Originally Posted by TimGFL

Well, maybe I spoke too soon. Ran fine for about a day, then started with the same symptoms, same code: P0505 (the only code). So I took the IAC back off, cleaned it to the point it looks brand new inside. Verified that it opens and closes by applying 12v to each side (B+ in the middle), put it back on.

I changed the plug wires with NGK (cylinder #6 wire was bad) and put on a new air intake hose because they were both needed anyway. I checked the plugs, and they were all pretty clean (NGK two-prong type, as spec for this motor). No evidence of arcing on the coils or boots. It does run smoother, but if I reset the code it still pops up P0505 after about 10 sec.

It throws the same code, even if I reset the code and unplug the IAC. If I understand correctly, it shouldn't throw that code with the IAC unplugged (if it's the IAC at fault).

I did the carb cleaner spray test for vacuum leaks, ALL the lines, all over the intake manifold, and couldn't find any that affected the RPMs at all. PCV valve jiggles freely. Throttle body is solid, no wear or looseness, opens and closes easy and smooth, and no air gaps around the throttle plate.

What's strange is two days ago, I stopped and put $10 worth of gas in it, and for about 2 miles to my house, it ran fine! Then, the next morning, the same problem. I just tried again today, put $10 in the tank, no change... same problem.

Here's the symptoms:

Cold start, idles low ~750RPM, and creeps up to ~850RPM as its warming up, then settles in at operating temp, idling normal ~650RPM. Before I cleaned the IAC it was idling ~450RPM at operating temp.

In park or neutral, gently revs up to ~1300RPM fine, but then starts to stumble. If I give it a bit more throttle (not wide open, but not casual either), it seems to even out. It's almost as if it's misfiring, but only after a certain point opening the throttle, and P0505 is the only code I'm getting.

In drive, it seems like the hesitation and stumbling is only occurring when I give it gentle acceleration, no matter the speed. Is it possible the throttle position sensor TPS is at fault, even though I'm not getting a TPS code?

I've got a Wix fuel filter, plan to swap it out in the morning. Just thought I'd post again in case anyone has any more ideas, and hopefully we'll solve it and it may help someone else in the future. This is a weird one, and I haven't been able to find anything exactly like this in all my forum hunting so far.

It's due for a timing belt, but could a jumped timing belt cause low idle and P0505? It seems to rev flawlessly up until about 1300RPM. The fact this happened all of a sudden makes me think it's something electrical, but... I'm stumped at this point, and I don't want to throw too much money at it with 280K miles.

Thanks for any help you could offer!