How to Tune for Fuel Economy

[donnyeagle]

I have a 85 Sedan with the 258. I have done the ECM Test Bypass, Motorcraft Distributor cap, new plugs and wires, Petronix coil, rebuilt BBD stepper motor carb, torqued intake bolts, new EGR valve, choke operates correctly, timing set to 9 degrees at idle. It has 2.35 gears. It pulls a solid smooth 20 psi vacuum at idle. I just did a 400 mile trip and only got 18mpg. What else should I do to get better gas mileage?

[eagle87]

what brand/model of plugs did you install?

[donnyeagle]

Autolite 5224, 2 ranges colder than oem, I seem to have a pinging problem.

[JSK]

...and at what speed did you drive? 60mph might be as low as practically (and safely) possible, and that makes a HUGE difference compared to 70mph or possibly even 65mph.

[txjeeptx]

You could install a vacuum gauge and try to keep the needle as high as possible. The more throttle, the less vacuum, the lower the economy, and the gauge will show it. Some vacuum gauges in the 70's were designed with economy markings on them. A vacuum gauge would also prove useful if the engine developes a running problem. Good diagnostic tool. If you do mount a vac gauge in the car, consider sticking it somewhere that you can see it while not taking your eyes off the road, like the windshield pillar.

Also, tire size, tire type, and tire pressure play a huge role in fuel economy. You might shop for a lighter wheel/tire combo.

18mpg isn't bad for a non-overdrive 4x4 with a big honkin inline six engine. Try to get that out of a 4x4 XJ with a 4.o and an automatic. At least the Jeep has OD, but it'll still barely make 15mpg at best.

Swap in a OD 5-speed manual to see improvements in economy . . and to make it more fun to drive.

[donnyeagle]

It's an automatic with factory size tires and 2.35 gears. We live in a very hilly area. I ran at 55 mph. 25% hilly highway driving the rest was flat highway. Some are saying they are getting 24 on trips. Just want to make sure the engine is setup right to get the best fuel economy. I do run vacuum gauge. On the flat highway I was pulling around 12psi steady and smooth needle. Does a Motorcraft carb get better economy? What is the best way to determine if I have a clogged CAT?

[eaglefreek]

The ones I have seen with high MPG numbers had properly working Carters. They are supposedly a really good carb if you can figure them out. I've had 2 Eagles with the 6. My first one, when I lived in CO, had a 4 speed with 2.35's. The best I ever got was 16 mpg. My current one ,when I first got it, had an auto and 2.73's. I would get 13-15 mpg. I had the carb rebuilt with no success. I installed a 4.0L head, Comp cam camshaft and Motorcraft carb and 3.07 gears. It had A LOT more power and would get 21mpg. Then I added A/T tires, roof rack and heavy custom bumpers and I'm down to 16 mpg. I have an AW4 waiting to go in and plan on installing 3.55 gearing.

[donnyeagle]

Thanks for the great reply. I hope some others might chime in if they did anything to their Eagles to improve their fuel economy. I am becoming concerned with the choke. Might switch to a manual one.

[ammachine390]

My Concord has 2.53 gears with an automatic and carter carb with feedback system hooked up and functioning, as well as all original emission components hooked up. I have also done the MIU upgrade to pertronix coil and have a pcv modification. Before I rebuilt the carburetor, I would get about 12 mpg city, and the best I ever got highway was just over 19 mpg. I rebuilt the carb over the summer, and I believe I get about 14-16 mpg city, and now I can get over 26 mpg highway. However, when I got over 26, I was going 65 mph and was behind trucks for a good amount of the drive. A few weeks ago, I did the same drive, and got over 23 mpg, and this was at 76 mph. Speeding over 65 really takes a toll on the gas mileage. When I rebuilt the carb, I set the fuel mixture towards the leaner side hoping for better mileage. The carb gaskets were also slightly leaking before I rebuilt it, that's probably where the increase in mileage came from.

[donnyeagle]

Does anyone have any experience with how to setup the tune for max MPG on a 1985 258.

I have rebuilt the Carter BBD feedback carburetor. I believe I have the choke, choke pulloff, accelerator pump and enrichment needles all set right. If I set the stepper motor towards the lean side it stumbles badly. It doesn't seem to have any vacuum leaks (pulls steady 18psi at idle, 12psi flat highway). The choke kicks out about 2 minutes down the road so that seems right. It seems to me like this motor has to work so hard to make it up hills that it will never be able to get good MPGs. I wonder if good MPG only happen in flat areas. 30 mpg highway is really one of the main appealing things about this AWD car. Otherwise I would have found a Cherokee. A Cherokee has more room and is a better AWD vehicle. If I can't get at least 25 MPG highway I will be selling this car. I used to drive a 69 Riviera (about 5000 lbs) with a 455 that got 14mpg on the highway. How can a car this small with an engine half the size only get 18 on the highway.

Help!

[Amc1320]

I have owned a couple of Cherokees, they will NOT get 30 MPG, try about half of that

An Eagle is a very heavy car, good if someone runs into you, bad if you are trying to obtain maximum gas mileage

If you can get 20MPG on the highway with an Eagle 6 cly auto, you are about where you re going to be short of adding things like overdrive transmission or taking a lot of stuff off the car to make it lighter

I have the window sticker for mine, it says 16 is the av fuel economy, and thats when it was NEW!

The only AMC i ever had that got 30 MPG was my 81 Spirit with a 4 cyl stick, no air, no PS, no PB, no nothin to steal HP and economy, and that was on relatively flat road interstate driving at 60MPH (it wouldnt go much faster than that!)

My 78 Concord AMX 258 Auto gets 17-18 around town, havent been on a trip yet, but its a lot lighter than an eagle and not AWD of course

Sounds like you have it set up right,

[BenM]

If you have a working stepper motor carb, I think having the ECM working is the best for economy. That motor tries to keep the carb in tune without you. It adjusts for everything but altitude since carbs are volumetric devices. It's very similar in concept to a TBI setup.

If you're set on not using the ECM, you can get or build a GM TBI to replace the carb. You can get a setup new for around $1,000, or 2-3x a good non-stepper BBD or about twice a Webber 32/36 kit.

In cost/benefit I like the TBI unless you're going to put a whole 4.0 in and have a donor vehicle available.

Also, make sure your EGR is working properly. EGR effectively reduces the displacement (and reduces lean knock) at highway cruising speeds.

The Eagle is also a bit... brickish. Putting a light rock plate under the engine, adding a front air dam spoiler, and putting wheel spoilers on will improve highway fuel economy.

Off road tires can eat a couple of mpg, find some 195/75, or at most 205/75 tires and plain steel wheels to fit if you have some off road clodhoppers on. Narrower tires are also better in the snow. Moving down to some very common 195/65R15s may make a difference in the city, you may have to adjust the speedo gear a tooth to match and I'd only consider it with a 2.35 geared car.

[donnyeagle]

Here is a link to some actual MPGs

http://www.fuelly.com/car/american%20motors/eagle

There is a post on this site where people are getting good MPGs.
I'm just wondering what it takes to get the kind of MPGs others are getting.
Thanks for your replies and suggestions.

I'm wondering how much of the stuff I have to re-hookup to get the stepper motor working again.

[rollguy]

If you have a working stepper motor carb, I think having the ECM working is the best for economy. That motor tries to keep the carb in tune without you. It adjusts for everything but altitude since carbs are volumetric devices. It's very similar in concept to a TBI setup.

I was thinking the same thing.  Taking the ECM out of the system will make the stepper and distributor adjustment  inoperable.  My '83 got an average of 21 MPG on it's first road trip in stock form.  I since did the ECM test bypass (made a wiring adapter, so no cutting/splicing of wires) and the "big cap" ignition upgrade (from GRONK), and now the car gets less MPG.   It does have a LOT more power than before, but the MPG is suffering.  I think I am going to remove the wiring adapter, plug the stepper back in, and see if the MPG increases.  I believe it will.  Just having the higher voltage coil, cap and wires, will help even with the ECM and stepper hooked up.

[eaglefreek]

I would agree that taking the timing control and stepper operation out would most likely result in less MPG. However, to have those functioning, your almost 30 year old components, hoses and wires under the hood have to be in proper working order for those to operate properly. To get the carb set up correctly, a dyno with a competent operator could set your carb where your O2 levels need to be for optimum MPG (if you can find someone who will touch the Carter). However, you mentioned hills. With a 235 gear ratio, I don't think you will do very well if you need to ascend hills often in your commute. Everything has to come together, carb, engine health, igniton, tires, aerodynamics, proper gearing, etc. to achieve the highest MPG. I knew I would never get decent MPG's out of my Eagles, so I bought a cheap Ford Festiva that gets 40 mpg for my daily commute.  I have just put 30,000 miles on it since I got it almost two years ago and figure I have saved about $2200 in fuel after deducting what I paid for it, maintenance and insurance. Not to mention the savings in maintenance on the Eagles, such as tires and other wear.