Measuring Injector Duty Cycle. MPG Different Speeds

[landtoy80]

I have a lot of free time in my 171 mi commute and want something to do so I have been driving at different speeds to see if I can increase my MPG.
So far I have got:
65 mph = 15.282 mpg average
75 mph = 14.482 mpg average
This is with the O2 sensors unplugged and EGR disabled.

I use to run the same route but the commute was 138 miles my average about 14 mpg with working O2 and EGR at 80 mph.

I am looking to hook up some sort of meter to measure the rate of fuel used in real time. I know there are some meters like Fuel Injector Duty Cycle Meter $75, mpguino $64. But all my $$ is being dumped into the tank every day.

My though is to measure the Duty Cycle of the fuel injector. I have read that you can use a dwell meter to measure the duty cycle of the injector. Short of tapping into the injector wire with the dwell meter, is there a port somewhere on the harness to hook up the dwell meter to? Do one of the electrical ports, that you hook up a Toyota hand held scanner for OBD1, go to the injector so that you could read the duty cycle from it?

Is the fuel pressure constant at different rpm's? If not, then using a dwell meter to measure the duty cycle will not work to measure the amount of fuel used.

 

[nodaksn3]

Duty cycle!!!! Wish I understood that concept. I had to take my 1984 380SL to an M-B tech to set it. Unfortunately I forgot what tools he used. I will review the CD and maybe I can figure out how to do it. If I do I will let you know. Ned

 

[Howard705]

Does the regulator have vacuum to it-adjusting the pressure based on load?

 

[elhombre]

I have two Ford 7.3 diesels and bought Auto Enginuity to read the computers in them. I have become a definite Nerd as I have tried to learn more about them and the fuel systems. I have a few questions. Why isn't your O2 sensor hook up? The sensor is what tells the computer how much fuel is being burned. If there is too much O2, that means more fuel can be injected into the engine. If not enough O2, then the computer needs to decrease the amount of fuel injected. The computer will change the amount of fuel injected by changing the Duty cycle (percentage of time the injector is buzzing). This happens many times per second while driving. The fact that your O2 sensor is not hooked up, your CEL is on, and the computer is in "safe" mode. It is running a pre-programed safe mode "map" that puts fuel in at a extra high rate (greater duty cycle than needed) so that the engine will not run lean. Lean engines run too hot and destroy themselves. Mr. T would rather you run down the road with poor fuel economy than burn up your engine, so it is programed that way.

The fuel regulator does have a vacuum line on it. It tries to keep the fuel pressure at a constant psi, and it does a fairly good job at this. But it is not rock solid constant, and the computer will adjust the injector duty cycle to compensate for the variable psi by looking at the O2 sensor.

Just because I am a self proclaimed nerd, doesn't by any stretch mean I know all there is to know about fuel injection, but when the duty cycle of an injector becomes too high, it usually means there is a fuel pressure problem, or an air intake problem. In diesels, it usually is a fuel pressure problem. In our Toyota, it usually is an air problem ie. Air intake tube torn, vacuum leak, EGR stuck open. The computer knows what the duty cycle should be, and it can adjust to 30% above and below that value until it will set a code and go into safe mode.

Are you thinking of trying to change your duty cycle for better fuel economy? The computer does know the duty cycle value, and it is read by plugging into the OBD port. After all this, I didn't answer the question, did I?

Drive slower, you'll get better gas mileage. It's all about wind resistance.

 

[greentruck]

Duty cycle!!!! Wish I understood that concept...

I'm definitely no injector expert, despite having passed many sets of Cummins injectors on to the rebuilders in a previous career. But duty cycle is basically a data set that shows how much time they're on and how much time they're off, if I have the concept even half right. Obviously more Off = better MPG. I wonder how wide the part of the curve is that gets the best MPG?

I know people often comment that they believe they see improved MPG when moving up to 33s from the stock tire Michelins. Seems counter-intuitive, until you consider the truck's gearing is said to be optimized for 33s in the world market. This could provide real-world evidence that the perceived MPG increase isn't a fluke, but built into the truck's DNA.

 

[landtoy80]

The O2 sensors died, again. I have run with them unplugged and plugged in off and on for the last 5 years or so. I have seen no decrease in gas mileage. I have a log of every gallon put in the tank for the last 195,000 mi.

I drove a couple regular commute routes for the last couple years and wish I would of unplugged the O2's while I was still running the same routes but it didnt work that way. I now drive a 171 mi round trip commute. Several years back I drove the same route but it was 138 miles. I will compare the MPG from that commute to this commute and see how much difference there is between them. When I ran the 138 mi commute all emissions were working. When commuting the 138 miles in 2007 I can see when I started to run the AC as my mileage went from 14.xxx to 13.xxx.
This time there is no O2 or EGR.

The first 2 weeks I was testing different speeds and this last tank I got 16.458 mpg going 60 mph. The speed limit is 75 and most go over 80 mph, I found it very dangerous to run at anything under 70 mph in western colorado on I70.


I was just looking for a cheap way to read my fuel consumption to see, in real time, how different driving styles, mph, timing advance changes the fuel consumption. Having an OBD1 vehicle limits the choices of electronic fuel monitoring. My 87 Supra had a readout that read MPG, Gallons Used and how many miles to empty. So I know the technology is out there to monitor fuel usage, I just want to do it on the cheap till I get back into $$$$ making mode.

You would think that one of the ports on the harness or ECU would send out a signal from or to the injector that I could hook up a dwell meter to so I could read the duty cycle as I drive. I really dont want to cut an injector wire as they are hard to get to and very brittle.

 

[scottryana]

I would get out the diagrams and look for anything that was related to Injector Pulse Width. The injector duty cycle is related directly to the pulse width, the pulse width is the signal that opens the injector, by monitoring it's length you could roughly gauge usage, but it isn't going to be exact because fuel pressure isn't always the same.

But really we all kind of know the reality is that at 50-55mph you're going to get the best gas mileage it is just a trade off on going that slow. lol

 

[landtoy80]

but it isn't going to be exact because fuel pressure isn't always the same.
. lol

This answers the main reason for the post . I wanted to know if the pressure to the injectors was constant. Without a constant fuel pressure, there is no way to use the duty cycle of the injectors to measure fuel consumption accurately.

Guess I could mount a fuel pressure gauge and an injector duty cycle meter but that would distract from driving and would be a pain in the a$$ to monitor.


How do OBD2 vehicles read out MPG? Do most modern motors have a constant fuel pressure or does the ECM/ECU take fuel line pressure reading into its calculations? Many modern gas and diesel motors have a fuel usage meter and I assume they are quite accurate.

 

[TrickyT]

There is no direct MPG readout in OBD-II equipped vehicles, at least none that is part of the standard "legislated" PIDs. The formulas I've seen for calculating it use the mass air flow rate and vehicle speed, both of which do come directly from the OBD-II port, and assume an ideal fuel stoichiometry of 14.7.

 

[Grim Reaper]

I'm surprised you haven't burned a valve. EGR at hwy speed will help you make mpg. When it's not functioning the O2 will compensate for it not working by Balancing the mixture but should throw a EGR or o2 light. The reason it might throw a o2 light is that's where it see the mixture out of expected range.
With both not functioning I would think the ECM would go rich to avoid burning valves but with the mpg your getting being so close to your previous observations I'm a bit surprised.


How do the plugs look?

 

[scottryana]

Yeah me too every time I disconnected the O2's it defaulted to open loop and was running 10:1.

 

[landtoy80]

My last O2 lasted 36,000 miles. They stared dieing last fall/winter. Got the off and on CEL and 25 26 code. Then it started running rough at idle and got worse.

I haven't checked plugs but after last time running O2-less they looked fine for the miles I had on them.

IF I get a local job I will be able to see how short runs compare to when ran short runs with the O2's.