Riddle me this...plugs, coils, and fuel mileage - JeepForum.com
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post #1 of 14 Old 07-21-2021, 06:31 PM Thread Starter
soler300
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Riddle me this...plugs, coils, and fuel mileage

2004 GC 4.7...was getting 18.5 mpg. Not terrible, but with the dinky tank size, range is not so great. Wanted to see if I could squeeze a little more out of her. Changed plugs and coils. Now getting 15-16.5 mpg. What the dang heck?!

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post #2 of 14 Old 07-22-2021, 10:17 AM
Bigrigr
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How far have you driven since the plug change? It may take a few hundred miles to get the PCM to adjust to the new parts for accurate fuel mileage.



If your getting 18.5 mpg's then leave it alone...It dont get better than that...

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post #3 of 14 Old 07-22-2021, 10:39 AM
jtec
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lets start simole-
What plugs did you use?
How did old plugs look- wet, dry, black, tan?
Any codes (scanner)?
Is engine reaching correct temp?
DO you have a scanner? lots we can look at then.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #4 of 14 Old 07-23-2021, 07:46 AM Thread Starter
soler300
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Plugs looked okay. Like they needed to be replaced, but not horrible. It's been several months. I thought the same thing about it taking some time to work itself out, but now I'm done waiting. Going to look at what plugs I installed. And going to put old coils back in and then start troubleshooting from there. BTW, since we're on the subject, what's the correct plugs? Irridium, platinum, or what? I did research when I did the plug change, but it's been almost a year.

Thanks!
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post #5 of 14 Old 07-23-2021, 07:58 AM
GmanWJ
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I would like to know if you come up with anything here.

Once upon a time, I got 19-20 mpg. Normal has been around 17. Lately I think I am closer to 15, and when towing it was 12. Now I have bigger tires and the last time I towed I was hitting 10, and even dipped under 8 when my right foot was too heavy.

If replacing anything can improve this (injectors, coils, plugs, sensors), I would be all in. At $5/gallon and 10 mpg ... well that's $0.50/mile isn't it?
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post #6 of 14 Old 07-23-2021, 09:12 AM
WJ60
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From WJJeeps.com:

4.7 Plug: SPRC12MCC4 Gap: 0.040


The standard 4.7L engine uses copper core ground electrode spark plugs, they must be replaced with the same type/number spark plug as the original. If another spark plug is substituted, pre-ignition will result.
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post #7 of 14 Old 07-23-2021, 11:34 AM
jtec
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4.7L vin N RC12MCC4 (.040 in.)
4.7L vin J RC7PYCB4 (.040 in.)

04 OEM manual.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #8 of 14 Old 07-24-2021, 07:14 AM
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I would be looking at maybe a seized brake caliper. It may be dragging a bit. After a drive check your rim near the center if one is warmer than the others. Don't touch the disc unless you want a burnt finger
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post #9 of 14 Old 07-24-2021, 07:35 AM
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Send me your old parts please, if they are getting you 18.5 I'll install them on mine in a heartbeat. I can get that on the highway with the cruise on 70, but around town I'm hitting around 14.4 according to the onboard MPG counter. I do feel it is optimistic however due to the speedo being off from the tire size. I highly doubt going from a 29" tire to a 32.5" tire increased my fuel mileage by almost 1.5 mpg's.

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post #10 of 14 Old 07-25-2021, 08:55 AM Thread Starter
soler300
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Thanks for the replies!
Brakes...I put new brakes on several months before doing plugs. No issues, but I'll definitely check them out.

I'm gonna check the plugs I installed against the part numbers posted

I'll let you know how it goes.
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post #11 of 14 Old 07-26-2021, 08:18 AM
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Pull the TB off of the intake, clean it. Pull the IAC, clean it. You'll see why once you get in there. Swap out your fuel filter too and toss in a bottle of Techron.

Make sure your alignment is good, tire pressures are set right and try to drive so that you keep the RPMs at 2200 or less while accelerating. You'll easily reach 70MPH at 2200 RPM.

The fuel tank is a little over 20 gallons. That's actually a decent size for a mid-size SUV.

"We have it totally under control. Itís one person coming in from China. We have it under control. Itís going to be just fine." ~D. Trump, Jan 22, 2020.
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post #12 of 14 Old 07-27-2021, 07:22 AM
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I hope you aren’t counting on the digital EVIC to provide accurate mpg. It’s horribly inaccurate and inconsistent. Miles driven per gallons pumped in is the only way to get data you can trust. I posted a long write up on this years ago with mpg over long interstate travel with various octanes. It’s here somewhere and may be helpful for you to refer to.

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post #13 of 14 Old 07-27-2021, 08:11 AM
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My evic shows 17 liters per 100. When I hand calculate its getting 13.6 liters per 100. Just under 18 mpg but im mostly short trips and tbh I'm not trying for the best I just drive it like I normally do.
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post #14 of 14 Old 07-27-2021, 10:15 AM Thread Starter
soler300
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Quote:
Originally Posted by leadsled jeep View Post
I hope you arenít counting on the digital EVIC to provide accurate mpg. Itís horribly inaccurate and inconsistent. Miles driven per gallons pumped in is the only way to get data you can trust. I posted a long write up on this years ago with mpg over long interstate travel with various octanes. Itís here somewhere and may be helpful for you to refer to.
I always hand calculate. It's the only way.
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