Rough Idle (with video)

[ItchyDong]

2006 LJ I’ve been experiencing a rough idle with no codes. My engine mounts and tranny mount are brand new. CKPS, CPS, TPS, MAF, IAT, all tested good. Decided to throw a Fuel Pressure gauge on it and this is what I came up with (see video). If I’m not mistaken, I shouldn’t have such a wiggly needle

 

[Jerry Bransford]

None of those are what I would have suspected early-on as possible causes of a rough idle.

1) Exactly what brand and model spark plugs are you running? A common cause of a rough idle is having installed a single-tipped platinum spark plug like an Autolite AP-985 or Champion 3034. Double-tipped platinums like Autolite APP-985 or Champion 7412 are fine.
2) Another common cause of a rough idle is a bad upstream O2 sensor. A bad O2 upstream sensor can trick the PCM into setting the air/fuel mixture way too rich which will cause the engine to run rough and wreck the fuel economy as well.

 

[ItchyDong]

It’s been a year since I replaced plugs, but I used whatever ones you suggested after some digging on the forum. I also used Chris’ write up and replaced all 4 O2 sensors with the NGK’s. Long term fuel trim is sitting at 4%

 

[Jerry Bransford]

It’s been a year since I replaced plugs, but I used whatever ones you suggested after some digging on the forum.

I suggest several different plugs depending on the model year and engine. Which brand and model plugs did you install?

 

[ItchyDong]

I honestly don’t recall. Which ones should I have for a 2006 LJ and I’ll just replace them for fun?

 

[ItchyDong]

So, bad spark plugs will cause the pump to fluctuate? I would understand if it were cam driven but I’m having a hard time seeing that being that it’s electric. I hope there’s no snarky tone in any of this. I’m legit just trying to wrap my head around it.

 

[ItchyDong]

I should also say that I went over, what I believe, to be all of the vaccum lines with propane and soapy water and couldn’t find anything

 

[ItchyDong]

So I should get Autolite XP985s? Pretty sure that’s what I got, but I’ll replace them anyway for fun.

Also I just read that on returnless fuel systems, it’s not uncommon to see fluctuations at idle.

 

[Jerry Bransford]

As above, the two most likely causes of a rough idle are, in no particular order, a bad upstream O2 sensor or the wrong model spark plugs.

Installing Autolite's XP-985 plugs would eliminate the incorrect spark plug possibility. If that doesn't help I'd next suspect a bad upstream O2 sensor. Especially if you have noticed a reduction in mpg and/or the aroma of fuel which would be caused by the fuel mixture being too rich. If you decide to replace your upstream O2 sensors make sure to go with NTK and avoid Bosch.