What Octane For 02 R/T? Please Help!

[SOHCKing03]

The "Magnum" version of the 2.0 engine is described above, but we feel we should emphasize that it takes premium gas, and that gas mileage is probably the lowest in its class

Does the R/T really require 91 octane gasoline (Premium)?

I never knew this for the first few months I drove the car (was not told by previous owner) and then read this and changed up to "super" (89 octane).

I always got a CEL when my car hit 85mph or got around 5K rpm in 3rd or 4th gear. Could this be because I was not running 91 or 92 octane ("Premium" octane rating varies from station to station around here)?

I have really noticed my mileage not being great lately. It fluctuates from between 20 and 30mpg each week. I go the exact same route to school everyday and it totals about 70miles/day (generally 325-375 miles a week). I drive the same way everyday too, and yet I still see a great difference in gas mileage each week?!

Another thing, the CEL gives me a P0300 Random Misfire code after the above CEL situation^^. Is this because of my octane gas? Or should I look at my plugs? Could this be the cause of my bad gas mileage? Wrong octane--> misfires--> bad mileage?? Chain reaction maybe?

Please help with any advice!

-Brad

[racer12306]

Any chance the car has an underdrive pulley? That was a common problem on 02 r/t's with UDP's.

Also, if you don't use 91 octane then you don't get to take full advantage of the aggressive timing curve.

[Diablo0]

Not really required, more recommended. Without the 91 octane the computer pulls timing which lowers performance.

[2001rt32]

from the factory a dealer does recomend 91 or higher octane the only thing that is said is that lower octane will affect acceleration times. i have never ran anything else in my r/t but i do still have that same problem as you with the gas milage problem exept myn is worse. i get like 230 to 270 miles to a tank. the cel could be because of the lower octane because of the timing advance and the random misfie code could be from the coil plugs/wires or an injector. i had that code and i changed the plugs and wires and it went away.

[silverneontuner]

Use diesel. Just kidding . A friend of mine uses just regular 87 octane fuel in his 02 and has never had any running problems.

[SOHCKing03]

Okay well I will run this tank down to almost empty and start running 91 for the better acceleration.

I can usually get around 275-300 miles to a tank.

As for the plugs/wires, I will change them here shortly if the problem still exists after I run 91.

Thanks for all the help guys.

-Brad

[racer12306]

does the car have an UDP?

[racer12306]

also, you can't judge by miles per tank. You have to do a true miles per gallon calculation to determine if you are getting reasonable fuel economy.

[oklamopar]

I always run 91 or better. i noticed power/performance and mileage difference when ran lower octane.
But then again if i used Higher octane it ran better so i tend to drive more aggressively and then mileage went down.
this is all in the 02 R/T.
Don't wanna talk about the SRT mileage

[occasional demons]

Not really required, more recommended. Without the 91 octane the computer pulls timing which lowers performance.

Higher octane burns slower, to prevent preignition, thus allowing more spark advance. This is also why most cars designed for 87 get worse mileage on 89-up because the fuel isn't completely burning before the piston is at bottom, resulting in less effiecient output. I will agree with 2001rt32 as far as the wires and plugs if they are getting alot of miles on them or if you have the UDP. I wouldn't think it (87 octane)would cause misfires, detonation yes, but the PCM will/should pull timing so the duration of knocking would be prolly less than a second or so.

[SOHCKing03]

Where should I look for a UDP?

I don't think I have one, but I'm not sure. I was just gonna buy the Unorthodox one lol.

-Brad

[Canada]

Keep in mind guys that he is in MI, so this time of year the weather gets much colder, with snow occasionally, which would greatly contribute to lower mileage.

I would also assume, being one of the colder states this time of year, they use "winter gas" formulas at your stations which encourage worse fuel economy.

[occasional demons]

If you can't see well enough by looking down from the top, you can remove the splash gaurd if there's one there (two 10mm head screws/plastic push clips) and check the crank pulley. If it is black with a rubber ring sandwiched between the hub and pulleys, it is OEM. Modern, E bay, Summit Racing, Jegs, etc. all should carry them. But you may get the misfire code as a result.

Edit:

Keep in mind guys that he is in MI, so this time of year the weather gets much colder, with snow occasionally, which would greatly contribute to lower mileage.

I would also assume, being one of the colder states this time of year, they use "winter gas" formulas at your stations which encourage worse fuel economy.

This is true too, and one wonders what the formula does to the octane rating, it should be the same, but reality is a different matter. But if he actually does have a misfire....

[zandifam]

One thing that has not been mentioned is that if you change from low octane to high octane gas you might not notice a performance difference right away. If your engine did detonate on the low octane gas causing the computer to pull timing it will take around 50 starts for it to return to the stock timing curve unless you clear the codes by disconnecting the battery. In my opinion the magnums knock sensor is extremely sensitive to knock or vibration. In fact I believe that is one of the reasons that some magnums seem to perform inconsistently or underperform. No proof of course but maybe someone who has seen this on the dyno or a scope could chime in.

[neon2ner]

i know this is off topic, but i see that people on here are running UDP's on their 02's. i was told that it throws a code about 5000 rpm on the 02 (or 01) and up pcm? is there a way to get past this?

[occasional demons]

i know this is off topic, but i see that people on here are running UDP's on their 02's. i was told that it throws a code about 5000 rpm on the 02 (or 01) and up pcm? is there a way to get past this?

I have not seen anything yet. I'm not sure anyone has conclusively found the actual reason it does it - alot of theories tho.

[kc2005ptgt]

I do not htink there is any way past it short of running an 01 rt pcm...

BRR says the reason is because of a change in the sensitivity of the misfire programming (somehting along those lines) for a change from 01 to 02... that is why the 02 is only one who throws this code with a udp...
I am however going to try an Adam K. ODP (alt drive pulley) and see when if any misfire code appears... I get the misfire code usually in 3rd gear above 4500 rpms... sometimes in 2nd gear, and usually when I stay above 4500 for any sustained period of time from 2 seconds on up...

[Canada]

i know this is off topic, but i see that people on here are running UDP's on their 02's. i was told that it throws a code about 5000 rpm on the 02 (or 01) and up pcm? is there a way to get past this?

I have not seen anything yet. I'm not sure anyone has conclusively found the actual reason it does it - alot of theories tho.

We know the reason, its due to the PCM mis-interpreting the extra vibration caused my it as a misfire.

As for a fix, simple, ask BlackRose, AF-X PCM.

[SGT BRAD]

the sensitivity in the 02 pcm was cranked up so that if you add an udp the rpms rise faster than the "window" that the pcm is expecting to see. it doesn't know whether the rise is faster or slower than it should be, it just knows that it's outside of the acceleration "window" that the pcm was programmed to see. the 01 had a more leeway, ie: the window was larger. it gets even worse if you put in a lightweight flywheel. supposedly the cel comes on at 3k rpms at wot instead of 5500. hope this makes sense as it's the way it was explained to me.

[kc2005ptgt]

I never had a CEL at WOT with the stock pulley on. I ran it like that for over a year with numerous sprints to redline in several gears as well... it was only after the install of the udp did I get it...

I have also heard of people getting fewer CEL's with the more 'expensive' pulleys like UR as opposed to an afx or mpx...

[titansxt]

Lots of answers but I'll give mine. The code you are throwing is probably from the lower octane gas igniting under compression rather than ignition. Thus you get a knock, that leads to the code.

[SGT BRAD]

if you are getting the code only at wot acceleration past 5500 rpms and you have an udp, it IS caused by the udp. 02's had this issue. i had the same issue with my non-r/t after installing the udp and so has every other 02 owner i know.