[ike84]

Blue -

Spot on Spot on.
Yes misfire, backfire , detonation, knock, pre ignition,
All misused and abused terms.

One thing I forgot in my Diagnostic process of a lean condition is>>>exhaust leaks before the O2 sensor and at the bung weld. Amazing what a pinhole leak you can't even see does to the O2 sensor when it is at the bung weld.

Yes the engine must be fully warmed up for the Vac leak testing I described. That's why the idle, look, listen, feel, smell period followed by the test drive.
Another thing I like to do if time allows is:
have good strong flashlight, and a cold drink, (insert favorite alcohol here). pop the hood and just look, then look some more. don't touch anything, then look some more.
Look and don't tough anything until the drink is gone.
Amazing how many times a person has brought there car me and I found the problem that way.

You mentioned difficulty in testing MAF Sensors.

So a very important concept to try to use in diagnostics:
Is how things correlate to each other. Things that work in tandem and things that work in opposites or ratios.
The example I gave you of the ratio of metered air to unmetered changing as you open the throttle is an example of opposites or ratios of opposites. So you use that to diagnose Vac leaks.

So if you look at the MAF sensor and the TPS (throttle position sensor) there is a tandem or a parallel correlation between the two.
That correlation is voltage as it relates to Air flow.

The more air past the MAF the higher the MAF signal voltage. The more you step on the throttle the more air you get and the higher the TPS voltage goes.
There is a direct correlation between the two.
So the voltage signals will parallel each other on a graph.

So here is how I test a MAF sensor.
With engine off and at ambient temp hook up a scanner that has graphing capabilities (The Durametric)
look at Intake Air temp with engine off. Is it within 3-5 of ambient temp.
If yes next step.
Intake air temp is measured by the MAF.

Now get the engine full warmed up.
Find TPS voltage. put it on the graph.
Now find MAF sensor voltage. (not air flow not resistance).
MAF voltage.

Now when you step on the throttle you will see both voltage signals rise on the graph parallel to each other almost like a set of railroad tracks.
Let off the throttle the signals will drop again parallel to each other.
If there is a problem with the MAF you will most times see one of three things. The signal is biased high, The signal is biased low, or there is a stutter it doesn't run parrallel to the TPS signal.
Two other things to be aware of: there is a little lag time in signal response from the MAF when you go WOT... Air flow and RPM's increase slower than your foot moves.
The same thing happens when you come off throttle quickly to closed throttle decelerate.

So hook up your Durametric and play with it. once you get used to what you see you will understand how the test works.

When I do it I get everything set up and go for a 50 mile drive (loop). With the laptop on the passenger seat so I can watch it.
This Lets me see the two signals under all driving conditions.
If there is any kind of anything not right with the MAF I will see it at the same that I feel the car not respond correctly.

It is not an absolutely fool proof test. But it is pretty damn good

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