[Cutlass327]

I'm wanting to swap in a 4.0L eventually, I want EFI - tired of fighting carbs every time the weather changes...

Just curious though what those with this swap noticed in MPG change. I'm guessing it would go up, efi is much more efficient, but I am wondering just how much. I figure with the 4.0 and a 5sod, I'll gain quite a few.

FYI.. running 31x10.5's in 3rd I am at almost 3k rpms at 60 mph - the MPG sucks!

[wendell]

I don't have carb problems anymore. I swapped on a holley 470 TA 4 barrel, offy intake, comp cam, header, CRT HEI and MSD. This summer it's getting a P&P head with rollor rockers for better flow. My 258 screams and still gets a round 16 mpg romping a round. Your not going to get that great of mpg in a jeep no matter what engine you have. What I don't have to worry about is a computer, harness or sensors failing. Still have a mechanical fuel pump thats easy to get to and the engine screams. The holley TA series 4 barrels are made for off road, they will take exteme angles without flooding or fuel starvation. There easy to adjust and maintain. All these go fast parts are still cheaper than the mopar efi kit, pretty easy to install with better performance. The 258 will run circles a round a 4.0 with the right upgrades and not all the hassles of a bunch of sensors or the 4.0 install it's self. The 4.0 is an awesome engine, but with the things that can be done with a 258, I don't think it's worth all the trouble of a swap.

[Mike Romain]

No, the mileage should go down. The CJ's with their 258's get better mileage than a YJ or a TJ with the 4.0 engine all the time.

I get a consistent 21 mpg running fully loaded on the highway, so do the friends with their 258's tuned the same as mine. We only turn about 2300 rpm at 65 mph though... I don't use 5th, it puts me at 1750 rpm @65 mph which lugs and costs me 4-5 mpg.

I regularly wheel with a bunch of 4.2 and 4.0 folks and we do the same highway run there, then run the same trails and we (4.2 engines) use as much as $15.00 less per fill up.

[Happy Joe]

I had several 258s with carburetors and got from 14 to low 20s mpg (depending on gears/tires and transmission). I converted one (16 mpg) to FI and got 14 mpg, highway, I replaced a 258 with a 4.0 and get 14+ to low 20s on that engine (currently in my CJ-7) depending on use (worst mileage is on curvy mountain gravel roads going to and from wheeling).
Overall because carburettors can be easily tuned/modified I can get better mileage with the carburettors (when everything is working right) but the fuel injection has so much better performance off road and so few problems/ hassles I will never go back to carbs.
Given a choice I would stick to a port injected 258, or go for a port injected V-8 for the additional torque and horsepower (the FI 4.0 was one of the worst mistakes that I have made, engine wise, for off road use. It is, overall, better than a 4 cyl or a carbed I-6 however; primarily due to its adequate (barely) torque and the hassle free FI).

Enjoy!

[Cutlass327]

So what if I was to modify the 258 intake for port fuel injection from a 4.0 from the JY? I know I'll need the sensors and such too. How hard would that be to do?

Why do you think the carb gets better MPG? Do they make tuners for the 4.0 efi? I used to have a Holley 600 vac secondaries on my 350 in my Cutlass, but it seemed to always need tuned every weather change. It got old doing that. I want to be able to go out, climb in, hit the key, and be able to drive it anywhere I want without it idling at whatever RPM it wants to, stalling when I put the clutch in, smelling of fuel when I pop the hood, and all of this after I pump the pedal repeatedly and holding it at just the right spot for it to start first thing in the morning.

Carburetors became obsolete during the last century... do what ever it takes to get fuel injection...It makes bigger grins off road.

I feel the same way.

[Mike Romain]

Originally Posted by Cutlass327

So what if I was to modify the 258 intake for port fuel injection from a 4.0 from the JY? I know I'll need the sensors and such too..

No mods needed, you can move a 4.0 intake and exhaust over to the 4.2. Some folks even move the whole head over.

Why do you think the carb gets better MPG?

The carb tunes up better, the FI doesn't tune up.

I want to be able to go out, climb in, hit the key, and be able to drive it anywhere I want without it idling at whatever RPM it wants to, stalling when I put the clutch in, smelling of fuel when I pop the hood, and all of this after I pump the pedal repeatedly and holding it at just the right spot for it to start first thing in the morning.

You just need to put a carb kit into the stock carb then and maybe a manual choke on it because mine, the stock Carter BBD starts up every time even at -40. It 'always' starts faster with less cranks than our 4.0 engine in the Cherokee we have. It also never stalls, even on pit wall and ravine climbs.

The carbs need a $20.00 kit every time you need spark plugs or every major tune up. The kits are easy to put in and they then just purr. I can have a glass full of beer on the air filter without a ripple in it.

[JoonHoss]

Originally Posted by Mike Romain

...I get a consistent 21 mpg running fully loaded on the highway, so do the friends with their 258's tuned the same as mine...

I think this is the big variable alot of people miss. I'd venture to say (having read/followed most of Mike's posts), his tuning is what allows him to achieve these kind of consistant results. (which anyone with time and patience should be able to emulate)

the problem is, most stockers or non maintained Jeeps likely run too rich, or haven't been regeared or driven properly to compenstae for larger tires, and mileage suffers as a result.

most mileage comparisons to the 4.0L come from stock set-ups. undergear or fail to maintain that motor, and it's just as nig of a pig as any other, if not more so.

Hoss

[Cutlass327]

I've got the 1bbl YF from what I can tell.

I've seen the head swap mentioned - haven't researched it much yet. I know many say they get more power with it. My question is how about the MPG?

I know these beasts are poor for MPG, I knew that when I though about buying one years ago, which is why I went with the Ranger. Now, I want a reliable vehicle that the body won't rot out on (fiberglass!) and I feel that the Jeep can be it, once I get rid of the ancient technology they call a carburetor, but I also want to squeeze out any MPG I can.

[Mike Romain]

Originally Posted by Cutlass327

I've got the 1bbl YF from what I can tell.

I believe the Carter BBD 2 bbl gets the better mileage, but could be mistaken there. I think it has to work less for the same power.

FI is nice because you don't have to tune it, Jeeps still need things like cap and rotor, wires, etc...

I only tuned mine every two years because of the need to pass emissions. I just let the last tune go 4 years and gas mileage suffered.

[86cj74.2L]

Mike,

If you had a O2 sensor.....what would your tune be reading to get your MPG?

Vacuum leaks around the intake manifold to head will make the BBD/YF run poorly.

Wonder how many replaced a perfictly good card thinking it was the problem and all along it was the gasket leaking at the manifold. I know I did once and it ran just as bad with the new reman carb on it.

[Mike Romain]

Originally Posted by 86cj74.2L

Mike,

If you had a O2 sensor.....what would your tune be reading to get your MPG?

I do have one in there still blocking the hole but don't know. Here is one set of readings I got on an ASM 2525 emissions dyno, 589 NOx, 16 ppm HC and 0.11% CO if that helps.

I run my air mix pins the stepper motor used to move, in the center of their travel or with their shoulder about 1/8" from the back wall.

[wendell]

I have never had to retune a carb with every weather change. I haven't tuned it any since the holley was installed. Even back in the day when all there were was carbs, we never had to keep tunning them. We would buy a car and never touch the carb untill we sold it. I think we may get carried away with carbs and tunning. I live in Ohio and wheel alot in the mountains of W,Va and still don't have to mess with the carb. It maybe different in higher elevations like the Rocky's, I don't know. Here in the east it's no biggy, at least in my experience. I think people to a large extent have forgotten how to properly tune a carb and that causes a lot of problems. They blame every issue on the carb and are allways screwing with it. Not saying you, but a lot of people in general.

[Cutlass327]

Well, I've gotten rusty on the tuning knowledge, I will admit..

[86cj74.2L]

Originally Posted by Mike Romain

I do have one in there still blocking the hole but don't know. Here is one set of readings I got on an ASM 2525 emissions dyno, 589 NOx, 16 ppm HC and 0.11% CO if that helps.

I run my air mix pins the stepper motor used to move, in the center of their travel or with their shoulder about 1/8" from the back wall.

If you get board sometime and have a DVM can you watch the voltage at 40 to 60mph.

I bypassed the ignition part of the computer but am letting the ECU still control the mixture.

[xeroOTG]

i did a megasquirt conversion recently, from the carter yf. mine was way outta the acceptable spec range, and i didnt want to buy a new carb. being the geek that i am i did the swap. i definately have a huge power increase and im positive my mileage has gotten better even though it runs rich right now due to having too big injectors. this summer i will properly tune it and do some other stuff when im not bogged down with school.