We can’t see the ocean of air through which we move and breathe, but we can feel it and we can see what it’s doing. At 78 percent nitrogen and 21 percent oxygen, the atmosphere blankets our planet, about 350 miles deep if you count all four layers of it.
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Our vehicle engines breathe the same air we use in our lungs, and with modern emission controls, those engines breathe out carbon dioxide just like we do. That’s a good thing, actually. That means we’re going green; the plants love CO2. Why? Well, it’s because they breathe in that carbon dioxide, which is more than three times heavier than oxygen and nitrogen so that it hugs the ground, and that’s where the grass and trees live. And they breathe out oxygen, which is good for us and for the animals.
We gearheads all know that pistons and valves on one end of the manifold create low pressure in an intake manifold. And as we increase the flow of air by opening the throttle, the PCM modifies the flow of fuel proportionally so that, if things are working right, the engine picks up speed and power. With that power applied to the wheels through the flywheel, transmission, and final drive, we finally drive away.
Of course, because of chemistry and heat, there’s some NOX, water vapor, oxygen and a couple of other gasses that leave the combustion chamber, too, and there are ways of dissolving the bad elements of combustion that prevent them from ever seeing the light of day on a properly operating system. The point is that, in order for this carefully executed dance to work as intended, accurate calculations have to be made from reliable inputs, and we’re here to talk about the most important input on most of today’s car engines.
Density vs. Mass The first fuel injection system I knew anything about used manifoldThen there was the Bosch mechanical fuel injection known K-Jetronic on VWs, Audis and Deloreans and the movement of incoming air is important even on that mechanical fuel delivery system. These provide a steady spray through injectors that are more or less glorified atomizers.
The heart of the fuel distributor on those engines is a piston-type valve that moves inside a slotted sleeve that allows fuel to flow through tiny .02 mm vertical slits (one per fuel injector feed line). A lever that is hinged on one end and has a large round air-driven plate on the other end moving in a funnel shaped air passage controls the position of the piston. The more air that flows into the engine, the more the plate moves and the more of each slit is exposed, thus fuel volume is increased to those gloriously simple fuel atomizers.
As for more contemporary electronic fuel injection, before Mass Airflow Sensors (MAF), the old MAP sensor started outThe closer to the manifold that sensor is mounted, the better it works, and because Chrysler has never been comfy with MAF, they finally mounted the MAP sensor right on the manifold with its sensing port reading right out of the plenum. Interestingly, Chrysler never followed Ford and GM to the fixed orifice refrigerant systems either – they stayed with expansion valves in their A/C systems, but that’s a digression.
Higher manifold pressure (low vacuum) means the engine is under a heavier load, and for the PCM on vehicles that use it, that’s a dandy way to decide how much fuel the engine needs. The MAP sensor also provides barometric pressure information, and a MAP that doesn’t tell the truth about barometric pressure will cause fuel trims to go wacky. Besides the MAP, there are other modifiers, too, though. Engine Coolant Temperature, Intake Air Temperature, throttle plate movement and position, etc. But MAF or MAP most normally are the priority inputs for fuel delivery.
MAF – Order of the Day
MAF sensors are virtually always mounted between the air filter and the throttle plate and they don’t have any moving parts. They just measure the temperature, humidity and flow of the incoming air, and the ones we’re most familiar with are the hot wire MAF sensors. This type of MAF sensor has two wires; one that senses ambient temperature and the other one, called the hot wire, is kept at a fixed temperature above the measured ambient temperature. The Ford hot wire sensors produce a voltage from about 0.4 to 4.5 volts, varying with the airflow.
The earliest hot wire MAF sensors appeared on some imports as far back as 1979. That’s the most common type of MAF sensor used today. There are two tiny bridges, each with some platinum wire wrapped around something that looks like a piece of plastic. The amount of current the sensor has to feed to the hot wire to keep it at the calibrated temperature is then converted to a voltage signal.
Rochester introduced hot-film MAFs on several 1984 model GM V6 engines. Like the later hot wire sensors used by GM, these hot film sensors produce a square-wave variable frequency output with a frequency range that varies from about 32 Hz at idle to 150 Hz at wide-open throttle.
In 1990, GM went back to MAP on most but not all of its engines. And if you buy a white box part to replace a GM hot film sensor, your MIL light might not go away even though the hertz reading is within specs, because the ECM sometimes just doesn’t like anything but a Delco sensor. Been there, done that.
Mitsubishi had oddball MAF sensors on their late 1980s and early 1990s cars that basically used a tiny speaker and a matching receiver with a honeycomb-like grid to direct the air so it passed straight on its journey through the chamber between them. Since air deflects sound waves, this $700 sensor uses that principle to report airflow.
Ferreting Out a Problem
GM’s earliest MAF system came out in the mid-eighties, with sensors that turned out to be so unreliable that a retrofit was released to convert those early MAF equipped cars back to MAP sensor systems. The troubleshooting technique on the early GM MAF sensors was to gently rap the sensor with something and see if the engine stumbled. If it did, you replaced the MAF.
Every sensor I’ve seen on more contemporary vehicles reads grams per second (gm/s) and a V6 engine will typically read 3 to 4 grams per second (gm/s) at idle depending on temperature and engine size. The grams per second reading on your scan tool should pretty much mirror the Throttle Position Sensor voltage and react quickly with throttle angle changes, but it should remain stable at a given rpm and load.
It feels almost like a clogged fuel filter and you can see the MAF top out on the scan tool. Disconnect the MAF and drive it (the PCM makes it calculations a different way when the MAF is absent) and if it runs like a race car, you need a MAF.
Once a guy came to the Ford dealer complaining about an intermittent knock on his Explorer – sometimes the engine would run just fine and other times it would ping like crazy. I removed the air cleaner housing and found a roach’s wing lying on the air filter. Removing the roach’s wing fixed that one – sometimes the wing would get sucked up to the MAF inlet screen and block the air from passing through the small hot wire chamber. Sometimes it wouldn’t.
When the airflow was blocked by the wing, it would cause the engine to run very lean. Look for spider webs in that little hole, too, because that happens, and any kind of crack in the air inlet tube or an intake vacuum leak will allow unmetered air to enter the intake stream. That fouls up the PCM’s fuel delivery. Since MAF is the dominant input, that can cause nasty issues. If the intake air tube is cracked and the engine opens the crack wider with torque movement, it can cause a drive-away surge that will jerk a crick in your neck, so consider that as well.
Sometimes the PCM will internally short the MAF signal and act on the bad reading – this can cause rough idle and even low vacuum. In that case, I’ve disconnected the harness connector and wired the MAF directly to power and ground while reading the MAF signal with a meter. If I get normal readings that way and the signal wire isn’t shorted to ground between the MAF and the PCM, I know the PCM has gone stupid. I’ve seen that a couple of times, too.
The best way to handle any kind of scan tool reading is to read the stream on good cars so you’ll be able to recognize the bad ones. That’s the way I teach it and that’s the way I do it. Get used to the numbers. It’s surprising what a difference that makes in the long run.
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