I noticed the other day that when my 3.0 Ranger idles, it runs a bit rich, the Long Term Fuel Trim on Bank 1 tends to fluctuate between -7.8% at worst and -4.9%. Bank 2 is more consistently subtracting around 4%. Driving down the road, both Long Term Fuel Trims run closer to 0, Bank 2 is almost always 0 on Long Term. I also noticed the MAF at idle fluctuated in a range of 6-7 g/s at 880-900 RPM. I pulled the MAF, sprayed it down with MAF sensor cleaner a few times, let it sit overnight and reinstalled it the following morning. After doing that, the sensor fluctuates between around 4.9g/s and 5.5 g/s, it will randomly bounce up almost a gram and come back down, without me touching the throttle, while the RPMs fluctuate in that same 20 RPM range. The air filter is still in decent shape.
How consistent is that general rule of 1 g/s per liter of displacement at idle? I could see the rich idle being caused by slightly more dirty fuel injectors on the left side, and the mixture leaning out as I drive due to the fact you need more fuel to drive faster, and the imperfect spray pattern is providing it. I threw a thing of Royal Purple in to see if that helps, but I'm also wondering if the MAF sensor is starting to show its age at 275k miles and 23 years and it could be contributing to the slightly rich idle. I have no drivability problems, but I thought this would be interesting thing to see if I can correct.
The way to test your MAF sensor is to look at the voltage output signal. It should be approximately 1.0 VDC at warm idle and steadily increase as RPMs increase, likewise steadily decrease as RPMs decrease. Erratic voltage fluctuations indicate a problem with the MAF sensor.
I have found with my Fords the g/s is a higher value than the engine displacement in liters. I recently saw my 4.6 idling around 5.5 g/s. The long term fuel trims were around -4% as I recall. Considering age and mileage (21 years and 155K) I believe that's fine and the engine runs like a top. If I were concerned I would perform the voltage test described below.
I have seen from google searches that the 1g/s per liter rule of thumb is for a warm engine idling at 500 RPM. My car is a manual trans that idles at 750RPM warm so the 5.5 g/s seems reasonable. That value is depended to a small degree on atmospheric pressure so it will change from place to place. This could be one reason why the voltage output signal testing method is preferred as a more definitive way to check MAF sensor performance. It's pretty simple and all you need is a multi meter. It think your cleaning helped and the 23 year old MAF sensor is still working correctly from what you've said. It would be a shame to buy a new one just to see nothing significantly improved.
I used Chevron Techron fuel system cleaner a few times thinking it was doing some good, maybe it did, but when I removed the injectors they were still pretty gooey. A little throttle body spray cleaner, new O rings and they were happy again.
When I tried measuring MAF voltages with a multimeter I personally did not succeed. Meaning that having 2 MAF sensors at hand - a bad one and a new and good one - I could not see any difference in measured voltages. Although the ECU of my car could perfectly tell between the two sensors: with the new one, the car was running fine, and with the old and bad one the car was idling roughly and the ECU was throwing MAF sensor related errors 🙁
I can't speak for Ford, but I know that most of the sensors on my GM do not generate a DC voltage. Engineers long ago found that frequency or duty cycle modulated AC signals are far more robust, reliable, and travel farther distances in the noisy automotive environment. My MAF sensor output looks more like this:
There are digital and analog signal MAF sensors. An oscilloscope would be needed to test the digital type. A volt meter only works for analog.
Nope. Not digital. it's still analog. Just analog AC.
My multimeter can measure frequency and duty cycle.
I was referring to an analog signal is a continuous signal and a digital signal as a time separated signal like in the example above. Yes, some type of frequency or duty cycle measuring device would be needed there.
The signal in scope image I posted cannot be digital. Not only is the frequency is changing , but it does not represent digital data. The frequency maps directly to an analog value.
Great video!!! Thanks
Is your MAF sensor reading airflow at KOEO?
I don't recall if it has the coolant temperature data or not.
I guarantee you it does.
It does. I didn't pay much attention to that. Drove 28 miles on the highway and 50+ mpg country roads. ECT is around 190 F, 90C, which is pretty close to normal operating temp.
when my 3.0 Ranger idles, it runs a bit rich
was it fully warmed up?
Yup, it better be after driving 28 miles on the highway, lol. I have actually watched my Mustang go from cold to warmed up on a scan tool, the it's cool how the fuel trims do what they do, rich when cold, leaner when warm. The theory of a carburetor in action on a screen.
What year is your ranger. ..My Mercedes did something similar when my maf started to go bad. You may have a vacuum leak. Try Disconnecting your maf see if that helps. If so then the maf is bad. This technic work 90% of the time.
It's in my signature underneath my post. It's a 1999. I thought about trying that trick of disconnecting the MAF. I may do that.
It is counter-intuitive, but on German cars, for example, a richer idle and a leaner mixture while driving can well be a sign of an engine | coolant temp sensor being faulty (or starting to get old / off). These sensor readings (and not only those the MAF delivers) are also involved in the process of the correct air-and-fuel ratio determination done by the ECU, so if the ECU is being misinformed of the actual temperature of the engine and/or of the coolant, the air-and-fuel ratio will be off even with a perfectly working MAF sensor.
I'll see if my scanner will get that info. I was thinking about that but I don't recall if it has the coolant temperature data or not. I may take part of my income tax and buy myself a higher level scan tool for a birthday present. Lol.
There is unfortunately a pretty wide grey zone in the temp sensor readings, as they get old, before those generate an error readable via scan tools. You might however want to try using an external calibrated temp gauge and compare its readings with those the car temp sensors deliver.