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Discussion Starter · #1 ·
in the 98 Grand Cherokee 5.9 limited package, premium fuel is required. However, in 1998, the ram, the durango, and the dakota all have the 5.9 magnum engine as well, but states recommended fuel is regular. does anyone know if there are differences between these other than the computer? In an effort to make the horrific mileage of my grand a little better, first i plan on doing the 242 select trac swap, and if it was as simple as a plug and play swap, it would more than pay for itself in time to switch to a dodge ecm/pcm. has anyone done this swap, or attempted to run the 5.9 as is on 87 or 89 octane?
 

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i'd was wondering this myself ?
what is this the intake different or what ?

I know on reg fuel they spark knock like a bastard!
 

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Discussion Starter · #3 ·
i found a little more info on this myself that there are actually at least 2 versions of the computer for a 5.9 magnum equipped vehicle. The first came in the 5.9 limited GC in 98, and came in a bunch of 96-01 dodge "R/T" offerings, up to 10/01 production month. This pcm had more timing and made a little more power (on the order of 5-10 horsepower in the tune), but required premium fuel as a side effect. All non R/T's got a pcm that worked on 87 octane at the expense of a bit of power. Apparently both engines are 100% identical otherwise, and in fact after 10/01, all 5.9 limited vehicles R/T and non R/T alike got the 87 octane pcm.

If i can find an OBD2 dodge pcm that's set up for the 87, i'd like to try it out.
 

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You can always have the dealer flash the 5.9 LTD PCM to run reg. It will be around a $100 though. It's not really worth it though. The diff between a tank of premium & reg is not that much. It would take 25 fill ups to equal $100.
 

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Yes,different between Magnum engines is in the spark advance.Jeep have 15° more advance than others Magnums. More advance need premium gas.For ex. like a higher compression.
Advance is possible to change by a DRB II. I heard also,that some ride 5,2 PCM. But I don´t know if true or how it works
 

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Discussion Starter · #6 ·
Colorado 5.9 said:
You can always have the dealer flash the 5.9 LTD PCM to run reg. It will be around a $100 though. It's not really worth it though. The diff between a tank of premium & reg is not that much. It would take 25 fill ups to equal $100.
That's pretty short sighted to say it's not worth it. if you're daily driving a 5.9 limited, you can make your money back in 6 months, save $100 the first year, and save $200 each year after. even for someone like me where i only daily drive mine in winter, and it might see 4000-5000 miles a year on it, i'll still probably roughly break even the first year and save close to $100 each year after.

that being said, i probably wouldn't just go to the dealer and get it flashed. I'd either want to buy a tuner to do it myself or just have 2 pcms, so i can swap back and forth if i feel like it.
 

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Discussion Starter · #7 ·
ok i did some research more, and found that the hypertech power programmer would be ideal for this, but they only list the part number for all the 98 dodges with the 5.9 magnum - no listing for the jeep. :( The nice part about it is that it actually makes considerably more horsepower and torque on 87 octane than the stock ecm program does on 93. And if you want to run a 93 octane based program, you get more power yet. I'm going to email them and ask what's different there. Even so, for $350+, that's rough.

jet offers a stage 1 chip which says in the generic description of a stage 1 chip that it will run on 87 octane, but 89 is preferred for best performance. It's $300+ though.

i did find something on another message board where someone plugged in the ecm from a dodge into a jeep, and gauges and such did not work, so I think I'm pretty much sol there.
 

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You are SOL. Only way is to have the dealer flash it. Or buy another 5.9 ECM and have it neuter flashed and switch between the two.

On the flipside, I would agree that it is NOT worth doing the flash to save a few bucks. Buy a commuter car if you're that worried about saving money on gas.

The Jet chips are junk, don't buy it.
 

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ok i did some research more, and found that the hypertech power programmer would be ideal for this, but they only list the part number for all the 98 dodges with the 5.9 magnum - no listing for the jeep. :( The nice part about it is that it actually makes considerably more horsepower and torque on 87 octane than the stock ecm program does on 93. And if you want to run a 93 octane based program, you get more power yet. I'm going to email them and ask what's different there. Even so, for $350+, that's rough.

jet offers a stage 1 chip which says in the generic description of a stage 1 chip that it will run on 87 octane, but 89 is preferred for best performance. It's $300+ though.

i did find something on another message board where someone plugged in the ecm from a dodge into a jeep, and gauges and such did not work, so I think I'm pretty much sol there.
hypertech planned that for the dodge that was already using 87 not the jeep that from the factory uses 91 or 93.

instead of a tuner or a chip (which no way lower required octane) and a dealership flash, maybe try a fitch fuel catalyst. ive seen some tests that show that they lower mileage and can make 89 octane clean enough to burn like 91
 

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Its a 5.9 people. You all know its a box with a wonderfully old-tech, pushrod V-8... right? Buy a KIA if you want mileage. I jump in mine and drive and wheel it like gas is free!
 

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I realize that this thread is a little dead but I have some questions kinda going the opposite direction as the original poster. 🤪

What, if anything, do y’all know about using performance parts for a 1998-2002 dodge 5.9 Magnum on a Grand Cherokee 5.9 Limited?
 
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