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Old 09-14-2018, 08:45 AM   #1
Bucksaw
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Default 6.4 Hemi fuel Question

This is my second 6.4 Hemi, and I have always run 87 octane fuel in them. Recently I have noticed an odd behavior by my truck when accelerating uphill under light throttle. It feels like it is stumbling and I can watch the RPM gauge bounce up and down a couple hundred RPM when it happens. If I make it downshift it does just fine, but under light throttle it acts like its missing. No check engine lights, and if it was missing, I feel like it would do it regardless of RPM, right? I have tried running mid-grade fuel, and that seems to help some, but not all the time. I have also noticed that on the new models it says that they run premium fuel.

Just wondering what octane rating that you guys running the 6.4 put in your truck, and if you have noticed any similar problems.

FYI, my truck just rolled over 58,100 miles.
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Old 09-14-2018, 08:50 AM   #2
WItoTX
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Clean injectors and maybe replace spark plugs. The ECU should auto reprogram to the lower octane if premium is recommended, but not required. However if the manual says premium required, then I would run premium, and suspect that is your problem. On cars where its recommended, but not required, then I would say you can get away with running a lower octane, but its still going to be harder on everything.
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Old 09-14-2018, 08:57 AM   #3
BBReezen
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I want to say my 5.7 did that. Like it’s lower RPMs and “bogging down” going up hill and indecisive about whether it wants to down shift or not? Quick poke on the accelerator and it down shifts and runs fine?

I was never happy about how that truck liked to stay in a higher gear than it needed to be in. It’s shift patterns were weird. I could be driving around at the lease and start to go up a hill and it would **** near stall itself out and I had to put my foot in it just to get it to downshift.
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Old 09-14-2018, 08:58 AM   #4
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Originally Posted by WItoTX View Post
Clean injectors and maybe replace spark plugs. The ECU should auto reprogram to the lower octane if premium is recommended, but not required. However if the manual says premium required, then I would run premium, and suspect that is your problem. On cars where its recommended, but not required, then I would say you can get away with running a lower octane, but its still going to be harder on everything.
I will have to check my manual, but Im pretty sure it says "recommended". Im gonna run some injector cleaner through it this weekend and see if that helps anything. If it is required to run premium, then I might as well be running a diesel, because the cost of premium is well over $3 around here.
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Old 09-14-2018, 09:01 AM   #5
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Originally Posted by BBReezen View Post
I want to say my 5.7 did that. Like it’s lower RPMs and “bogging down” going up hill and indecisive about whether it wants to down shift or not? Quick poke on the accelerator and it down shifts and runs fine?
Thats exactly what its doing, and it only occurs on the highway. Dont have any problems offroad. My first truck with the 6.4 didnt do it though. I wonder if its an intermediate gremlin.
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Old 09-14-2018, 09:02 AM   #6
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I run 87 mostly, 89 if I'm on a long highway trip. My 2015 manual says "87 to 89" with 89 being recommended. The manual says that over 89 has no benefit. The 2018 is the same I believe. I have had no issues like what you describe with 32k. That said, the transmission tuning on these trucks isn't the best. I've had a couple times when I was cruising and had it hesitate on a hill. Had to put my foot in it to wake it back up.

Last edited by FVR JR; 09-14-2018 at 09:07 AM.
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Old 09-14-2018, 09:04 AM   #7
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Have 2017 3/4 ton with 6. 4 hemi. Run 87 octane and no issues.
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Old 09-14-2018, 09:10 AM   #8
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Originally Posted by FVR JR View Post
I run 87 mostly, 89 if I'm on a long highway trip. My 2015 manual says "87 to 89" with 89 being recommended. The manual says that over 89 has no benefit. The 2018 is the same I believe. I have had no issues like what you describe with 32k. That said, the transmission tuning on these trucks isn't the best. I've had a couple times when I was cruising and had it hesitate on a hill. Had to put my foot in it to wake it back up.
OK, good to know. Mine's a 15 too so now I dont have to go look it up.
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Old 09-14-2018, 09:15 AM   #9
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I have a 2015 6.4 with 70,000 miles and no issues. Been running 87 octane since day one.
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Old 09-14-2018, 09:47 AM   #10
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Check your fuel filter would be my first guess?
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Old 09-14-2018, 09:51 AM   #11
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is that the MDS, if you're on light throttle might it be in "ECO" mode? Not sure if 6.4 has that or not.
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Old 09-14-2018, 10:11 AM   #12
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is that the MDS, if you're on light throttle might it be in "ECO" mode? Not sure if 6.4 has that or not.
It does have it, but that isnt it because it wasnt in ECO mode. There are only a few instances where it seems to happen all the time. It never has happened when towing or under load (which I guess is inaccurate since with my flatbed its always under load), which leads me to believe it has something to do with the programming of either the fuel system or transmission codes if its not an octane problem. I was just wondering if anyone else had this issue or a possible fix.

Last edited by Bucksaw; 09-14-2018 at 10:19 AM.
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Old 09-14-2018, 10:23 AM   #13
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Originally Posted by Bucksaw View Post
It does have it, but that isnt it because it wasnt in ECO mode. There are only a few instances where it seems to happen all the time. It never has happened when towing or under load (which I guess is inaccurate since with my flatbed its always under load), which leads me to believe it has something to do with the programming of either the fuel system or transmission codes if its not an octane problem. I was just wondering if anyone else had this issue or a possible fix.
10-4, hope you get it all figured out. I know my truck when in that eco mode has a slight delay when I punch it. Like it takes just a second to kick back into 8 cylinders or something.

I'm not much of a mechanic so I'm not sure how it all works.
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Old 09-14-2018, 10:47 AM   #14
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10-4, hope you get it all figured out. I know my truck when in that eco mode has a slight delay when I punch it. Like it takes just a second to kick back into 8 cylinders or something.

I'm not much of a mechanic so I'm not sure how it all works.
Yeah, its more of a nuisance than anything. I figure if it causes a problem, Im under warranty until 100K. I know the delay you are talking about, and I guess Ive just gotten used to that.
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Old 09-14-2018, 11:26 AM   #15
Javelin
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sell it and buy a cummins like I did lol
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Old 09-14-2018, 12:10 PM   #16
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sell it and buy a cummins like I did lol
Budgets too tight for now. Wife just got her first paycheck since August of 2017. Been living on student loans for a year while she went back to school, so its gotta do me until we get some stuff paid off. Shes got it all planned out to be mostly paid off by about 2020. I did tell her yesterday though, that as soon as we get her car and student loans paid off, Im making the switch to a good used diesel.

Last edited by Bucksaw; 09-14-2018 at 12:13 PM.
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