Random rough idle on startup

[Mitchs07explorer]

On occasion maybe once every couple of weeks I’ll start my Ex (07 4.0) and for about the first 30 seconds while it’s on I’ll get an erratic idle with the rpms bouncing between 600 and 900 and then it smooths out. I’ve been chasing a small evap leak for about 6 months until I recently bought a smoke machine. I hooked up the machine and smoke was coming from my EGR, so I replaced the EGR as well as the throttle body spacer because the gasket/ spacer was worn out. I put it all back together but didn’t have time to use the smoke machine to check my work. I’m confident where I worked is put back together and sealed correctly but yesterday on my lunch break after the vehicle had already been warm I got in to start it up and got the erratic rough idle until about 15-30 seconds later it smoothed out. It was raining yesterday with lots of moisture in the air and I’ve heard people say that can be a factor but I have started/driven my Ex plenty of time in the rain and humid conditions and have not had the rough idle occur upon startup cold or warm.

The cel hasn’t come back on for the evap leak so far, I’ll hook the machine back up this weekend just to check. But my question is what else should I be looking at for the cause of this random rough idle. I’ve heard there are O-ring gaskets in the upper and lower intake that dry out over time. My Ex is 12 years old and I’m wondering if I should change those gaskets anyways. Anyone have any thoughts on what might be causing the weird idle that only occurs randomly?

 

[dacaur2]

I had something similar. Not exact so probably not your issue but there is a chance.... I would randomly and rarely have a rough idle on startup, but I never waited to see if it went away, I would just shut down and restart and it would smooth out. Mayby once a week for a couple months. Never got a CEL and never happened other than on startup.
Then one day re-starting didn't fix it, I was at work so just drove home, and got a CEL on the way home. Code showed misfire cyl 1. Swapped plugs with the cyl next to it and the code followed the plug.... Long story short, it was a bad spark plug.... my next step if it didnt follow the plug would have been to swap coil packs and see if it followed....

 

[Mitchs07explorer]

I had something similar. Not exact so probably not your issue but there is a chance.... I would randomly and rarely have a rough idle on startup, but I never waited to see if it went away, I would just shut down and restart and it would smooth out. Mayby once a week for a couple months. Never got a CEL and never happened other than on startup.
Then one day re-starting didn't fix it, I was at work so just drove home, and got a CEL on the way home. Code showed misfire cyl 1. Swapped plugs with the cyl next to it and the code followed the plug.... Long story short, it was a bad spark plug.... my next step if it didnt follow the plug would have been to swap coil packs and see if it followed....

That’s really interesting and actually sounds very similar to my issue. Mine is only on start up as well. I’m at 123k and I did a tune up at 100k, probably wouldn’t hurt to take a look at the plugs now they’re about 1/2 way through their recommended life span. It’s so intermittent when it happens it not an issue but I’d really like to know what’s causing it and if I can remedy it.

 

[Mitchs07explorer]

I had something similar. Not exact so probably not your issue but there is a chance.... I would randomly and rarely have a rough idle on startup, but I never waited to see if it went away, I would just shut down and restart and it would smooth out. Mayby once a week for a couple months. Never got a CEL and never happened other than on startup.
Then one day re-starting didn't fix it, I was at work so just drove home, and got a CEL on the way home. Code showed misfire cyl 1. Swapped plugs with the cyl next to it and the code followed the plug.... Long story short, it was a bad spark plug.... my next step if it didnt follow the plug would have been to swap coil packs and see if it followed....

But you’re also saying that the idle never smoothed our until the car was restarted unlike my issue.