ZJ Octane Question

[Comanche County]

I've got a 98 ZJ Limited with a 5.9L. It requires 91 octane or better and usually I end up paying for 93 octane. Does anyone know how I can reprogram the computer to allow for 87 or 89 octane?

 

I'm in TX at about 1200 ft elevation and will move back to Savannah, GA later this summer - sea level.

 

I'm hoping I could do this without springing for a Superchip or Flashpaq, (the MJ is taking all the money right now). But it may be the easiest. I know some of those chips can be very quirky and I don't want to mess up the computer or sacrifice dependability, power, or what little economy it has.

 

sorry, not an MJ question....

[dasbulliwagen]

As a Chrysler tech, I know of NO reprograms available to do what youre looking for. Aftermarket might be a different story, but I have no idea. Usually octane is set going by compression ratio, and timing, but mostly compression ratio. I don't think theres much you can do. Sorry.

[terrawombat]

I'm not so sure that altering the ECU/ECM programming is going to lower the necessary octane level your vehicle requires as I'm pretty sure it's based on the compression ratio of the engine and that's entirely mechanical.

 

You can wire in a piggyback computers such as a GReddy E-manage, SAFC-II, or similar and adjust airflow and timing signals to run a more conservative tune. I've done that on a 3000GT VR-4 I own, but I was aiming for more power, so the HIGHER the octane the better in my case...

[Pete M]

the 5.9 is prone to pinging. My Dakota was supposedly able to run 89, but even after the dealership reflashed the computer it'll still ping a small amount on anything less than 91 during hard acceleration. I guess it's sorta a pay-to-play thing, but since the difference is like 10 bucks per tank, I have learned to accept it.

[Comanche County]

I thought that I saw a thread somewhere about another 5.9 owner who did it. Oh, well I'm certainly pleased with the power it makes and the acceleration that comes with it. I can live with it for sure, but I'm doing a lot of driving lately,,,,if you want to go somewhere when in Comanche County, TX, then you really have to GO somewhere. I guess I shouldn't complain about the more expensive gas, it could be a lot more expensive like back in 08, and heck, it is a 5.9 Limited - its the Jeep Carroll Shelby should have built.

[aemsee]

One of my want to have vehicles...Deep Slate 5.9 Limited. A couple of 5.9 owners said they used to do 1/2 93 octane and 1/2 89 when 91 wasn't available. But have no first hand knowledge if it actually worked or not? Since you are in the heat, make sure you aren't running hot. That was always an issue with the 5.9 in the ZJ. They barely stayed cool right from the factory.

[dunl]

Well, according to the guy down the street, it's more than $10/tank....his mileage went down to something like 10mpg when he tried 89.

 

So it's actually cheap IHO to run the higher $$$$.

[Comanche County]

I remember the octane mixing question from high school science. According to my teacher you could mix it exactly like you describe - equal amounts of a low and high result in a mixture right the middle.

 

I think I'll just keep pumping the good stuff. I already get 17.5-18 on a long trip and I average 16.8 with city and country driving. Of course, I'm easy on the skinny peddle (most of the time). :thumbsup:

 

Thanks for the replies!

[jimoshel]

In 1960 when I was in Spain a lot of the guys brought American made cars over. They ran terrible on the Spanish gas available. Could drive 90 kilometers into France and get better gas but that was inconvenient. What I did to my '52 Mercury was install 2 head gaskets under each head and retard the timing as much as possible. Helped some but not a viable option with modern computerized cars. Viva Le Progress.

[HOrnbrod]

In 1960 when I was in Spain a lot of the guys brought American made cars over. They ran terrible on the Spanish gas available. Could drive 90 kilometers into France and get better gas but that was inconvenient. What I did to my '52 Mercury was install 2 head gaskets under each head and retard the timing as much as possible. Helped some but not a viable option with modern computerized cars. Viva Le Progress.

 

Yep. I love flatheads. You could do about anything to a Ford flathead and it would still run smooth as silk. One of the favorite cars I have owned was a 53 Ford Victoria with a 51 Lincoln 337 flathead engine. It used to eat the early 55 and up Chev V8s for lunch. Ah, you reminded me of yesteryear Jim. Gracias. :D

[Automan2164]

In 1960 when I was in Spain a lot of the guys brought American made cars over. They ran terrible on the Spanish gas available. Could drive 90 kilometers into France and get better gas but that was inconvenient. What I did to my '52 Mercury was install 2 head gaskets under each head and retard the timing as much as possible. Helped some but not a viable option with modern computerized cars. Viva Le Progress.

 

Yep. I love flatheads. You could do about anything to a Ford flathead and it would still run smooth as silk. One of the favorite cars I have owned was a 53 Ford Victoria with a 51 Lincoln 337 flathead engine. It used to eat the early 55 and up Chev V8s for lunch. Ah, you reminded me of yesteryear Jim. Gracias. :D

 

Hey oldtimers! For those of use who weren't born yet...

 

:needpics:

 

Rob L. :D

[Sir Sam]

In 1960 when I was in Spain a lot of the guys brought American made cars over. They ran terrible on the Spanish gas available. Could drive 90 kilometers into France and get better gas but that was inconvenient. What I did to my '52 Mercury was install 2 head gaskets under each head and retard the timing as much as possible. Helped some but not a viable option with modern computerized cars. Viva Le Progress.

 

Yep. I love flatheads. You could do about anything to a Ford flathead and it would still run smooth as silk. One of the favorite cars I have owned was a 53 Ford Victoria with a 51 Lincoln 337 flathead engine. It used to eat the early 55 and up Chev V8s for lunch. Ah, you reminded me of yesteryear Jim. Gracias. :D

 

Hey oldtimers! For those of use who weren't born yet...

 

:needpics:

 

Rob L. :D

 

Or if our parents weren't born yet.

[dakal]

haha

 

you all will get hwere.

[whowey]

I remember the octane mixing question from high school science. According to my teacher you could mix it exactly like you describe - equal amounts of a low and high result in a mixture right the middle.

 

I think I'll just keep pumping the good stuff. I already get 17.5-18 on a long trip and I average 16.8 with city and country driving. Of course, I'm easy on the skinny peddle (most of the time). :thumbsup:

 

Thanks for the replies!

 

 

That's how alot of stations do it. Use two tanks. One low octane and one high. When you buy the middle grade it is mixed at the pump.

[Automan2164]

In 1960 when I was in Spain a lot of the guys brought American made cars over. They ran terrible on the Spanish gas available. Could drive 90 kilometers into France and get better gas but that was inconvenient. What I did to my '52 Mercury was install 2 head gaskets under each head and retard the timing as much as possible. Helped some but not a viable option with modern computerized cars. Viva Le Progress.

 

Yep. I love flatheads. You could do about anything to a Ford flathead and it would still run smooth as silk. One of the favorite cars I have owned was a 53 Ford Victoria with a 51 Lincoln 337 flathead engine. It used to eat the early 55 and up Chev V8s for lunch. Ah, you reminded me of yesteryear Jim. Gracias. :D

 

Hey oldtimers! For those of use who weren't born yet...

 

:needpics:

 

Rob L. :D

 

Or if our parents weren't born yet.

 

I was going to say that initially, but hit the backspace.

 

Rob L. :D

[HOrnbrod]

Hey you young whipper snappers, the old 53 flathead wasn't NEW when I had it. I also had a 36 5-window Ford coupe at one time that wasn't new. I'm old, but not that old. :banana:

[Automan2164]

Hey you young whipper snappers, the old 53 flathead wasn't NEW when I had it. I also had a 36 5-window Ford coupe at one time that wasn't new. I'm old, but not that old. :banana:

 

So... Do you have any pics buried anywhere Don?

 

Rob L. :brows:

[HOrnbrod]

Wish I did Rob. All that stuff went to the ex unfortunately........ :(

[Automan2164]

Sorry about that Don.

 

Bloodsucking leaches. :fs2:

Rob L.