Rough idle at stop lights unless I turn on my HVAC (non-A/C TJ)

[Ejforan]

Hello!

I have a bit of an interesting issue that I don't really know where to start. I have been noticing a bit of a rough idle when I am stopped at stop lights while still in drive. Nothing crazy but the rpm sits at slightly below 750rpm and has drops to a little above 700 every couple seconds then bounces right back up. It feels almost like a mini misfire but I have no codes.

The interesting part is that this goes away completely if I turn my hvac system at any speed or temperature EXCEPT it still idles rough in the foot vents only position. I have heard rough idle when the AC is turned on but this is the opposite and I don't have AC.

My gut says vacuum leak but this just isn't the behavior I am familiar with for a vacuum leak. The rough idle sure but not the rough idle going away when a vacuum actuator is actuated.

Has anyone had this issue or know more about the hvac vacuum controls?

I haven't had a chance to clean the IAC or investigate for any leaks yet and since it is my daily driver I can't pull it apart much during the week.

'98 Sport ~155k miles

 

[macleanflood]

Confirming what you're thinking...clean the IAC and check for vacuum leaks. Maybe plug off the HVAC line to see if it's leaking inside the cab.

-Mac

 

[Chris]

Yep, gotta be either IAC or vacuum line related. A smoke test could help in this situation!

 

[Ejforan]

Ok, thank you both! Plugging the HVAC line is a good idea.

I don't know enough about vacuum lines but know all the concepts from college. Could there be a vacuum leak but when I add a bit more load, that load cancels out the leak? Intuitively I would expect the exact opposite but that is what it almost feels like is happening

 

[B1Toad]

There could be a leak at the supply on the back of the selector switch when the switch is off or on the footwells and goes away when selecting any other modes. Plugging the line before it reaches the switch as mac suggested is the fastest way to rule that in or out. Note that the selector switch is designed to leak while switching between settings so when you go from one setting to another the old setting will release the vacuum motor immediately. When the switch is off the supply should be effectively capped by the switch but could have failed on yours. If capping the line solves the rough idle it might be time for a new dash control.

edit: I just saw you have a 98, the info I posted is for 99-up but I suspect the vacuum works the same way. I recall that the dash controls for 97/98 are hard to come by.

 

[macleanflood]

I happen to have a spare 97/98 non AC HVAC panel. I have not vacuum tested it.

I have AC on my 97. Been hoarding this part JIC mine dies and I have no other options.

Found it at a junkyard.

-Mac

 

[Cedarhouse]

Confirming what you're thinking...clean the IAC and check for vacuum leaks. Maybe plug off the HVAC line to see if it's leaking inside the cab.

-Mac

I have the same problem with my Daughter's TJ. Start it up, runs great, put it into gear, runs great. Come to a stop sign, shakes & stops. I wanted to buy an IAC valce in Amazon, but everyone says, "it is not the real deal" and ist makes things worse. Amazon claims it is MOPAR. Where can I find the REAL DEAL??

 

[macleanflood]

I have the same problem with my Daughter's TJ. Start it up, runs great, put it into gear, runs great. Come to a stop sign, shakes & stops. I wanted to buy an IAC valce in Amazon, but everyone says, "it is not the real deal" and ist makes things worse. Amazon claims it is MOPAR. Where can I find the REAL DEAL??

Have you cleaned the IAC orifice?

-Mac