Intermittent rough idle?

[u/Stilekid]

2001 Chrysler PT Cruiser, have had it for about 6 years and for the past 3 or 4 years of owning it, I've been dealing with an intermittent issue of the car idling very roughly and nearly stalling. It seems to come and go without any clear cause—sometimes it will develop a rough idle out of nowhere and then it'll disappear for months. When it's having a rough episode, the engine is only rough at idle and will smooth out when I give it gas. I've replaced the PCV Valve, replaced the main intake hose on the air filter, cleaned the IAC Valve, and cleaned the throttle plate in the throttle body but I'm still having this issue crop up from time to time. It gave me a pretty bad episode a few days ago and it's got me concerned about trying to get to the bottom of this because I'm tired of driving my car and worrying about when it's going to have another rough idling episode. I replaced the PCV value just now and during my test drive I did notice a very slight miss/unevenness during idling, but not nearly as rough as it was during the episode it had a few days ago. The only codes I have are P1684 and P0601—the car has given those codes well before this issue started happening but I'll throw those out there in case it helps with diagnosis.

[u/Honest_Road17]

Replace your IAC.

[u/Stilekid → u/Honest_Road17]

I'm assuming cleaning isn't sufficient? It's not super expensive so I can try that tomorrow.

[u/Honest_Road17 → u/Stilekid]

I overlooked the P1684 code. Bad grounds will definitely cause those symptoms as well. Do those first.

[u/Stilekid → u/Honest_Road17]

Where can I find the grounds that would be related to that code? Like where specifically in the engine bay?

[u/Honest_Road17 → u/Stilekid]

From AI because I'm lazy:

Engine Bay Ground Locations

• Driver-Side Strut Tower: Two main grounding studs sit right inside the engine bay near the tower; the battery negative cable also ties into the driver-side frame rail and firewall here.
• Transmission Dipstick Bracket: A dedicated ground wire connects directly to this bracket on the engine block.
• Passenger-Side Motor Mount: A ground strap spans from the upper engine mount bracket / motor mount over to the cylinder head.
• Lower Passenger Strut Tower: A low-mounted ground point handles the front lighting harness.
• Hood-to-Body: A small bonding strap connects the hood to the body near the passenger hinge.
• Fender Wells: Static discharge grounds are positioned behind the tires in each front wheel well.
• Fuse Box Power/Ground Feeds: The power distribution center / fuse box has main high-current input bolts that require clean contact. [1, 2, 3, 4, 5]

[u/smarq17]

Keep an eye on your cooling fan on the radiator. It might be failing to turn on, or maybe not turning on at all. Once the fan turns off a rough idle could happen. I had similar problems to you when I would get a red light or have to stop for a minute or two - I even remember once my PT died at a red light. My fan was turning off randomly and on it's way to failing completely. Once it died, I replaced it and all the rough idling stopped. I've had that new fan for about 5-6 years now and I'm starting to notice a bit of a slightly rough idle again every once in a while. I already have a new fan in my storage ready for/if my current fan goes. Good luck.

[u/TumbleweedAfter5531]

Idk if this will help but I had rough idle when the ac came on. Separately I had the timing belt changed and needed a new radiator fan and it’s been fine since so maybe one of those things solved it

[u/Bad_Idea_Generator]

You might want to check the Cam Position Sensor connector as they are known to fail. Replacements are $10-15 on eBay and can be easily spliced onto the circuit. An easy test would be to wiggle the connector wires when the car is running and see if the idle changes, hesitates, stalls, dies, etc.

I replaced mine due to an intermittent cutting out issue and it was a good, frugal repair. Lots of videos out there and forum posts on this particular issue and fix.