Is it bad to use cruise control when going downhill? - JeepForum.com
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post #1 of 17 Old 07-06-2013, 08:49 PM Thread Starter
JeepLiberty2011
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Is it bad to use cruise control when going downhill?

I never gave much attention to it till now but noticed recently that my RPM's are way higher if I have cruise control engaged while going downhill...

ie: If I am going down a slight slope and have CC on, my RPM's seem to sit around 3k, if I turn it off it goes down to 2k, if I put it in neutral with the CC off it goes down to 1k.

Is this completely normal? The RPM's staying that high even when going downhill? Surprised the vehicle doesn't notice the lower RPM need and adjust?

Sidenote: I think I noticed an odd sound/feel when using CC the other day while going downhill but haven't been able to replicate it.

Sidenote: Had a shop I take my jeep to when I want independent (non-dealership) opinions tell me a month ago that it had a "rough idle" that I think I can notice/hear. Had the dealership check this out, they said they couldn't find it/or any problems.

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post #2 of 17 Old 07-06-2013, 08:58 PM
Gageraid
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Quote:
Originally Posted by JeepLiberty2011 View Post
I never gave much attention to it till now but noticed recently that my RPM's are way higher if I have cruise control engaged while going downhill...

ie: If I am going down a slight slope and have CC on, my RPM's seem to sit around 3k, if I turn it off it goes down to 2k, if I put it in neutral with the CC off it goes down to 1k.

Is this completely normal? The RPM's staying that high even when going downhill? Surprised the vehicle doesn't notice the lower RPM need and adjust?

Sidenote: I think I noticed an odd sound/feel when using CC the other day while going downhill but haven't been able to replicate it.

Sidenote: Had a shop I take my jeep to when I want independent (non-dealership) opinions tell me a month ago that it had a "rough idle" that I think I can notice/hear. Had the dealership check this out, they said they couldn't find it/or any problems.
I won't be able to answer this fully, but will be able to suggest a few things.

I think your cruise control symptoms are normal. Cruise control will always try to keep your at a steady speed, and to do so will increase the RPMS. Are you using cruise control to save gas? It's been proven that you can save more gas by not using it...

Now to tackle your idle issue. We need more information. How many miles on the Jeep? How many miles on the spark plugs? Are they gapped correctly? What kind of gas are you using?
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post #3 of 17 Old 07-06-2013, 08:58 PM
Ted15
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That "high" rpm is just the cruise control keeping the speed from increasing while going downhill. It's fine, every vehicle I've ever had did the same thing. Don't know about the rough idle though.

2010 KK Sport, Detoures Knucklebone & backbone, smittybilt XRC 10, rigid led lights, R/L sliders & rack, cb, EMT2, R/K 15x7, OME, JBA, mopar skids & hitch, headlight covers, 4.10's and TruTracs
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post #4 of 17 Old 07-06-2013, 09:05 PM
huntb
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Downshift/disable overdrive to keep speed from increasing downhill
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post #5 of 17 Old 07-06-2013, 09:16 PM
Ted15
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Just let the cruise control do it. There's no need to do anything.

2010 KK Sport, Detoures Knucklebone & backbone, smittybilt XRC 10, rigid led lights, R/L sliders & rack, cb, EMT2, R/K 15x7, OME, JBA, mopar skids & hitch, headlight covers, 4.10's and TruTracs
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post #6 of 17 Old 07-07-2013, 07:25 AM
lfhoward
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Even though the rpms are high in that downhill situation, my Scangauge usually shows 9999 miles per gallon: in other words, there is no gas going to the cylinders so for a time you are getting infinite MPGs! I wouldn't worry about it.

2008 Liberty Sport 4x4 Automatic
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post #7 of 17 Old 07-07-2013, 07:30 AM
Gageraid
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Quote:
Originally Posted by lfhoward View Post
Even though the rpms are high in that downhill situation, my Scangauge usually shows 9999 miles per gallon: in other words, there is no gas going to the cylinders so for a time you are getting infinite MPGs! I wouldn't worry about it.

Don't only the newer KKs have the fuel shut off on deceleration? I know the 2008s don't. That's another reason I think the Hypertech won't work with the newer KK's.

On deceleration down a hill with my EcoGauge, the highest reading I have ever gotten is 114mpg . Without a hill the highest I see daily is no more than 90mpg.
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post #8 of 17 Old 07-07-2013, 09:54 PM
Cobradwight
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I coast downhill every day for 5 miles . I shift to n at 70 mph ,i usually reach 85/87 mph with rpm at 1000 . If I had been on the gas im sure it would be around 3k rpm . So I figure im saving gas for a few miles.
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post #9 of 17 Old 07-08-2013, 02:26 AM
tjkj2002
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Quote:
Originally Posted by Gageraid View Post
Don't only the newer KKs have the fuel shut off on deceleration? I know the 2008s don't. That's another reason I think the Hypertech won't work with the newer KK's.

On deceleration down a hill with my EcoGauge, the highest reading I have ever gotten is 114mpg . Without a hill the highest I see daily is no more than 90mpg.
All multiport OBDII fuel injected engines have decel fuel shutoff.

'02 Liberty sitting on 35" tires,HP44,RockJock60,and AtlasII t-case
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post #10 of 17 Old 07-08-2013, 08:25 PM
Gageraid
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Quote:
Originally Posted by tjkj2002 View Post
All multiport OBDII fuel injected engines have decel fuel shutoff.
http://www.allpar.com/trucks/jeep/liberty-2008.html

It's says 2010 they started it, but not sure if they mean they just started adding the green ECO dash light, or the actual fuel shutoff. I can never find a solid answer to this.
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post #11 of 17 Old 07-08-2013, 10:30 PM
tjkj2002
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Quote:
Originally Posted by Gageraid View Post
http://www.allpar.com/trucks/jeep/liberty-2008.html

It's says 2010 they started it, but not sure if they mean they just started adding the green ECO dash light, or the actual fuel shutoff. I can never find a solid answer to this.
Mine has it and it's a '02,just not as effective as the eco mode version on newer KK's.They just advertize it as a "new" feature when it's not to bump sales.The "ECO mode" version just uses it more even when not decelerating.

'02 Liberty sitting on 35" tires,HP44,RockJock60,and AtlasII t-case
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post #12 of 17 Old 07-09-2013, 05:06 PM
Gageraid
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Quote:
Originally Posted by tjkj2002 View Post
Mine has it and it's a '02,just not as effective as the eco mode version on newer KK's.They just advertize it as a "new" feature when it's not to bump sales.The "ECO mode" version just uses it more even when not decelerating.
Can you show me a reliable source so I can believe you?
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post #13 of 17 Old 07-09-2013, 05:23 PM
tjkj2002
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Quote:
Originally Posted by Gageraid View Post
Can you show me a reliable source so I can believe you?
The FSM

'02 Liberty sitting on 35" tires,HP44,RockJock60,and AtlasII t-case
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post #14 of 17 Old 07-09-2013, 06:13 PM
Gageraid
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Quote:
Originally Posted by tjkj2002 View Post
The FSM
Okay, can anyone else verify a 2008 Liberty has "Decel Fuel Shut Off"?
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post #15 of 17 Old 07-09-2013, 07:46 PM
Ted15
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Quote:
Originally Posted by Gageraid

Okay, can anyone else verify a 2008 Liberty has "Decel Fuel Shut Off"?
Push the odometer button and it will cycle through total miles, two different trip odometers and the ECO. While driving, the readout will change depending on what it's doing.

2010 KK Sport, Detoures Knucklebone & backbone, smittybilt XRC 10, rigid led lights, R/L sliders & rack, cb, EMT2, R/K 15x7, OME, JBA, mopar skids & hitch, headlight covers, 4.10's and TruTracs
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