MtnRDX

I've searched and really can't find anything.

I don't primarily drive this RDX but I have noticed a few times driving it that the idle isn't consistent. I noticed during hot weather that sometimes it would be a little rough when the AC kicked on while it was idling in park, more so than when idling in drive at a stoplight. Now that it has cooled off, I've noticed it idles a little rougher in park and when it sits long enough to cycle the cooling fan it stumbles when it kicks on and then recovers.

Nothing really worrisome until tonight. Tonight I went to stop by a curb to wait to pick someone up. Radio on, heat running. I realized in coming to a stop that I rolled to a spot that there was radio interference. It was a slight uphill so I just shifted to neutral to roll back a little from the bad spot. Man it really stumbled when I shifted from drive to neutral. Like for a second there I thought it was going to die on me. I fiddled with it again and it repeated. I put it back in drive. Then thought well not out here yet so I put it in park and it stumbled pretty hard again. So I just put it back in drive. I picked up the person, then drove home and tried to reproduce in the driveway and couldn't.

Parts I have replaced in recent years... air intake boot, alternator, transmission position sensor, battery and starter... in case maybe I might have been around a vacuum line or something that might be a culprit.

I find it strange, because typically a basic problem like with ignition parts or vacuum it will run consistently bad, and sometimes even the reverse is true as in it gets worse the farther you drive rather than bad after 3 miles of driving, then better after 3 more miles, or reverse is true in that it may idle OK in park or neutral but pull down once shifted into drive or reverse. I'm kind of at a loss for a starting point. Surprisingly there is no CEL either even as badly as it idled. I think it's the worst I've ever had a car run and not pop a light, or stall out. And then it was fine. So weird.

MtnRDX

Forgot I also replaced the AC clutch but don't even think anything is near that.

Tomtwtwtw

Have you looked at the idle air control valve? I believe it's at or near the throttle body, and the symptoms you're describing could definitely be caused by a dirty or malfunctioning IAC valve.

yanga001

What are your fuel trims. Are you running lean or rich?

I dont want to sound smooth brained but I did not recall the RDX having an IAC (idle air control valve).

Could be a bit of anything in the engine department. What is the output voltage of the battery, when was the air filter last changed, have you ever cleaned the MAF sensor?

Almost sounds electrical, like your battery was leaning on a connection, and when the rpm's dropped in neutral and the alternator could not compensate.

Tomtwtwtw

You very well might be right. That might be a more common thing before every car got drive by wire. But perhaps the throttle body itself is dirty, or any of the items you mentioned as well. It sounds like a mild restriction of either air or fuel. Maybe fuel pump on its way out? Though I thought more often than not they just crapped out vs withering away.

MtnRDX

Well there has not been another instance of the worrisome hard stumble almost dying during shifting from drive to park or neutral since.

The rougher idle in park than drive persists and so does the mini stumble when the cooling fan kicks on, and when shifted into park.

The battery is new and good. The alternator was replaced not that long ago and it also checks out good. The only thing more I'd like to do there is find my darn analog meter with the needle gauge and watch the voltage with it when the fan kicks on, and when someone shifts it... as those react faster than digital. I can't see any drop with the digital.

I have also replaced the throttle body boot not all that long ago. It had a crack.

I'll pull the intercooler and pull the spark plugs to see how they look. I don't have the fancy scan tool to look at the fuel trims, but I am very good at reading spark plugs. I used to do some off road racing and learned to tune that way with carburators and EFI before hauling stuff to the dyno tuner became the norm. While I am at it I will go over the throttle body, air filter, sensors, and try my best to trace the vacuum stuff around the engine bay.

It's not been that long since the air filter was serviced. But never know what my wife may have done. For all I know she could have parked it over a dry dusty spot again and left it idling and plugged the filter up good. She did that once before a few years back when my daughter was attending a different school. She would go park in this one spot every day waiting for her to get out of practice. They redid the curb island, took out old trees and roots, planted new trees, and the new grass got sun baked before it ever got started. It was full of powder clay dust and powder sawdust.

I was like how did this happen??? Until the next week when games started. She had just kept parking there idling with the AC on every day for weeks.

It may just be a hair slow on idle causing it. Maybe gunky throttle body. But I really was thinking something electrical. I kind of expected to see voltage fall on it's face for a couple seconds even with a digital meter. But did not see it. Hopefully I can find my old analog and just rule that on out.

Tomtwtwtw

For fuel trims, nothing fancy needed. You can get a generic bluetooth reader off Amazon for 15 bucks or so, and use the free Torque app (Android only last I checked) to read all kinds of sensors and codes, including short term and long term fuel trims.

John from PA

What year?

MtnRDX

2007

John from PA

The 2007 uses an electronic control throttle body and I think one function of the device is to compensate for varying loads on the engine at idle, like when the AC compressor kicks in. A P/N might be 16400-RWC-A02, check your specific vehicle. Unfortunately it is expensive and sometimes difficult to replace. What might be worth a try is a can of Techron gas additive. Some people swear by it but in my personal experience I’ve never had a need.