Rough Idle

[tenortech]

Still chasing my tail on this rough idle.
It all started the Monday after a weekend of work. On a Saturday I replaced the transmission mount, Sunday I wired in some bumper lights, and on Monday I started it up and once it warmed up I got a rough idle. I noticed the idle RPMs used to sit right above 750 and now it's right below the 750 line. Every once and a while I can feel the computer catch that it's running too lean and will kick up the RPMs to above 750, but that only lasts about 5 seconds. The manifold holds pressure after I shut the engine off after about 10 minutes. I've sprayed starting fluid all over all the vacuum lines and no difference in RPMs, I've replaced the IAC and gasket, TPS, throttle body was cleaned with and put back with a new gasket, new camshaft sensor, new fuel injectors, new spark plugs (fuel pressure is like 54 psi), inspected all the vacuum lines and replaced any that looked iffy. Under the dash vacuum lines are a little hard to inspect so I unplugged the vacuum line going through the fire wall and plugged it going to the manifold and didn't notice any difference. The rough idle gets slightly better with the heater running.

I'm looking for any and all possibilities of what it could be at this point. I'm going crazy!!
Pic for attention. Thank you for anything you might be able to give!!
*Not sure if this is related, but I didn't notice this before the issue of the rough idle started.

 

[Chris]

Still chasing my tail on this rough idle.
It all started the Monday after a weekend of work. On a Saturday I replaced the transmission mount, Sunday I wired in some bumper lights, and on Monday I started it up and once it warmed up I got a rough idle. I noticed the idle RPMs used to sit right above 750 and now it's right below the 750 line. Every once and a while I can feel the computer catch that it's running too lean and will kick up the RPMs to above 750, but that only lasts about 5 seconds. The manifold holds pressure after I shut the engine off after about 10 minutes. I've sprayed starting fluid all over all the vacuum lines and no difference in RPMs, I've replaced the IAC and gasket, TPS, throttle body was cleaned with and put back with a new gasket, new camshaft sensor, new fuel injectors, new spark plugs (fuel pressure is like 54 psi), inspected all the vacuum lines and replaced any that looked iffy. Under the dash vacuum lines are a little hard to inspect so I unplugged the vacuum line going through the fire wall and plugged it going to the manifold and didn't notice any difference. The rough idle gets slightly better with the heater running.

I'm looking for any and all possibilities of what it could be at this point. I'm going crazy!!
Pic for attention. Thank you for anything you might be able to give!!
*Not sure if this is related, but I didn't notice this before the issue of the rough idle started.

Here are a few more ideas and possibilities to consider:

Vacuum Leaks: Even though you've checked most of the vacuum lines, it's worth checking the intake manifold gasket and other related seals. A small leak might not show up while spraying starting fluid, especially if it's intermittent.

Fuel Delivery Issues: You mentioned that fuel pressure is at 54 psi, which is generally good, but consider checking the fuel pump and filter if you haven't already. Restricted fuel flow can sometimes cause irregular idling.

Engine Grounds: Sometimes a bad ground can cause all sorts of electrical glitches. Make sure all your engine grounds are clean and securely connected.

Exhaust Restrictions: A partially clogged catalytic converter can sometimes lead to poor idle and erratic behavior. If you can, check the exhaust flow to see if it feels restricted.

IAC: Since you replaced the IAC, make sure it is calibrated correctly. Sometimes the PCM needs to relearn the idle after changes.

PCM Codes: Even if the check engine light is not on, it can be helpful to scan for codes using a diagnostic scanner to pick up any potential issues the system might be trying to flag.

Throttle Position: Sometimes the TPS can need adjustment, or there may be a miscommunication between the TPS and the PCM, so checking the voltage readings can help.

You've definitely done a thorough job so far, but hopefully, one of these suggestions points you in the right direction. Keep us updated on what you find!

 

[suicideking]

Following because one of my TJ's has a rough idle. Only thing I've tried is spraying TB cleaner in the intake. That seems to make it stop for a few months.

 

[Lemmy]

What kind of spark plugs are you running? Make and Model. I’m asking because with Iridium plugs our TJ also developed had a rough idle.

 

[Jeepoftheseus]

So the AC and heat in the jeep are vacuum actuated. If it's true that the idle genuinely improves when the heat is running, there could be a relation. If the vacuum leak were in your AC's vacuum lines, then the heat would only blow in de-frost mode so thats probably not it. We also know the heat works by blowing over a heater core in yojr dash, which is essentialy a small radiator. So another possibility here is that when your jeep's operating temperature lowers the idle evens out. Is it possible that you cracked the exhaust manifold or one of the upstream o2 sensors has gone bad? You've basically covered everything else that it could be. Maybe something dramatic like a valve cover gasket? What if it's something mundane like the whole geometry being upset by the replacement of a mount, though I doubt that's the problem. Just spit-balling here.