End of my rope on Rough Idle - please read - JeepForum.com
 
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post #1 of 9 Old 06-11-2019, 09:20 PM Thread Starter
bparsel
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End of my rope on Rough Idle - please read

2005 LJ I am bringing back to life. I started with about 9 codes and have whittled them down.

New Air Filter, Spark Plugs, TPS, IAC, Crankshaft position sensor, Cleaned throttle body, all 4 new O2 sensors, replaced OPDA with Cam sensor and clocked at Top Dead Center when replacing. It even has a new PCM.

I now have zero codes. It idles fine when started but once put in gear with a load on will start to idle rough around 5-700 rpms and sounds almost like it has a miss and wants to die, it also stumbles between 45-50mph. The cats seem fine.

I will be replacing the fan clutch this week as well but that shouldnt effect the idle.

WHAT AM I MISSING?!?!? Ive read a ton of rough idle threads and this thing should be cured by now. This is my 6th jeep so not exactly a rookie and Ive completely restored 2 before but this is getting frustrating.

Any ideas are welcome.

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post #2 of 9 Old 06-12-2019, 01:41 AM
mukluk
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What spark plugs did you install?
How is the battery and charging system? Cables in good shape with clean and tight connections?
Have you tried connecting a vacuum gauge to see if it shows anything amiss?

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post #3 of 9 Old 06-12-2019, 04:49 AM Thread Starter
bparsel
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Quote:
Originally Posted by mukluk View Post
What spark plugs did you install?
How is the battery and charging system? Cables in good shape with clean and tight connections?
Have you tried connecting a vacuum gauge to see if it shows anything amiss?
NGK Spark plugs and O2 sensors. Battery cables are clean, alternator is charging fine.

I havent connected a vacuum gauge but have done brakecleen all around to see if it picked up and got no change.
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post #4 of 9 Old 06-12-2019, 08:20 AM Thread Starter
bparsel
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I guess I will go after the ignition coil next with a multimeter.

1&6, 2&5, 3&4 firing order....should be about 1500 ohms correct?
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post #5 of 9 Old 06-12-2019, 12:25 PM
mukluk
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Type of plug was what I was looking for, not brand. You cannot use single platinum plugs with your coil rail ignition as they are known to cause misfires and rough running.

Coil resistance should be 0.71-0.88 ohms on the primary, and 11-15 Kohms on the secondary.

Vacuum gauges can show you much more than just if you have a leak.
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post #6 of 9 Old 06-12-2019, 01:11 PM Thread Starter
bparsel
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NGK 7866 Plugs...so those are covered.

I'll slap a vacuum gauge on it tonight and see what I get.
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post #7 of 9 Old 06-12-2019, 02:11 PM
Jerry Bransford
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When the spark plugs are correct and the O2 sensors are in good condition and either NTK or NGK, my next suspicion would be a bad TPS (throttle position sensor) or a dirty IAC (idle air control). The IAC can be cleaned by spraying throttle body cleaner into the throttle body while the engine is at idle (only!) rpms... don't rev the engine or the spray won't pass through the IAC. The cleaner will flood the IAC so the engine will die but that's normal and ok, just restart the engine and spray more into the TB while the engine is idling. Again, don't raise the rpms trying to keep the engine from stalling, the IAC will only pull the cleaner in through itself at idle rpms which is when the IAC is operating. I'd keep at this until at least 1/3 of the can has passed through the IAC. The IAC will suck the TB cleaner in via its 'Idle Air Control Passage Inlet' slot shown below.

I'd clean the IAC first and if that doesn't help then I'd replace the TPS with a quality replacement, Mopar if you can get it. No, not a store brand.
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post #8 of 9 Old 06-12-2019, 02:39 PM Thread Starter
bparsel
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Quote:
Originally Posted by Jerry Bransford View Post
When the spark plugs are correct and the O2 sensors are in good condition and either NTK or NGK, my next suspicion would be a bad TPS (throttle position sensor) or a dirty IAC (idle air control). The IAC can be cleaned by spraying throttle body cleaner into the throttle body while the engine is at idle (only!) rpms... don't rev the engine or the spray won't pass through the IAC. The cleaner will flood the IAC so the engine will die but that's normal and ok, just restart the engine and spray more into the TB while the engine is idling. Again, don't raise the rpms trying to keep the engine from stalling, the IAC will only pull the cleaner in through itself at idle rpms which is when the IAC is operating. I'd keep at this until at least 1/3 of the can has passed through the IAC. The IAC will suck the TB cleaner in via its 'Idle Air Control Passage Inlet' slot shown below.

I'd clean the IAC first and if that doesn't help then I'd replace the TPS with a quality replacement, Mopar if you can get it. No, not a store brand.
Thanks Jerry, you solved an issue on a '99 TJ I restored a few years ago so I appreciate your input.

Both Plugs and 02 Sensors are NGK/NTK and the IAC and TPS are brand new however the TPS is not a Mopar unit so I may need to try another one.
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post #9 of 9 Old 06-12-2019, 03:06 PM
Jerry Bransford
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Try this with that TPS... it may be dirty inside from its variable resistor carbon having been scraped off inside and causing intermittent connections. Try cycling the throttle lever or accelerator pedal fully rapidly 50 or so times to see if that temporarily helps or at least changes the rough idle symptom. If it does, odds are it's dirty inside and intermittent from the variable resistor's carbon flaking off inside. Kind of like old-time volume controls making a scratchy sound when adjusting the volume which can be temporarily helped by rotating the volume control back & forth a bunch of times to help clean up its interior variable resistor.

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