deal. Cannot do idle test in a CL on 2k5 rpms, as mixture gets too lean. But I guess, my observations on LTFT that goes over 20% on acceleration and -20% on deceleration are sufficient.

EDIT: I think I'll take the MAF out and give it a bath in an isopropylene alcohol. I'll keep You updated.

Cheers,
Mike

 

So, to keep You updated.
Although the article gave an info on how to hook up to a MAF, there was no information how the proper traces should look like (no reference). Therefore, I had nothing to compare my traces with. At least, I've seen MAF seemed to be transmitting data.
So, I have decided to give it an alcohol bath:

I've been shaking the MAF in a bath to get at least some cavity. I didn't see a reason not to wash also the electronic circuit unit, as IPA fluid is quite often used to clean PCBs after manufacturing process.
I haven't find a lot of crap at the bottom on the end:

I left the MAF for a bit to dry and mounted back on the intake.
Luckily, I could increase on idle up to 6-7k without any significant problems. I hooked up with Torque Pro to see the fuel trims on 2k5. This time the conditions let me to do so. On 2k5 my STFT/LTFT are about -8...-10%. Now the question - what's an acceptable range? I know, in ideal they all should be zero, but, hey, there is no ideal conditions, so they used to deviate with some values. What is Your experience?

 

On fuel trims, take another look at post #16.

George

 

So, if I have my trim up to -8% on 2k5, then I should assume my engine is running OK? Great So the problem is solved! Thanks to You all for a great support!
Cheers, Mike