Hello Folks,

Thinking about changing things up on my truck and looking for thoughts and opinions.

Right now my truck is running well, with no problems to report other than chasing seemingly ever present coolant leaks. Since, resolving the IAC issues earlier this year I have quite enjoyed the results of my build. However, my in-town mileage seems to be pretty well stuck at 10 mpg or less depending on how I drive, which is lower then before the rebuild, and I have to admit to being disappointed.

My build, largely is in the pattern of a Lightning, but I am still using the stock dual bore throttle body, and I am thinking about how that may be pushing me further up the fuel tables then is necessary. It occurs to me that there is probably a sizable mismatch between the TPS reading versus the MAP reading, compared to the programed expectation, especially at lower speeds. Seems like the fuel and timing tables would all be a bit off from ideal. So, my thought was to use a smaller throttle body, like a stock Lightning part to get things more in line with the computers expectations, and hopefully burn a little less gas.

Based on the research I have done, I would end up feeling like I have to use more throttle, and the truck would feel softer at low speeds, which I think would be compounded by my current gearing compared to the computers expected 4.10 ratio.

I have collected the majority of the parts to do this, which would include swapping the upper intake to an edelbrock 5.0 performer, which throws another variation into the set up. Parts and time aren't really an obstacle, but I could put my energy into other things if this isn't going actually make a difference, and sell what I have.

Your thoughts and opinions please.

 

I think you are making incorrect assumptions about how the EFI system works. Does this motor still have the stock cam in it? The motor has no restrictions on the intake or exhaust so if anything it should be running lean, but that is just a guess and guessing isn't a good way to make effective decisions. What have you done with the air injection system... deleted it?
The right way to go about optimizing your setup is to capture some data, but you need a tuning system for that, have you considered buying a Quarterhorse? A wideband sensor would also be a useful tool, with that alone you can see where the fuel mixture is in real time and also use to effect some broadband tuning based on the A/F ratio by redirecting the EFI system to use that sensor as it simulates a narrowband sensor. How does that make any difference? The narrowband output can be offset from 14.7 to make the system run leaner or richer.

 

My assumption is that it is lean and the computer is responding with fuel, based on the O2 sensor and on the MAP versus TPS feed back. It seems to me the computer can see that there is more air, and that there is more air faster than expected and it is going to try to compensate with more fuel. It seems to be happy with its own ability to trim it out as it has not thrown any codes.

The cam is the stock roller truck cam, and the 1.7 roller rockers. I deleted the air injection pump but left the solenoids in place.

I guess, my thought was less to optimize the tune, and more to nudge it nearer to the stock L tune...

 

10 mpg around town seems pretty normal to me? What does it get cruising a highway in overdrive at 65 mph or so?

 

I don't know how much more you stand to gain but there has to be at least a little left on the table. I think you'd be better off installing an air fuel ratio gauge and seeing what the truck is actually doing, vs throwing parts at it and hoping for improvement. I think you could definitely make improvements to your current selection and gain some fuel economy, but you could buy a lot of fuel with the money you'd spend to gain even 1 or 2 MPGs. If you do a lot of in town driving and almost no extended commutes or highway drives, I don't know that you'd gain anything at all.

The 351w in my '89 F350 is quite stout, with 35" tires and a motor assembled for low end torque to haul loads (fuel use calculation shows it should be at or just over 400HP peak) with an emphasis on decent fuel economy, I average around 11 in town and 13 on the highway, 10 if I'm pulling a 10k trailer. If I can get that from my rig, you should be able to do better. I guess it should be stated that the biggest change you can make is obviously in the way you drive the thing.

 

My engine is setup very similarly to yours (see signature line below for specs) and I'm getting 16.5 mpg... but most of my driving is highway, so I'm not really sure what I'm getting in town.
My truck is also similar to yours except mine is 2WD and short bed... same size tires... geared similarly (mine is 3.27:1 vs your 3.55:1)... mine has a 5-speed though.

So, it seems possible that you should be getting a little better mpg.
I like GNR22's suggestion above about putting an air/fuel ratio gauge on it to see what exactly is going on.
Also, you could try adding a vacuum gauge... You should get your best mileage when driving at the highest vacuum reading (i.e. lowest load).

 

That's what I did : another-better-your-fuel-economy
Works very well, kinda feel like an grandpa on a Sunday drive, gauge is very sensitive to throttle response, and I am getting better fuel economy.

@R&RFord do you have a tuner and/or data logging capabilities? Rather than changing hard parts, you should be able to better economy with your tuner.

 

The A/F ratio gauge would be connected to a wideband O2 sensor so it would seem we are all suggesting the same thing. When I had the warmed up 5.8 in my '90 the city milage was about 12-13mpg so I think there is a little bit of room to improve your setup, the trick is figuring out what that move is.

Where do you have the O2 sensor installed right now, and is it new?

 

I have always had good luck with AEM widebands when I tune. They are around $200 shipped for the gauge, sensor, and harness. No goofy calibration like the Innovate.

No clue what my '88 did when it was on the road. I remember 16-17mpg freeway was about it going around 65-70mph. Just cannot recall city, but I would venture to guess lackluster. The batchfire EFI is not helping consumption, but I would not expect 5mpg gain going sequential.

Window sticker from my '92 4.9/five speed 2x4 claims 15mpg city and 19mpg freeway with a 3.08:1. Figure a V8 and auto will do a bit less, yours is far more efficient than stock so that helps. Efficiency yields better economy. Seems Lightning was rated at 11 and 15. So...yah, you are in that area plus you weight more being 4x4 and a long bed.

 

Agreed... mine is still batch fire and getting 16.5mpg (mostly highway).

 

I figure most everyone went SEFI to help with emissions, not much other reason. Better economy was a byproduct, just like more efficient heads and exhaust systems make for better emissions/fuel economy and more power. That is why we had AIR pumps forever, leaned out mixture on rich cold starts...looks better for tailpipe emissions.

 

Actually the SD version historically produced a little better fuel milage on these motors than SEFI which tends to run a little richer and make better power. The air injection does not lean out the mixture either, as soon as the engine hits closed loop from a cold start the system redirects that air outside of the EFI management zone downstream of the O2 sensor so that it doesn't interfere with mixture control.The catalyst uses the oxygen being pumped in to burn off excess cold start HC's but it can't do it for very long, it normally operates with lean combustion gasses so if the fresh air isn't re-routed once it comes up to temp it would quickly overheat and melt.

 

Same here. I went through 3 Innovate gauges on 2 trucks. Every one of them would work just fine for a few weeks and then would start throwing some error code. When I called Innovate they suggested perhaps the wideband sensor itself was too dirty, so I cleaned it up, and it worked again for a week. Replaced it, good for another 2 weeks. They finally warrantied it and the next gauge did the same thing. After the gauge on the 2nd truck did the same thing after a couple months, I just tossed it in the garbage. Swapped to AEM gauges and haven't had any issues in 3 years.

 

So, trying to respond to folks questions and to provide more information.

Most of my driving is stop and go trips less than 10 miles, with occasional jaunts at interstate speeds to nearby towns. It is killer on mileage, but the same truck in the same conditions got 12 to 13 mpg's before the build. I haven't had any reason to take it on a long drive, so I don't really know what it will do on the hwy, My guess would be 14/15 mpg's. I know my truck is heavier and higher geared than the L trucks, so it seems my mileage is in the ballpark, based on what I could find... I have also really been considering lower gears as the bulk of my driving is in town.

No tuner or data loggers installed. Running a stock Lightning computer, with the stock tune.

The O2 sensor was new when installed, last year, and is in the stock location in a Bassani Y pipe. Considering the issues I had early on, it is possible the sensor is fouled. I have installed a new bosch this morning.

The vacuum gauge is something I have thought about before, and will consider.

To the suggestion of putting a wide band on there... I get that collecting more data would be ideal, but I don't know that I want to go down the path of playing with the stock tune or going full aftermarket with something from stinger... I have gone to a fair amount of trouble to not have to do that.

I will think on things. Thank you for all the feedback so far!

 

Installing a wideband might keep you from spending a ton of money for no reason. If the trucks fueling map doesn't match its actual needs, you will see which way it needs to go. If the truck is always lean, then it wouldn't make sense to throw better air flow components at it, as it would only make the condition worse. Plus, it is a valuable diagnostic tool even without the desire to tune the truck.