I have a 1997 Mazda with the 2.3 liter in, my issue is that it has an intermittent miss that I have been having on and off for several months now. it will run fine for a couple of weeks and then act up for a few days then go away, it will also stall occasionally at stop lights, the miss happens at different speeds and makes the truck jerk pretty bad. I do have a Bluetooth OBD scanner, it does not show any codes, I did see while driving and watching the mass air flow sensor that the reading would drop to zero when the miss happens and when sitting at a stop light the reading jumped from 3.1-3.2 up to over 100 when it was trying to stall. Could these numbers mean that the sensor is going bad? any help will be appreciated.

 

Yes, that reads like a failing MAF sensor, unplug it and go for a drive, computer should switch to OPEN loop and run from Factory air:fuel mix tables, which don't need MAF data, unless you are at a high elevation, which Florida ain't, lol

If engine runs OK, no stalling out, then replace MAF sensor, engine won't run perfect with no MAF data, but should run OK

 

thanks RonD I'll give that a shot today, I also got the spark plugs, wires and coils from Rock Auto yesterday, I've had the truck for 2 years and don't know when the last time they were changed.

 

Tried unplugging MAF sensor and starting the engine, would start but not idle. Had to keep my foot on the gas to keep it running and while it was running it was really rich, could see black smoke coming out if the exhaust. Any other things I can look at?

 

See if you can get the LTFT(long term fuel trim) numbers, my guess would they would be +20's or higher

LTFT is used when engine is cold OR when a main sensor like the MAF is unhooked.
The purpose of LTFT is offset small vacuum leaks, lower fuel pressure and other small issues as a vehicle gets older, so computer uses factory air:fuel mix tables but offset using LTFT when cold

Was test with MAF unhooked when engine was cold, if so do the test with warmed up engine

 

yes the MAF was unplugged when it cold, I got a chance today to let it warm up and check the LTFT numbers, they were between 14-15 with it plugged in and 19.5 with it unplugged and the water temp at 186. I did take it for a drive and it had a little more power when on the throttle, idle was a little rough, when I got back home I let it idle for a few minutes, it idled fine for a minute then got really rough and stalled.

 

LTFT is high, assuming those are + numbers
STFT and LTFT are fuel injector open time DIFFERENCES from Computers calculation of open times, which is 0 STFT
Computer uses engine RPMs, MAF sensors data, and throttle position to calculate the amount of fuel to add to the intake to get the 14.7:1 air:fuel ratio need for gasoline engines
That calculation is 0 fuel trim, after this mix is burned in the engine, the Computer gets O2 sensor data, O2 detects Oxygen not fuel, high oxygen is a Lean burn, low oxygen is a Rich burn
If oxygen level in exhaust is high computer adds more fuel, opens injectors longer, +1 STFT, is 1% longer, +10 STFT is 10% longer, so 10% more fuel is being added than computers original calculation called for
RPMs are RPMs, Throttle Position is Throttle Position, no real guess work on these, lol
MAF data and O2 sensor data however can be subject to problems, these are the main sensors in the Computers Closed Loop operation
O2 sensors generally last about 100k miles
MAF sensors, well they rarely fail, they do get dirty but are easy to clean, but rarely fail does not mean never fail

STFT numbers are averaged and become LTFT, so higher LTFT numbers means STFT has been higher for awhile
Vacuum leaks are the most common cause for higher + STFT and LTFT numbers, extra air is coming into the engine, bypassing MAF sensor so Computer's 0 number is wrong

But there is another often overlooked factor................
1997 will run a Return Fuel system which should have fuel pressure of 30-40psi at the injectors(1998 and up is 55psi)
Computer has no "Fuel Pressure Sensor" it assumes 35psi engine running in its calculations, if pressure is say 20psi then when an injector opens LESS fuel is flowing into the engine, so this would through off Computers 0 calculation and you get higher STFT and LTFT numbers
So if possible check fuel pressure with engine running, also hold engine RPMs at 2,000 for 30seconds or so to make sure pressure is not slowly dropping

 

I think I finally caught the problem, there were no codes showing this morning, I had some errands to run today and it decided to act up. It is showing P0102 (MAF circuit low) under pending codes, I went ahead and ordered a new one from RockAuto ($38.00). I also installed a different app on my phone that lets you see all of the sensors live, Will wait for the MAF sensor to be delivered, hopefully it solves the issue. By the way both O2 sensors looked good, no fluctuations in voltages, they both were pretty consistent with each other, there is not a cat in the exhaust, it wasn't there when I bought the truck.

 

I received the MAF sensor last Thursday and put it in that day, I also disconnected the battery for 30 minutes to reset the computer. All I can say is wow what a difference, the STST is now at -2 at idle and LTFT is at +5ish instead of 19.5, the only thing is now when starting the RPM's will go to about 1200 like normal but will drop really low and it will stall, doesn't matter if it's cold or already warmed up. I can press the throttle slightly and it will start fine and idle fine. It always started fine before the sensor was replaced, it's not showing any codes, any ideas on what this could be?

 

Does it have idle air controller and erg valve? either could cause idle issues.

 

It does have an IAC valve, it idles fine after I start it with my foot on the throttle slightly.

 

 

It will relearn on its own
Reset is done by unhooking 1 battery cable for 5 or more minutes, it clears KAM(keep alive memory) in computer, the same as it clears Clock and Pre-sets in the Radios KAM

 

The stalling at startup has been cured, it was self inflicted, when I bought the truck 2 years ago it would idle around 550 rpm when the A/C was on,, me thinking I knew what I was doing messed with throttle plate adjustment screw trying to raise the idle. Whoops that came back to bite me. Thanks for the help RonD you pointed me in the right direction.