Hi All, I've been having trouble with engine surge or fluctuating idle on the 05 RL. I first noticed it a few months ago, but it would only happen very sporadically like once a month and was fine otherwise. Lately, it's an issue constantly- possibly due to colder weather or maybe just passage of time?

When operating normally, the idle rests at about 680 rpms or so. When I'm having the surge issue, it rests higher at around 800 and moves to 950 rpm every few seconds or so. it doesn't really go past 1000 rpm at idle. I have to push the brake harder at stop lights because it feels like the car is trying to pull forward slightly. It happens in drive and in park. If I turn off the heat and all electronics, it still does it. I notice it the most when the car is stopped at a traffic light, but it also acts funny on the highway or at higher speeds if I let off the gas. when it's sitting at 800, there is a low rumble like I know it's about to start surging. When it's at the normal 680, it's very quiet and smooth.

So far to address the issue, I replaced the throttle body with a junk yard unit from an 08 RL so I don't think it's the IACV- I think that's built into the throttle body on our cars. I also put on a junk yard EGR valve from an 06 MDX. I did an idle re-learn procedure. The coolant is properly burped and the coolant temp sensor appears to be working fine based on my OBD reader. These didn't fix the issue. I don't think it's a vacuum leak because why would it only happen sporadically? Plus I did a visual inspection and didn't see any issues. There are no error codes.

I'm almost at the point to take it to a mechanic, but I'm concerned they won't put in the adequate time to figure it out properly. I figured I would ask the forum before taking it to a mechanic. Thanks in advance.