Unstable Idle RPM When Cold, 2005 GX470

[Corry El]

In neutral, RPM gradually falls off to 500 or less, then zooms up to around 1000. With the car in drive the variation isn't as great but the engine is 'bumping' against the transmission, very noticeably. It goes away when the engine reaches normal operating temperature. My 'trusted mechanic' has basically given me the 'can't reproduce' (meaning no engine check light or OBD2 fault code and he doesn't want to try to troubleshoot without that, fail to find the cause, then have to get me to pay for his labor). But it's definitely there, anybody else had this? 
Possibly relevant earlier problem: a couple of months ago I did get OBD2 code P1445 (secondary air injection system valve stuck closed) after engine check and VSC/off lights came on. But it went away by itself, first the warning lights disappeared but OBD2 tool still registered the fault, eventually it too showed all clear. But 'secondary air injection' is an emission system operating when engine is cold, now cold idle problem, related?

[RX400h]

Was there a history code? Often, a dedicated Lexus or Toyota mechanic would be more likely to have experience with such a code and how it relates to your model vehicle.

[Corry El]

20 hours ago, RX400h said:

Was there a history code? Often, a dedicated Lexus or Toyota mechanic would be more likely to have experience with such a code and how it relates to your model vehicle.

I don't think so, at least on my basic OBD2 tool as far as I can see. It seems to just say all OK now. My eventual course of action is to take it to independent Lexus specialist (rather than my neighborhood guy) to see what if anything they can make of the problem. I was just wondering if it was a common thing with a known likely cause. Thanks.

[RX400h]

This looks like it could cause idle issues.

 

[Ambgx470]

I have the same problem. Fees like a bump and the idle at stop varies from 1k to 500. Sounds like something turns on and turns off. Have you found out what it is? 

[Corry El]

5 hours ago, Ambgx470 said:

I have the same problem. Fees like a bump and the idle at stop varies from 1k to 500. Sounds like something turns on and turns off. Have you found out what it is? 

Tried cleaning throttle body intake and MAF myself, no change. Took it to independent Lexus specialist who also cleaned the throttle body (I didn't say I'd done it, maybe I didn't do it enough) but said a resistor in the fuel pump controller was registering low resistance and Lexus says that can cause idling problems in GX470. Doing both those things solved it he said (so probably the resistor). I'm picking it up today or tomorrow to make sure that's true and remains so.

[Corry El]

Got the car back and...that didn't fix it. Shop said they tested it cold and no idle variation but it also seems to vary with weather, hasn't 100% *always* done it with me either since it started. Today it does it again, maybe less so but again seems to vary anyway. I give up. I planned to sell the car before long anyway, this is by far the oldest car I've ever had, so I just will. I'll live with it in the meantime.