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  #313  
Old 04-29-2016, 12:21 AM
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Re: For those who drive a Hemi 5.7

I have 2012 which is every 30k for the plugs. Mine had over 60k when I purchase it. Yes I do think the dealer should have did the 60k service on the car before delivering to the customer. According to the description on the Jeep website all required service are to be done to a CPO vehicle. My air filter and cabin filter also appeared to be the original. They absolutely needed changing when I purchased the vehicle I only put about 1500 miles on it before changing the plugs.

Quote:
Originally Posted by leadfoot4 View Post
Lately, it's been my observation that most manufacturers are claiming 100,000 miles for spark plug life. Therefore, unless your CPO vehicle had 100,000 miles on it, why would you expect them to change the plugs? My '15 Hemi has 16 plugs, I believe. That's about $65 worth of plugs. Think a dealer would change them, if they didn't ABSOLUTELY have to?

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  #314  
Old 04-29-2016, 05:54 AM
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Originally Posted by sidebox View Post
Welp, I have 16 MOPAR spark plugs coming tomorrow. It looks like buying a certified used car from Jeep is horseshit. There is some offbrand "NKG" plugs in there now. I'm hoping I see some pretty fast progress on my MPG... Or I'll be out another $95 for nothing.

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  #315  
Old 04-29-2016, 06:34 AM
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Re: For those who drive a Hemi 5.7

I hope I'm wrong, but I think there are other issues going on here besides spark plugs. Remember we are talking 6 miles per gallon AVERAGE. That is horrific. By comparison the 1960's era, dual quad Chrysler 426 HEMI got better mileage than that. And it was known as one of the biggest gas hogs ever.

Even if replacing the plugs doubles the current mileage you're getting, (which is unlikely), you still will be getting no better than 12 MPG. Which is still horrible for a modern computer controlled, electronic fuel injected, mid displacement V-8, running through a multi speed transmission. The plugs are a good place to start, but you should be looking at mileage at least in the mid to upper teens. Higher if you drive with a soft foot.
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  #316  
Old 04-29-2016, 08:24 AM
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Re: For those who drive a Hemi 5.7

Quote:
Originally Posted by leadfoot4 View Post
Lately, it's been my observation that most manufacturers are claiming 100,000 miles for spark plug life. Therefore, unless your CPO vehicle had 100,000 miles on it, why would you expect them to change the plugs? My '15 Hemi has 16 plugs, I believe. That's about $65 worth of plugs. Think a dealer would change them, if they didn't ABSOLUTELY have to?
MY11-13 require the plugs be changed at 32K intervals. MY14+ it's 100K.

That said, if the CPO was sold at or close to the plug maintenance window, IMHO, the dealer doing the certification should have handled it.
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  #317  
Old 04-29-2016, 09:01 AM
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Re: For those who drive a Hemi 5.7

Quote:
Originally Posted by Jim_in_PA View Post
MY11-13 require the plugs be changed at 32K intervals. MY14+ it's 100K.

That said, if the CPO was sold at or close to the plug maintenance window, IMHO, the dealer doing the certification should have handled it.
This is because in order for the cylinder deactivation to work they needed to use copper core plugs and not platinum. For my14+ they figured out how to make the platinum plugs work.

That said, based on my experiences with the 2 hemi's I have owned a drop in mpg is likely one of 4 things:
1. time for plugs: at 30k when I swapped the plugs in my RAM I saw a significant improvement, so the 30k number doesn't seem to have any padding.
2. dirty air filter
3. less than 89 octane: I know other swear by 87, but where I live with the heat in the summer, 87 seems to trigger the knock sensor and retard the timing - thus less mpg. My best results have been with 91
4. Ethanol: Ethanol fuel has less energy per unit. This one is hard to put a finger on because most stations label their gas as "up to 10% ethanol". It take 1.5 gallons of pure ethanol to equal 1 gallon of gasoline. So even at 10% you are taking a hit in mileage. There is almost always one gas station around that sells ethanol free gas, run a tank or two of this through to find out your "true" mileage.

As to the guy who says plugs shouldn't cause such a drop on a modern computer controlled engine, I think that is exactly why you can see this kind of drop. On a old carb fed engine bad plugs would mean less fuel burned and less power. With sensors and computers a modern one will account for the loss of power caused by incomplete burn by sending more fuel. The end result being that the driver barely notices, but the fuel use goes up even more.
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  #318  
Old 04-29-2016, 09:26 AM
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Re: For those who drive a Hemi 5.7

Quote:
Originally Posted by crombenevolant View Post
....As to the guy who says plugs shouldn't cause such a drop on a modern computer controlled engine, I think that is exactly why you can see this kind of drop. On a old carb fed engine bad plugs would mean less fuel burned and less power. With sensors and computers a modern one will account for the loss of power caused by incomplete burn by sending more fuel. The end result being that the driver barely notices, but the fuel use goes up even more.
If the plugs are so bad, that they have dropped his mileage from the upper teens, (what he SHOULD be getting), to 6 MPG, that engine would be missing, idling, and sounding like a coffee can full of rocks rolling down a hill. According to the OP, it is running smoothly, except for the crappy fuel mileage. Fouled plugs are just that, fouled.
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  #319  
Old 04-29-2016, 09:58 AM
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Re: For those who drive a Hemi 5.7

Quote:
Originally Posted by billt View Post
If the plugs are so bad, that they have dropped his mileage from the upper teens, (what he SHOULD be getting), to 6 MPG, that engine would be missing, idling, and sounding like a coffee can full of rocks rolling down a hill. According to the OP, it is running smoothly, except for the crappy fuel mileage. Fouled plugs are just that, fouled.
Keep in mind that there are different failure mechanisms for plugs. Plug fouling is really a symptom (because the air/fuel mix is not fully burning and thus depositing on plugs). The electrode can erode causing greater gap and thus less spark, the core can break down and become more resistive causing less spark, etc. Modern engine control systems are pretty good at correcting for the plethora of conditions (heat, cold, altitude, bad gas, etc) that used to cause the same rough running symptoms you describe.

It is possible that there could be other factors in his mileage, but since he has already bought the plugs I am guessing he can tell us in a week or so if it did help or not. Since his ECU is not throwing any codes, the most likely suspect is one of the ususal suspects: spark, fuel, air. Or the most intangible of them all, the human factor. I know my mileage drops to about 10 each winter because I remote start my truck and let it warm up each morning. I saw something similar when there was a lot of road construction and I sat in traffic waiting each morning. Those are hard to pin down and often not thought about.
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  #320  
Old 04-29-2016, 10:13 AM
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Re: For those who drive a Hemi 5.7

You're right about one thing. We'll know as soon as he replaces them. I'm curious to know what the outcome is as well. That is unbelievably bad mileage for that engine.
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  #321  
Old 04-29-2016, 10:21 AM
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Re: For those who drive a Hemi 5.7

Quote:
Originally Posted by crombenevolant View Post
3. less than 89 octane: I know other swear by 87, but where I live with the heat in the summer, 87 seems to trigger the knock sensor and retard the timing - thus less mpg. My best results have been with 91.
Here I agree completely. If I was not happy with my fuel mileage, the first thing I would try is a tankful of 91 Octane Premium. I know out here in the extreme heat, it all but assures the best fuel economy, and power the engine is capable of.
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  #322  
Old 04-29-2016, 06:24 PM
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For those who drive a Hemi 5.7

I'm glad to have sparked this conversation. Oh man. That was really ****ing bad. Sorry... The point is, I appreciate the feedback. The benefit of this issue is I'm learning more about my engine.

I'm going to take care of this on Monday, so I should be able to report back MPG a few days after that. While the engine feels smooth, I did notice a change last night. When idling without the brake on (N or in D on a slight incline without braking) I am getting a sort of mini rocking/vibration. On my old all-used-up 99 Infiniti this would happen if I had a leak in my air hose. I double checked my air filter and all my hoses, and those are not the culprits. In double checking the plugs today I did notice that one has an ever so slight bend to it, as well.

Unfortunately, I just topped off with regular. Fortunately, I just have to drive to the corner to burn half a tank of gas. Once empty, I will be trying a clean tank of premium. I don't think I can legally find a tank of ethanol-free gas in NYC.


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  #323  
Old 04-29-2016, 06:47 PM
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Re: For those who drive a Hemi 5.7

Quote:
Originally Posted by sidebox View Post
I don't think I can legally find a tank of ethanol-free gas in NYC.
It's much the same here in the Phoenix Metro area. Everything out there has 10% Ethanol. I did find this website.

Ethanol-free gas stations in the U.S. and Canada

but when I punched in Arizona, the only non Ethanol gas stations there were sold racing fuel, (100-110 Octane). And none were near me. You might have better luck depending on your location.
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  #324  
Old 05-05-2016, 06:22 PM
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Re: For those who drive a Hemi 5.7

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Originally Posted by Blkfenders View Post
I was a long time skeptic of dash board MPG readings myself.
It's a popular misconception that EVIC is inaccurate compared to hand calculating MPG. The problem with hand calculating is that you will never "fill" the tank" to the same level every time. There will always be air pockets in your tank, and depending on a multitude of variables you will always have differing amounts that will be "full tanks".


If you don't reset you trip computer after each fill up (I never reset trip B)... let it run across 5-6 tanks and then compare to your aggregate hand calculated MPG based on fill ups. You will be surprised at how much closer the numbers are. In my case usually dead on or .1 +/-
You are completely correct on this. I just ran 4 tanks doing it this way and it was within .2 mpg cant get much better than that. so no need for the hand calc at all anymore on this vehicle just using the EVIC. Thanks for the tip
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