Re: OBD-II/CAN Reader at Costco stores for $35.

I have heard of them but have no experience with it.

I knew a guy that had one mounted to his dash. He liked it.

If the ScanGauge II gives actual sensor voltages and frequencies, then it is worth the money. I'm not that interested in all of the other stuff the unit displays, but it might be worth it for someone who is.

No problem leaving it plugged into the OBD II port unless you leave the vehicle parked for long periods, where it will probably drain the battery more quickly.

I'd like to know how they are calculating mpg just from ECU data. Makes me wonder how it can tell the mileage difference from say a

41 mpg Corolla, and a 27 mpg Chevy Impala for instance.. "actual real life ratings, I've driven both". The Impala had that mpg indicator built into the car on the headliner display. Seems to me you would have to calibrate the thing according to miles driven, and gallons of gas burned. They seem to let you input gas data.

The ECU now controls everything; the speedo, the odo, everything. It would be a simple calculation from the data to figure fuel economy.

In addition, the car must have some way to determine how many gallons of fuel (or more likely, how many pounds of fuel) are left in the tank. Otherwise, the car's computer would not be able to estimate how far the car can go until the tank is empty. It can use this and the miles driven to determine average mileage.

Jeff

Say what?

The ECU keeps a running calculation of the current consumption, then it calculates the average over time or miles. Since the ECU knows how far it has gone, and how much gas it is using and has used, then it knows the current fuel consumption rate and the average consumption rate.

It's pretty easy to calculate MPG if you know fuel consumption and distance traveled. I'm not sure how they calculate distance traveled but this can be done through tire rotations or by speed x time. My father-in-law's old Servile gave MPG figures as an instantaneous value which maxed out at 70 MPG when going downhill or as a cumulative value which went up to around 15 MPG on trips of more than 12 miles. I just made shorts trips so my average was about 11 or 12 MPG. The cumulative MPG could be reset much like a tripometer.

There's all kinds of neat information available at the ECU port, I can read all that on my dinky cheap reader but that has a dopey small display. I wish there was a big readout multi-function display made especially for systems monitoring while driving. A great thing is that the hook-up would be pretty much a snap - not the case with a standard after market gauge set.

MPG figures and absolute manifold pressure and would be great to know as well as the standard coolant temperature and oil pressure etc.

I'd like to know how they are calculating mpg just from ECU data. Makes me wonder how it can tell the mileage difference from say a

41 mpg Corolla, and a 27 mpg Chevy Impala for instance.. "actual real life ratings, I've driven both". The Impala had that mpg indicator built into the car on the headliner display. Seems to me you would have to calibrate the thing according to miles driven, and gallons of gas burned. They seem to let you input gas data. <JS>

It's an easy matter to calculate the Injector ON time and the number of ON Cycles to arrive at how much fuel is in demand. The ECU knows how far the tires have gone, so figuring out how much gas was used to go how far is pretty easy.

</JS>

Possibly for the factory computer. Not easily done with a scanner. Too many vehicle variables, injector flow rate, fuel system pressure, increasing on time linearly DOES NOT generate a linear fuel flow curve.

You can buy software and an interface for a laptop. Many functions can display at once.

Here's one:

It seems like a no-brainer to me that a cheap flat panel display with a simple ECU port connector and a 12 volt line for the onboard microprocessor and display backlight that would sell for around $40 or less would be a popular add-on and has the possibility to make some money. OTOH, there's probably some reason it's not done.

Yep, but the fuel amounts could vary to engine. And different cars have different tire sizes which could effect miles per rev. They would have to use a specially built unit for each type of car, unless the vehicles OBD-II ECU has the MPG calculating logic built into it, and calibrated for each specific engine. As far as the Impala, I don't know if the ECU provided that data, or if a 2nd unit did that using data from the ECU. I didn't check too hard, but I didn't notice different models of the Scangauge for each type of car. I think it's a one fit's all kind of deal, unless I missed something.

If you check out their forums all your questions are answered. You have to tell the scan tool the vehicle make and model etc.

Ya think! I guess you don't realize that EVERY make and model uses different ECU programming. While the sensors and control items (solenoids, injectors, and such) are somewhat interchangeable the ECU is not even close. Back in the 80's they used different PROM chips based on engine/trans/gearing/option packages. Now they use different flash programming in the onboard EPROM

All of the fuel data comes from the Engine Management Computer.

It is, However it only reads SOME of the OBDII data.

There are at least 10 different communications protocols in use on domestic vehicles currently. Those are ONLY the ones controlling the engines by the way, add a few more if you want to talk to the body control modules, ABS, Air Bags, Climate Controls, Entertainment systems and a few others. Now you also need the codes from every manufacturer to tell you what the data actually means. Making ONE machine that can read ALL the different data sources and communicate over ALL the protocols is darn near impossible. There are just to many. Sort of like a human trying to learn all the different languages in the world.

So they created the OBD standards. It is sort of a Morse code for cars. BUT the codes only give you the basics. Sort of like ... --- ... tells you there is a problem. It doesn't tell you WHAT the problem is.

Then they came out with the OBDII codes. Those narrow down the problems by using more codes. So now you get ... --- ... / .-- . / .... .- ...- . / .- / .... --- .-.. . / .. -. / - .... . / .... ..- .-.. .-.. Which the computer then translates to - SOS We have a hole in the hull

And it only gets worse from there. When you look at the protocols it's closer to someone talking to the Russian key operator in Chinese and the Japanese operator on the other end translating that morse code into English. You just hope the people KNOW the language and translate it properly!

If I didn't realize that, I wouldn't be asking this question.. :/

I'm talking about the data they are using to calculate mpg. They are going to need more than just fuel data.

That's what I was thinking, one fits all, but according to another poster, you have to tell the scangauge what model car it is, so it does seem they have data sets for each type of car.. Or according to Bob anyway.. You would have to use different sets to be able to do that in all the various types of cars that use OBD-II. They sure all aren't equal. That would tend to tell me that the ECU does not have logic to calculate mpg on it's own, if you have to set it up for the model car you have it on.

OK, I suppose that could work.

ALL of the data needed is in the ECU. Amount of fuel used gets calculated using the injector pulse timing, RPM, load percentage, throttle position, miles traveled and starting fuel load (calculated using the percentage in the tank) This get compared to a table in the ECU which it uses to determine mileage.

The reason you need to tell it the make/model is so that the machine knows which communication protocol to use. It then looks in it's memory and determines which code maps to use.

Pulse width is there (as is fuel trim and flow rate) RPM is there Load is there TPS position is there Miles is there Amount of fuel in the tank is there

Plus a LOT of other data. Use a good scan tool and look at the live data some time. The data is there on ALL OBDII ports.

Easy enough to program a chip with the proper communications protocol and tables for fuel usage. The tables are not based on stoichiometric ratio though.

Then how do you calculate the MPG on vehicles that do not use a MAF? And there are a lot of those out there. Which DO display MPG.