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Discussion Starter #1 (Edited)
Fast startup idle seems to be a common issue with Pentastar JKs. I have read many threads on this on Wrangler forum but will admit to still being confused as to startup RPM. Maybe some of you more knowledgeable JKers can help.

I live in south Florida, not exactly a challenging place for engine starting. Yet my 2014 JKU (6-spd, max tow package, otherwise normal) routinely starts at 1800-2000 RPM. It has done this since new; I now have 20,000 easy miles. My dealer says no problem. I assume it is intended to heat up the cat quickly for emissions testing. But 1000-1500 would seem more than adequate for this. 1000 is a plenty fast start idle in my experience.

2000 RPM seems like both a waste of gas and needless wear on a cold engine. EPA of course is far more interested in low cold start emissions than engine longevity; it's actually in EPA's interest to see the vehicle fleet turn over as rapidly as possible. Same with manufacturers.

I have found that I can 'cheat' the fast idle by quickly stabbing and releasing the throttle a couple of times before starting. The engine then starts at ~1200-1500 and settles almost immediately to 1000. This seems to indicate some play or stickiness in the linkage. I have tried to explain this to my dealer, but they just brush me off. I bought the lifetime warranty, but I would far prefer not to have to use it due to excessive engine wear.

Anyone else experience such startup behavior? Is there anything I can adjust in the linkage, without getting into engine programming? Or is really fast idle just the new normal??
 

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Not too sure about the "linkage", as I thought that it was a drive by wire setup. What you are doing by stabbing the accelerator is probably sending a message to the control unit, causing the lower rpm's.
If the engine races enough that you are uncomfortable with it, then I'd go to a local Jeep dealer and ask for a test drive on a new one, and see if it acts like yours. If it does, then it's probably OK. If it runs slower at startup, then I'd see the service manager.
 

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I have found that I can 'cheat' the fast idle by quickly stabbing and releasing the throttle a couple of times before starting. The engine then starts at ~1200-1500 and settles almost immediately to 1000. This seems to indicate some play or stickiness in the linkage. I have tried to explain this to my dealer, but they just brush me off. I bought the lifetime warranty, but I would far prefer not to have to use it due to excessive engine wear.

Anyone else experience such startup behavior? Is there anything I can adjust in the linkage, without getting into engine programming? Or is really fast idle just the new normal??
The reason your dealer is brushing you off is because there's no throttle cable.

Fast idle (automatic choke) has been around since the 1950's or 60's...

This is the norm...

.
 

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How long does your startup fast idle last before dropping off? Mine starts at about 1800 and within about 15-20 seconds it drops. About the same with my other three cars. Idle is controlled by the computer, not the pedal.
 

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VT is correct, your Jeep is fly by wire: there is not a throttle cable.

Your Jeep was designed to have a high idle at cold start.
Not every vehicle is perfect, but rest assured thousands of engineering and programming hours go in to making modern vehicles run as lean, clean, and as efficiently as possible. If the engineers could get .0001 more MPG out of a tank of gas due to reducing rich mix or the time it runs rich, they would.

I also think your rich start is taken over when the O2 sensors get a reading, and can adjust air/fuel mixture accordingly.
I travel some for work and any modern vehicle I've rented in recent years does this.
 

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Discussion Starter #7
Not too sure about the "linkage", as I thought that it was a drive by wire setup.
Good thoughts, thanks. Excellent suggestion on test driving a new one. Correct on the ‘drive by wire’, I know that actual opening of the ‘throttle’ on modern EFI engines is electronically controlled. But something connects the foot pedal to the ‘wire’.

What you are doing by stabbing the accelerator is probably sending a message to the control unit, causing the lower rpm's.
This is certainly what it acts like, but I seem to get the same effect whether the ignition is on or off. Doesn’t seem likely that the EFI controller would be operating with no electricity. But some modern auto electronics operate in the background (e.g. AC blend door actuators) so this could well be what’s happening. Interesting.


<snip>This is the norm...
Perhaps. But the last several road vehicles I have owned had EFI (LR, SAAB, Volvo, VW petrol); haven’t owned a carbureted auto or truck in over 30 years so I’m well accustomed to EFI and electronically controlled idle speed. Fast idle on the Euro EFI cars was generally ~1000-1200 RPM unless it was really cold and/ or higher elevations, e.g. Colorado in January, when the start idle would be higher. I don’t think US automotive engineering is that far behind our European cousins as to have a different ‘norm’ for idle speeds. But thanks for your thoughts.

How long does your startup fast idle last before dropping off? Mine starts at about 1800 and within about 15-20 seconds it drops. About the same with my other three cars. Idle is controlled by the computer, not the pedal.
This is essentially how my JK starts; same profile. Maybe closer to 2000 on start but your 15-20 seconds drop-off time is about the same. Probably just the way the EFI is programmed, and the faster than necessary idle speed has much to do with cat heating.

A brief tap on the gas pedal once my engine is running does cause the idle to drop immediately to ~600-800 RPM. Reduces wear on the clutch.

Thanks for all the good thoughts!
 

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Discussion Starter #8
<snip>Your Jeep was designed to have a high idle at cold start.
Yes, I think this is probably true. Although there enough other threads on this that I don’t feel alone in finding it excessive.

Not every vehicle is perfect, but rest assured thousands of engineering and programming hours go in to making modern vehicles run as lean, clean, and as efficiently as possible. If the engineers could get .0001 more MPG out of a tank of gas due to reducing rich mix or the time it runs rich, they would.
True, unless emissions considerations override good basic engineering. The cat heats up a lot faster with more fuel burning in it. Since most emissions occur on cold start, EPA really pushes engineers to reduce startup emissions.

Another example: If tetraethyl lead additives had not been outlawed, we would be running higher compression engines with significantly greater fuel efficiency. The Clean Air Act solved this problem. Higher carbon footprint in return for elimination of a specific air pollutant.

I also think your rich start is taken over when the O2 sensors get a reading, and can adjust air/fuel mixture accordingly.
Correct. As soon as the O2 sensor is happy, all is good!

I travel some for work and any modern vehicle I've rented in recent years does this.
Yes I have also noticed this. Always attributed it to typical rental fleet cars. Guess I just expect more from my Jeep!
 

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Don't try and reinvent the wheel, the amount of gas wasted & wear is miniscule. Surely you have something better to do with your life than think about something as stupid as this.
 

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In your experience the idle is too high? What experience? I rent 35-40 vehicles a year from crappy econoboxes to loaded out Suburbans. They all act the same and it is to achieve cat light off.

Linking the pedal won't do shit. There is no mechanical connection between the pedal and engine so pumping the pedal with the engine off does nothing.
 

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It never fails to amaze me the design features you guys will see as issues.
 

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Good thoughts, thanks. Excellent suggestion on test driving a new one. Correct on the ‘drive by wire’, I know that actual opening of the ‘throttle’ on modern EFI engines is electronically controlled. But something connects the foot pedal to the ‘wire’.
The wear from this is very minuscule.....almost nonexistent. The pedal connects directly to the 'wire' so there's zero cables or linkages for the pedal.
 
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