How would a bad barometric reading affect gas mileage?

Question

My wife drives an '08 Hyundai Azera Limited 3.8L with ~79k miles. Over the past few months she has been complaining that the gas mileage has gone down. Normal around town driving was getting us ~19.6 mpg (via the in dash meter). Recently fuel mileage is down to an even 17 mpg.

I started doing some diagnostics on the car to see if I could figure out what's going on. Using my trusty Innova 3160d scan tool, I used the live data mode to check the O2 sensors to see how they were behaving. They looked quite normal with readings all over the board between .100 & .900 mv. I started looking through the other readings which were available.

The Hyundai provides about 38 different items you can check through the DLC port (and I can see on this scanner). The only thing which caught my eye was that the barometric pressure was showing at 29.2 in/hg. The reason it caught my eye was that it didn't move. I didn't think it would move a lot, but maybe it would move some. With scanner in hand I started testing the other vehicles in my stable, only to find out neither of them showed the same reading. I had to go to the next door neighbors house and "borrow" their cars. I finally found one which showed a barometric reading. The vehicle I checked showed a pressure of 29.8 in/hg.

While .6 in/hg does not appear to be a large amount, in the world of climatology I believe it is fairly significant. What I'm very curious about, though, is if the car is showing this much of a difference from actual, how would it affect fuel mileage? Would it be enough to drop it like I describe above? Yes, I'm assuming the neighbor's vehicle to be reading correctly as it is a much newer vehicle than mine. It could be off, but I'm highly doubting it at this point.

Answer 1

If the location shown in your profile is accurate, ambient pressure should be around 29.7 in Hg, so your neighbor's car seems to reflect the correct altitude.

As @barbecue mentions, Wolfram|Alpha shows that 29.2 in Hg reflects an altitude of 205 m (674 ft) above sea level.

If you compare the air densities resulting from these two readings, the percentage difference isn't really that much: 1.6%.

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Some Azera-specific information

I found this extremely useful tidbit of information on the AutoZone site (emphasis my known):

The Manifold Absolute Pressure (MAP) sensor is a pressure sensitive variable resistor. It measures the changes in the intake manifold pressure which result from engine load and speed changes, and converts these to a voltage output. This sensor is used to measure the barometric pressure at start up, and under certain conditions, allows the ECM/PCM to automatically adjust for different altitudes. The ECM/PCM supplies 5 volts to the sensor and monitors the voltage on a signal line. The sensor provides a path to ground through its variable resistor. The sensor input affects fuel delivery and ignition timing controls in the ECM/PCM.

Based on this:

• I can understand why the barometric sensor reading doesn't change during the engine's operation - the MAP sensor is used to evaluate it just at startup.

• Stating the obvious, there is no dedicated barometric pressure sensor. The MAP sensor is used to determine barometric pressure. So if the barometric pressure reading is off, I have to question the integrity of the MAP sensor.

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So how could a bad MAP explain reduced fuel economy?

This was the reason why I was asking in the comments if the Azera has a MAF sensor. AutoZone seems to think so, so I'll assume it has both MAF and MAP sensors.

In such setups, the MAF sensor is usually used by the fuel management system to monitor and regulate AFR. The MAP sensor is only be used to regulate AFR if the engine computer finds fault with the MAF signal. So to say that fuel trims are impacting fuel economy would not explain it since the MAP isn't used for that purpose.

However, MAP sensors in these setups are usually used for secondary functions such as EGR.

I can only speculate at this stage that the faulty MAP reading is affecting the operation of the EGR valve (which operates off a solenoid as opposed to vacuum). It is speculation because I don't know enough about the specific implementation of the fuel management system on the Azera to confirm that the MAP is used for EGR. However, this would explain all of the observations and corroborate the hardware on this particular vehicle.

If the MAP-used-for-EGR assumption holds true for the Azera, the fault MAP would likely explain the reduced fuel economy.

Answer 2

If the Barometric sensor (BARO) is incorrect will the fuel mileage will be affected? It depends, and only rarely . In control systems that use MAF sensors such your Azera the BARO is not needed except as backup to the MAF sensor. The MAF sensor measures the mass of the air entering the engine which is used by the PCM to decide how much fuel to mix with it to achieve stoichiometric mixture. In this system the BARO can, and usually is, used only as a backup in concert with RPM, IAT and a MAP sensors to calculate air mass. That system is called Speed Density. Some vehicles do not have a MAF sensor and use this calculated air mass system. Also, it is common for PIDs that are not in use to display inaccurate data.

Even in the speed Density System if BARO was off the fuel control system would correct the mixture with the feedback given by the mixture sensors; whether the Air Fuel Ratio or Oxygen Sensor type. Check the Fuel Trim PIDs, if they close are to zero, less than 10, then the system is doing little to correct fuel mixture. If they are over 20 then they are getting close to the limit to maxing out what they can do to correct it. Fuel mileage should not be affected until the fuel trims reach the limit of available correction, usually over 30 total. But then the check engine comes on and fuel mixture type codes are set.

The onboard mileage calculators are not accurate in many cases. I always calculate actual mileage over at least three tanks the old fashioned way with pump and mileage readings and a calculator before spending time on diagnostics. In fuel injected engines 99% of the time the fuel system is not the cause of the problem, it is one of four things: Driving patterns changed, fuel type/quality changed, weather changed or vehicle modified from stock setup, such as air intakes or other inappropriate parts or programs.

Answer 3

Atmospheric pressure changes are rarely fast enough to see them happen on a gauge, it tends to take minutes or hours for them to be noticeable. Unless you're experiencing strong weather or rapidly changing elevation, you probably would not see any change at all, so that part doesn't surprise me.

Also, the sensors themselves are unlikely to be all that accurate. I'd expect a +/-5% variance at least.

I started thinking about this in terms of how a real change in atmospheric pressure might affect mileage. At sea level, atmospheric pressure averages around 29.9inHg (but can easily range from 29.7 to 30.2 with changes in weather.)

Generally speaking, lower air pressure would mean lower efficiency overall, and therefore lower mileage.

I used Wolfram Alpha to do some calculations. 29.8inHg corresponds to an elevation of 112 feet, and 29.2 corresponds to an elevation of about 674 feet. That's a difference of around 560 feet. By comparison, driving from Savannah, Georgia to Atlanta would be a change of around 1000 feet in elevation, significantly more. I wouldn't expect that kind of mileage drop just by moving from Savannah to Atlanta, so I don't think that much of a sensor variation should cause such a mileage drop either.

Based on this, I'd say it's unlikely that just the faulty reading alone is enough to account for this mileage difference.

That said, the sensor reading alone may not be all there is to it. It's possible that the sensor reading is multiplied by some other error, or that the sensor reading is a symptom of an electrical problem, for example.

I'd suggest you compare your car's reading with the barometric pressure reading from your local weather report, and if there's a big difference, replace the sensor.