I have a 2004 Santa Fe with the 2.4 litre engine with auto transmission. Recently I replaced the timing belt and shortly thereafter the engine began to "float" around 1800-2200 RPM. By this I mean that if I am cruising along and the RPMs are in this range the engine starts to drop RPMs if I'm keeping the pedal steady or using cruise control. As soon as I press the accelerator a bit the engine "wakes up" and the RPMs jump back up, often with a lurch from the automatic transmission. The engine purrs like a kitten and has no other running or power issues.

I disconnected the Throttle Position Sensor the other day and this alleviated the problem, though it doesn't shift properly at all. It will not downshift under hard acceleration unless I have it in manual shift mode and I downshift. I am wondering if this is a positive indication of a faulty TPS or if disconnecting it just alleviated some other issue.

The Santa Fe has 175,000 miles on it and otherwise runs great, we get 26-27 MPG on the highway and the car is otherwise reliable. I don't think the timing belt replacement had anything to do with the issue as it's obviously timed correctly given how smoothly it runs, but I thought I'd mention that as it's the last work done to the engine prior to the issue.

 

Brob,

Did you ever get a reply to your issue. My 2004 Sonata (2.4L auto) started doing the exact same thing this morning. Got me to work, but not sure if it will get me home. The check engine light flashed on twice during the half-dozen or so rpm drops, so I may be able to grab a code on it when (if) I get home. Just curious if you ever figured it out.

 

The OP's issues were probably caused by taking the battery off
The ECU reset and the transmission had to relearn all the settings again

Yours is a real issue as evidenced by the Check Engine Light.
Scan and see what the Diagnostic Trouble Codes are saying to look into

 

After driving the car home yesterday afternoon and having it shut off and jump back to life on me twice (it felt like I turn off the ignition in both cases and immediately turned it back on), I checked the codes when I got home. The only one showing was a 'pending' P0446 code. Since the car ran fine except for those two occasions (no hesitations, skips, lurches, or loss of power), and the check engine light was not on, I did some more research and finally decided to change the Ignition Failure Sensor, which had caused similar issues in other 2.4L Sonatas. Car started and ran fine last night, but on the way to work this morning, died about 5 min away from home. Shut off, and would not restart (like it wasn't getting fuel). Let it sit for 2 minutes and it restarted, so I drove it back home and swapped it out for my Silverado. There it sits while I'm here at work, with no real clue as to what's happening. Anybody have any suggestions?