MPG improvements for 89 4.2 - JeepForum.com
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post #1 of 15 Old 11-15-2019, 07:22 AM Thread Starter
Slyjj
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MPG improvements for 89 4.2

Greetings all. It turns out my old jeep frame took a massive **** on me and PO did a good job covering up rust with a paint job.... so after snapping a suspension mount and a leaf spring recommendations were for me to replace the whole frame. Nonetheless in the midst of difficulty lies great opportunity.

So that said I was playing with a couple of ideas for my jeep. Im going to get a frame from a 2.5 and just re-weld the motor mounts- cheap. Probably like $1000 for all of that from my local jeep guy. I was playing with the idea of an engine swap but realized theres so much that would have to be modded that cost would add up quickly- at least $4k on top of what I already have to get fixed. I lost interest in that. Thought about a stroker engine since I could just drop it in, but $8k for a stroker engine is pretty obscene.

So that said, primary reasons for wanting to do the above is MILEAGE. Right now that's my primary concern (after getting the jeep repaired of course). A larger, more capable engine will get more MPG than this gutless 112 horses will. I don't mind so little power, but Im only getting about 9-10 MPG at the moment.

Now I've got what appears to be a 4 inch lift and 33" tires on this. I love carbs- the simplicity of them and the lack of CPU's. What mods can I do that would improve the MPG on this thing? Im hoping I could get 14-15 MPG out of this thing. The next question is: is it possible? I've heard of several mods, such as a headers, holly or motorcraft carbs, etc. Shes being rebuilt from the ground up so this is the time to do it. I have a spare set of 4 cylinder axles with 4.10 gearing that Im thinking of swapping in (to replace the 3.07). The catch is, they have 200k+ miles on them. Intelligent swap or no?

Any help and suggestions are appreciated, Thank you

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post #2 of 15 Old 11-15-2019, 07:27 AM
jbolty
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there are lots of reasons to do an motor swap but none of them are for economy and nothing you can do, or buy, or add on is going to make much difference

if you want gas mileage, get a honda.

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post #3 of 15 Old 11-15-2019, 07:38 AM
jkbrereton
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A newer motor can gain you more HP and torque for the same mpg, just through efficiency.

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post #4 of 15 Old 11-15-2019, 07:51 AM
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Quote:
Originally Posted by Slyjj View Post
...appears to be a 4 inch lift and 33" tires on this.....What mods can I do that would improve the MPG on this thing? Im hoping I could get 14-15 MPG out of this thing..... suggestions are appreciated, Thank you
The current rendition of my 1989YJ is a 4.0 with AX15. I consistently get 15 to 16.7 mpg and depending on what I’m towing I can expect about 14 MPG. And that’s in hilly Vermont. Running 32” tires, 4.10 gear.
The previous rendition of my 1989 was a 2.5. I consistently got 16 to 17 sometimes 18 in the summer. And when I spent some extended time down in the flat regions (Virginia, Washington DC area) I even got almost 21 MPG a couple times on a tank. That was on 235s

But to me the discussion is going off the rails; if you have a rusty frame I’d see about fixing it or 1000 bucks to put another frame under it? I’d do it. Everything else in the setting you described is sortof off the tracks

[size=“3”]Shackles & D-rings are different things.
Cranking IS turning over
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post #5 of 15 Old 11-16-2019, 11:36 AM
shaggyjim
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4.0l head swap, Offenhouser (sp?) intake, 4.10-4.56 gears. That’s about the best your going to get as far a MPG on a budget IMO.

Jim...

"That's more fun than watching a hippie get mauled by a bear!"
......YJOTM June 18......
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post #6 of 15 Old 11-16-2019, 01:45 PM
bruinjeeper
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27" tires

Stop whining about the 'ride' - If your YJ ain't wrangling your soul free, then might I suggest you buy a stationwagon... at least you can fit all your bull**** in the back.
~YJOTM MAY '16~
~YJOTM JULY '19~
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post #7 of 15 Old 11-16-2019, 05:16 PM
Chrisnvegas
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I'll assume you have the stock ignition.
It makes the Jeep sluggish. My theory is it has a poor advance.

Installing a performance ignition is the most bolt-on power and economy you're going to get.
If it's still stock, carb, ignition and exhaust,
I recommend:
A CRT HEI (CRT or DUI brand ONLY. No elcheapo ebay models)
A Holley Sniper or Howell TBI injection
or
Motorcraft 2150 if you have no smog requirements
A free-breathing exhaust.

It will substantially increase HP with no downside.
Maybe a little better MPG increase, but you'll enjoy the increased throttle response so that will be minimal.

The Jeep is a short wheelbase cinder block on wheels. Pushing that much air takes a certain amount of energy regardless of the engine under the hood.

You can increase the power and drivability there. ^^^

And instead of an $8000 motor, you can get a used Hyundai, Subaru or Honda at get MPG that a Jeep is physically unable to match.

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post #8 of 15 Old 11-16-2019, 05:35 PM
fishadventure
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@Chrisnvegas so you don’t like the MSD stand-alone distributor?

[size=“3”]Shackles & D-rings are different things.
Cranking IS turning over
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post #9 of 15 Old 11-16-2019, 07:50 PM
Chrisnvegas
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Quote:
Originally Posted by fishadventure View Post
@Chrisnvegas so you don’t like the Mallory stand-alone distributor?
I never used it so I can't say for sure. I will say there is more than one option to improve the ignition.
The two examples I gave are the only ones I know of that have a non-hardened cam gear that will wear to the cam instead of vise versa. We don't want to damage the distributor drive on the cam. Also, the two examples don't require an external coil. That makes them economical and really simple.

One wire for the ignition, one vacuum line for the advance, one wire for the factory tach.

I think the Mallory will require a module to allow the factory tach to operate.

If I got anything wrong, feel free to correct. I don't want to give bad info.

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post #10 of 15 Old 11-16-2019, 08:14 PM
timatoe
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I'd do a HEI, ditch the carb all together and go FI and gear accordingly for the tires.

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post #11 of 15 Old 11-17-2019, 07:43 AM
fishadventure
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Quote:
Originally Posted by Chrisnvegas View Post
...examples I gave are the only ones I know of that have a non-hardened cam gear that will not wear the cam...don't want to damage the distributor drive on the cam. Also, the two examples don't require an external coil. That makes them economical and really simple. One wire for the ignition, one vacuum line for the advance, one wire for the factory tach. I think the MSD will require a module to allow the factory tach to operate....If I got anything wrong, feel free to correct. I don't want to give bad info.
Nope, good info.
Think I’m needing to do this for my nephew’s ‘88 so I’m getting myself tuned up too.

MSD apparently no longer has or never did have a stand-alone a.k.a. “R2R” pure centrifugal advance distributor for the 258 in their catalogue.
I thought I recalled a stand-alone 258 version but I’m not seeing it. The one they have looks good but isn’t stand-alone though it is comparatively expensive.

The CRT appears confidence inspiring.

If I can solve his no-start and grinding-if-it-does-run problem I’ll probably start with the CRT route to get him above 8/9mpg

[size=“3”]Shackles & D-rings are different things.
Cranking IS turning over
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post #12 of 15 Old 11-17-2019, 06:02 PM
timatoe
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Pretty sure MSD only offers the 6A controller and you run the stock distributor. I did that for awhile because that's how I bought it, the HEI was an improvement.

FWIW, when I did mine I bought a new distributor for a 80's I6 Chevy van and got the gear from CRT and used wires from an early 80's Ford Fairmont with the I6 (8 mm wires stock). I maybe saved $35 - $40, if I were to do it again I'd just order it from CRT rather than collecting parts.
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Quote:
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post #13 of 15 Old 11-17-2019, 08:37 PM
JarheadAl
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I'm not sure what size tire was stock but with 33s, your speedometer and odometer are reading low. Maybe 10-15%. Go to grimjeeper.com and run some numbers to find out. Plus being so over-tired is making your engine over tired. It is struggling to turn the wheels. Lots of good info above to make your engine more efficient but IMHO I think the 4.10s will give the best return on investment of money AND time. YMMV....literally
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post #14 of 15 Old 11-18-2019, 08:12 AM
fishadventure
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Quote:
Originally Posted by JarheadAl View Post
I'm not sure what size tire was stock
Usually 215/75-15

[size=“3”]Shackles & D-rings are different things.
Cranking IS turning over
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post #15 of 15 Old 11-24-2019, 05:02 PM
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Your tire size could be killing you.

Your mileage estimates may be off due to above. Already mentioned by previous posters.

Your electric choke needs to be functional and actually open.

Your feedback system needs to function in a few ways. It needs to reach closed loop. A couple of different temp readings have to be met. O2 has to work, Stepper motor in carb has to work. Other systems have to work after it goes into closed loop to really work correctly.

http://gleebledorf.com/

Lots of good information on this site. If you can process it than you can be helped with this system. If you cannot honestly process it than maybe an HEI and simpler carb are in order for you. You may also be missing a part that is no longer available on the older system that is failing you.

If you cannot fix it with a hammer then it has to be an electrical problem.
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