Rough idle on cold start (no codes)

[SteamboatJPN]

Looking for some guidance on a strange issue that has come up on a new to me TJ. The Jeep is a 2000 sport 4.0 5 spd w/ 34,000 miles on it. I bought it from someone who used it exclusively for their vacation home. It has been really well maintained and kept in a heated garage throughout ownership. No leaks at all.

Upon picking the Jeep up about 8 weeks ago, everything ran really well. No cold start issue no matter the wait. After getting the Jeep, I have done a bunch of maintenance Including, oil change (Conventional), plugs (Jerry’s recommendation) gapped and torqued to spec, trans fluid, air filter. New Napa battery. Alternator and battery fully tested and performing perfectly.

I have seen this issue on other threads, but no final remedy so looking for some help. A few weeks ago, out of nowhere and shortly following my plug change, I get A rough idle on cold start after sitting for 24+ hours. The Jeep has somewhat of a delayed start, but then runs rough for about 3-4 seconds. After that, the Jeep runs absolutely perfectly through the day and if it’s started again throughout the day. There is no other rough idle issues.

I bought some throttle body cleaner and plan to clean all of that for good measure. I am struck that the issue emerged shortly after plug change. Though I can stress enough how well the vehicle runs after the cold start. really scratching my head on this.

 

[SteamboatJPN]

Bumping for comments

 

[03coloradowrangler]

have had rough cold startups in the past after sitting overnight but mine also included immediate stalling unless I held the throttle open a little bit & intermittent high idle at stop lights. ran fairly perfect after roughly idling for a few seconds after startup. didnt have any codes but my problem was IAC motor failure.

based that your seeing the problem after changing plugs is a little suspicious but pulling the IAC and cleaning it is free and easy to test. worst part is getting the 2 screws out.

 

[Caleb_Bolin02]

When I first got mine It would always idle rough when it would cold start, but I changed the coil pack and the spark plugs and it seemed to start up not as rough

 

[Jerry Bransford]

A couple questions... 1) Is your temperature gauge indicating correctly? Cold when it's cold, warm when it's warm?

2) Does your ignition have a distributor cap or does it have the newer coil pack ignition with 3 coils on the rail above your spark plugs? 2000 is the transition year for going from the older distributor cap with ignition wires to the coil rail.

3) Exactly what brand of spark plug did you install, and exactly what model # is it?

The wrong type of spark plug with the newer coil rail ignition system can definitely cause misfires and a bad idle.

 

[SteamboatJPN]

@Jerry Bransford - really appreciate your inquiry!

1. need to confirm again when a I get back in town, but yes when I check it was very consistent. I had a 99 many years ago and temps track similarly.

2. Rails not distributor

3. APP985 - the platinum ones you have noted prior.

It runs near perfect after the first 4 seconds after cold start. It’s only right after startup it does this. Then for the rest of the day all starts are good as is idle. This can go on for 6-8 hours between starts. Seems only 24hr + do it which has me scratching my head.

 

[Ridge582]

any news on this issue being fixed ?

 

[SteamboatJPN]

any news on this issue being fixed ?

Yep, it’s perfect. Replaced IAC (not sure if I really needed to). Cleaned throttle body extensively (it was hardly dirty).

I think to be honest it was gas. Given I wasn’t sure how long the vehicle sat, I put premium in. I think that had something to do with it, but not sure. It been perfect since weeks after this original post.

For the first time, I had to leave it outside our heated garage and it cold started in -14 after sitting out overnight. Perfect idle! Impressive!