you might be able to do it digitally if you can tie a multimeter into your MAP sensor return wire, and run the multimeter wires into the cabin so you can spy on the relative voltage in different driving conditions.
you just need to know what the upper and lower limits are (which is easy to figure out in a few minutes of "extreme" driving).
(also, i understand not all cars have MAP sensors, so this might not apply to you - some use MAF instead.)
I tried this today -- but ended up with a problem. I assume I located the MAF sensor -- it was the only item on top of the intake manifold area, with a vaccum tube running to it, a tube sticking down into the manifold, and a 2-wire connector. The only other sensor nearby is what I think to be a throttle position sensor with 3 wires.
There are 2 wires: red and yellow. I hooked up the red wire and a miscellaneous ground to a multi-tester and got 14-volts, so I'm good there. I hooked the return line to the tester (yellow wire and ground) and still got 14-volts. I ran the wires into the car and drove around -- still 14-volts under all kinds of engine speeds and conditions -- warmed up and everything. I disconnected the sensor and got zero volts, but no check-engine light.
Could my MAF sensor have gone bad? I assume it's supposed to vary voltage based on the flow of air, vaccum, or something. At any rate it isn't. May need some help here.
I'm probably going to order a Vac-guage anyway, but should the MAF be reporting the same voltage all the time?
MAF should not report the same voltage constantly, you must've done something wrong, if you were running on a busted MAF you would be throwing code and the car would run terrible. You'd be spitting smoke you're so rich or choking the engine by running to lean, you'd definately know if the MAF was busted.
Someone on honda-tech mentioned a while back that they left their MAP sensor disconnected when they did their engine swap and as a result got 80 miles to the 12 gallon tank.
MAP sensors should have three wires (+5V, ground, and output). Often on the intake manifold is the intake air temp sensor, which is just 2 wires. If you were measuring the voltage across the 2 wires, it would make sense that it would always read 14V. No matter how its resistence changed with temp, it's the only voltage drop in that loop and will always drop whatever the supply voltage is. The MAP sensor is usually on the throttle body (which is also where the throttle position sensor is). Both are 3 wire sensors with supply, ground, and output wires. Be careful connecting power directly to these sensors. I'm 90% sure they are on 5V levels, not battery voltage.
Sorry for not posting in so long. I've been busy working a lot now that school is out (going back today though). Hopefully sitting in class I'll get bored and then start thinking about the MPG gauge and get re-inspired to do it. I don't know if I'd sell it or what, but you would need a specially programmed chip to do it, so maybe I could just sell that and some of the not so common supplies. Anyway, the project never left my mind, it just got put on hold. I ordered a PCB fab kit for my work-related microcontroller project that would be great for something like this, too. Scangauge, here I come.
Also, I've been doing the "pulse and glide" technique with my latest tank of gas. I'll respond once I fill up again.
Scouring the country for an excellent condition Civic VX
The MAP sensor connector and the TPS sensor connector on Honda/Acura cars look almost identical. I know on obd0 and obd1 cars (not sure about obd2, but probably the same) people often plug the TPS wire into the MAP sensor, and vice versa. In other words, they both have 3 wires.
On my intake manifold/throttle body the MAP sensor is on the front (towards the front of the car). The TPS sensor is on the back of the throttle body, near the firewall.
It sounds to me like you may be testing the wrong sensor.
There are lots of plans floating around for o2 sensor monitiors, but that's so simple since it's just displaying the voltage you get, but in this case you would need to make a chip for it. I wonder if I can use the chips I generally use for chipping ecus to do it.
I couldn't find it in time before the experiment, but I did some Internet research to try and find the MAF sensor. Turns out I tapped into the air-temp sensor. I may need to explore further around the throttle body when I get back (out of town). I should be reading Ohms of resistance instead of voltage, I expect.
SVOboy has commandeered by brain on this for a minute, and ordered me to post my thoughts on it. :P Basically, we need some way of determining the amount of fuel used, and the distance travelled. These can either be directly measured, or inferred from other methods. The Honda fuel economy displays in the Insight and all that good stuff use fuel injector duration, with an assumption of a specific fuel pressure and density. These could be adjustable parameters in whatever system we devise, as they can vary from car to car, and place to place.
My thought is to use a dedicated device, like a PIC or something similar. This would allow you to sample the injector interface very rapidly using most of the processor time without hurting any other stuff, like it would on a laptop. Care must be taken to use a very high impedance input for the fuel injectors, as you don't want to interrupt their operation. Distance is as simple as speed and time from the VSS.
The average fuel economy is easy to calculate, as it is the cumulative values of fuel used and distance travelled. The "instantaneous" fuel economy would have to be the average of the last X values the device recorded. The last 10 values should provide a reasonable value, but tweaking might be required.
I'm not a programmer, so my job sort of ends here.
I've posted my thoughts on using a PIC microcontroller for monitoring MPG and other variables extensively in the past. Theoretically it would be very simple to do. It's just getting up and doing it that's the hard part. I wasn't going to use any averaging in the instantaneous MPG though, just simply measure one cycle of the fuel injector, then measure one cycle of the VSS, and calculate MPG. The two would happen so fast there shouldn't be much error in the speed changing after the injectors are measured. The refresh rate would be maybe 4 to 10 times a second based off speed. A long term average might be more of a problem since the total sum would start to get very large. I think the compiler I use for PICs supports long ints, but it's something to consider. Slowing down the sample rate for the average calculation would also help.
Scouring the country for an excellent condition Civic VX