Mpg CJ 5, pollution stuff gone, I6, 4 speed

[Ctknorr]

I'm still a relatively new Jeeper and find I have lots of silly questions.

I just did an engine swap in my CJ to go from old AMC 258 i6 / 4.2l to a newer lower mileage used AMC 258 i6 / 4.2l . Jumped from 74 to 81 model years. Swap in has 63k miles on it. My old one unknown.

My old engine was getting 10 miles to the gallon. The newer swap in engine is averaging 14 with the first two tanks of gas, so I'm wondering what other people from the same vintage are getting for MPG?

I've been told 14 mpg should be really good, is that true?




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[gr8dain]

I am getting about 15 on my rebuilt 82 engine. And it is a AMC 258 i6 / 4.2l as well. Was only getting 10 on the more worn out 84 that had serious blow by.

 

[jzak]

14 MPG is really good! I'm getting 16-18 on my AMC 258 i6 / 4.2l , but I've got a TBI conversion kit.

 

[Lumpy Grits]

When I'm easy on the 'fun-pedal', I can get a confirmed 14-16mpg highway.
My '7' is OEM stock fuel system and Ca. SMOG legal.
LG

 

[elwood blues]

14MPG is a solid number to get from a carbureted AMC 258 i6 / 4.2l any day. It really depends on so many variables, the carburetor itself, what kind of gas you're running, your elevation above sea level, how much you use the skinny pedal, how much you're shifting up and down, etc.

 

[Ctknorr]

14 MPG is really good! I'm getting 16-18 on my AMC 258 i6 / 4.2l , but I've got a TBI conversion kit.

What is a TBI?

 

[mtnwhlr]

What is a TBI?

throttle body injection. our fuel injection.

 

[jzak]

What is a TBI?

Yeah, the previous owner put in a Howell Throttle Body Injection Kit. Generally improves fuel efficiency, and performance.

 

[Ctknorr]

14MPG is a solid number to get from a carbureted AMC 258 i6 / 4.2l any day. It really depends on so many variables, the carburetor itself, what kind of gas you're running, your elevation above sea level, how much you use the skinny pedal, how much you're shifting up and down, etc.

Elwood, now you've done it. I've got more questions.

I'm about 100 ft above sea level. My first two tanks of fuel have been 87 octane with
10% ethanol. I have access to real gas I used to run in my BMW. Better mpg and better for the car imo.

My carb is a reman 2 bbl Carter. I've been told it's post 79 the jets are not adjustable unless drilled out. Does that make sense? They are sealed in I guess.

Been driving sticks since I was 14. So a while now. I've always felt like I could get better mileage driving vs cruise control with an automatic. But that's me.

Based on these comments do you or any other forum members have suggestions or recommendations?

CK


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[jzak]

Elwood, now you've done it. I've got more questions.

I'm about 100 ft above sea level. My first two tanks of fuel have been 87 octane with
10% ethanol. I have access to real gas I used to run in my BMW. Better mpg and better for the car imo.

My carb is a reman 2 bbl Carter. I've been told it's post 79 the jets are not adjustable unless drilled out. Does that make sense? They are sealed in I guess.

Been driving sticks since I was 14. So a while now. I've always felt like I could get better mileage driving vs cruise control with an automatic. But that's me.

Based on these comments do you or any other forum members have suggestions or recommendations?

CK


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So what is your goal here? I think most of the commenters where saying the 14mpg is in line with what you can expect from a 2bbl carbureted AMC 258 i6 / 4.2l . I'm sure you could squeeze out a few more miles per gallon, but if you trying to get Honda Accord efficiency, your going to have to put in a new engine with a modern fuel injection system, which is probably going to cost you more then what the CJ is worth. Just my opinion.

 

[Ctknorr]

So what is your goal here? I think most of the commenters where saying the 14mpg is in line with what you can expect from a 2bbl carbureted AMC 258 i6 / 4.2l . I'm sure you could squeeze out a few more miles per gallon, but if you trying to get Honda Accord efficiency, your going to have to put in a new engine with a modern fuel injection system. Just my opinion.

I want 16 mpg. Lol. My goal is just to learn. My none jeep saves a ton on fuel so I get to save that way.

I am curios about how to jet a carb or keep it tuned for sea level and just tooling around town.

I've got to learn how to adjust old school again. First carb since my 75 Chevy monza.


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[hole]

Ton of variables here when it comes to MPG. Mine use to consistently get 18-20mpg if I cruised it at 55-60 on flat highways, AMC 258 i6 / 4.2l with MPI- T-5 -2:73's and 30" tires. Fast forward to present day, 14-15mpg AMC 258 i6 / 4.2l w/4.0 head and MPI-T19- 2:73's and 32" tires. I'm no genius or engineer, but losing OD and bigger tires were my MPG demise . The T-5 was a much closer gear ratio split then the T19 is so I wasnt revving the engine as high as I currently have to now either. You could probably swap to a different carb that was easier to fine tune, but at what cost? Anything near 15mpg out of a I6 is pretty good.

 

[elwood blues]

Elwood, now you've done it. I've got more questions.

I'm about 100 ft above sea level. My first two tanks of fuel have been 87 octane with
10% ethanol. I have access to real gas I used to run in my BMW. Better mpg and better for the car imo.

My carb is a reman 2 bbl Carter. I've been told it's post 79 the jets are not adjustable unless drilled out. Does that make sense? They are sealed in I guess.

Been driving sticks since I was 14. So a while now. I've always felt like I could get better mileage driving vs cruise control with an automatic. But that's me.

Based on these comments do you or any other forum members have suggestions or recommendations?

CK


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So what is your goal here? I think most of the commenters where saying the 14mpg is in line with what you can expect from a 2bbl carbureted AMC 258 i6 / 4.2l . I'm sure you could squeeze out a few more miles per gallon, but if you trying to get Honda Accord efficiency, your going to have to put in a new engine with a modern fuel injection system, which is probably going to cost you more then what the CJ is worth. Just my opinion.

Ton of variables here when it comes to MPG. Mine use to consistently get 18-20mpg if I cruised it at 55-60 on flat highways, AMC 258 i6 / 4.2l with MPI- T-5 -2:73's and 30" tires. Fast forward to present day, 14-15mpg AMC 258 i6 / 4.2l w/4.0 head and MPI-T19- 2:73's and 32" tires. I'm no genius or engineer, but losing OD and bigger tires were my MPG demise . The T-5 was a much closer gear ratio split then the T19 is so I wasnt revving the engine as high as I currently have to now either. You could probably swap to a different carb that was easier to fine tune, but at what cost? Anything near 15mpg out of a I6 is pretty good.

So, yeah, what these guys have said. There are so many variables, hole and jzak really hit what I was going to say. You can squeeze out 16mpg out of a AMC 258 i6 / 4.2l if you reduce the weight of your rolling stock, aluminum wheels vs steel, all terrain or even SUV tires vs mud terrain tires, what Transmission you have, my T-18 I usually start in 2nd gear, and 4th gear is 1:1 ratio, to run 55mph I'm pushing 2800 rpms, a Motorcraft/Autolite 2100 carb is better on fuel than a Carter, a correctly tuned Weber is better on fuel than a Carter, I ran a Holly 390cfm 4bbl carb for a few years and I got 16mpg as long as I stayed out of the secondaries, and the list could go on. My question to you is; is the pursuit of 16mpg or higher worth the amount of money it's going to take you to get there?

 

[Ctknorr]

So, yeah, what these guys have said. There are so many variables, hole and jzak really hit what I was going to say. You can squeeze out 16mpg out of a AMC 258 i6 / 4.2l if you reduce the weight of your rolling stock, aluminum wheels vs steel, all terrain or even SUV tires vs mud terrain tires, what Transmission you have, my T-18 I usually start in 2nd gear, and 4th gear is 1:1 ratio, to run 55mph I'm pushing 2800 rpms, a Motorcraft/Autolite 2100 carb is better on fuel than a Carter, a correctly tuned Weber is better on fuel than a Carter, I ran a Holly 390cfm 4bbl carb for a few years and I got 16mpg as long as I stayed out of the secondaries, and the list could go on. My question to you is; is the pursuit of 16mpg or higher worth the amount of money it's going to take you to get there?

You all are so right saving a few cents is not worth the cost in big dollars.

Carb tuning and such I feel like I could keep it in the 14-16 range. I was ecstatic to see I'm getting 14 over 10.

I do love the conversation this sparked. I have a lot to learn. Now that I have the jeep running again it's time to make sure everything else is solid.

Next up is the steering and tranny-Transfer Case .

Then we go on from there.

Thanks for the comments and thoughts/opinions. That's what I asked for.

CK




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[Hedgehog]

Learned something from the Carb guy I use (no it's not Rob). Hes been at the carb game for a VERY long time so he knows a few things about carbs. I run a 360 with a stock 2150 Carb and the thing gets 10.5 mpg on a GOOD day. So, I've ask very similar questions about carbs. His answers were enlightening and you almost never hear them on the boards. He says that Jets make very little difference on fuel economy. WHAT! I come from tuning carbs on Harley Davidson motors with jets being very important. Of course those were CV carbs which are a very different beast. In a 2100 or 2150 it's the ventury (?), those round hoop structures you see when looking in the carb. Those things can be had in different sizes for different motor applications. Different sizes, different air speed, better fuel atomization for different motors and uses. Unfortunately few if any carb rebuilders deal with those at all.

For my application, not at all like yours, I've tried everything I can to get better mileage, but the lessons are about the same.
- Rebuilt carb, good seals everywhere possible,
- different advance settings,
- HEI dist. (love what that thing does),
- I even went from 3.73 differential gearing to 3.31's which dropped RPM's at 65mph from 2800rpm's to 2400rpm's. The engine is much more comfortable to live with at 2400rpm's. Inexplicably enough, using the same tires and GPS for speed/distance calculations the changes did little to nothing for mileage. It is more consistent than it once was, but 10.5 - 11.0 is all I can expect.

Another words, chasing mileage is expensive and frustrating. About the only way to gain mileage is to go with one of the fuel injection systems on the market. I'm eying the 48 state Howel kit, there are others, but they are relatively expensive.

 

[Lumpy Grits]

Stay with the carb, HH. When the FI goes TU in the sticks. You are SOL and have a long walk.
With a carb-Much better chance to fix it yourownself.
FWIW: You should be get'n better that 10 mpg with that 360. You do have some weight with the bumpers/winch etc.
Just for 'grins'-See what your Jeep weights now.
LG

 

[Hedgehog]

No question, it is weighty. It's around 4,000 lbs. Last time I went through emissions it was determined that the motor PO installed a cam of some sort during the rebuild. It's no race engine, but it's lumpy and snappy on the road. I'd be happy with 15-16 mpg the same as the OP is looking for.

Lumpy - you are the first one to mention equipment reliability issues with an HEI. Probably a good point. Mileage is only one of the attractions for the HEI though. Consistent emissions, and the ability to remove several emissions components, mainly the air pump system. It's not that the pump hurts mileage, it's just in the way and has come off its brackets. On a long trip it was found just hanging off the bracket and lost belt. Shortly after, it froze up requiring a replacement. I don't trust is anymore. When it happened the engine never even burped.

 

[Lumpy Grits]

HH-VERY good point about FI, as I do recall the frustrations you have had with SMOG testing in the past.
PLZ, don't let price point 'drive' your decision here.
Are there any Jeep clubs near you with older CJ's that may have done the swap? What does your friend at the Jeep shop recommend?
Do you think the non std cam in your 360, will affect the FI and SMOG?
LG

 

[Hedgehog]

We aren't nearly as tight about smog as you guys are. On the older vehicles, we do test every year, but for us on the visual inspection close counts. It was the tail pipe that got me. In the end we had to INCREASE the idle speed to ~1,000 rpms and it passed easily. The shop said that a bit of a lumpy cam shows as a bad miss at lower RPMs.

Several guys here have done the Howel TBI upgrade and like it. It's time proven and I'd get the 48 state legal model. The bad part for me would be the need for an electric fuel pump, I'm not a big fan of electric fuel pumps.

 

[hole]

HH, been running a quality electric pump for 10+ years with no issue. I do carry a spare under the seat to help me sleep better though