2002 Sequoia mystery idle issue

Sorry for the length, but this way the details are there.

Hello all, thanks for looking at this. I'm trying to troubleshoot an issue with an '02 Sequoia (268,000 miles) that I started maintaining earlier this year for a family that's friends of mine. They don't know much about its history, as the dad/husband took care of the maintenance, and he died in February. What I've been able to find out is that it hasn't ever had (or needed) any major repair, and it has “had some oxygen sensors replaced last year.”

So, here's what's going on. In February, the lights were left on for an hour or so, the battery was old, and so they had to get a jump start. After the jump start, when you'd let off the gas, the engine would usually quit, but if you coaxed it, you could sometimes get it to run a low rough idle. (Additionally, the check engine , VSC, and Trac lights stayed on. The check engine light had always been on, and I'm told the VSC and Trac may have gone one and off.) So, they took it to the Toyota dealer, who replaced the battery and “reset the computer” (they didn't know that's a matter of disconnecting the battery) and said that it should be fine. Well, as you might assume, nothing changed. So, I offered to work on it for them, and so I cleaned the throttle body (REALLY needed it), and changed the air filter, and reset computer. Problem solved, it ran like NEW, the VSC and Trac lights have been off, and its been perfect since then until July, I changed the oil (Mobil 1 extended) and changed the transmission filter (never been done), cleaned the pan, new gasket, and added new fluid (I did not flush the transmission or mess with anything in any way. For the sake of the following, I did not disconnect the battery (so no reset).

Here's where the problem starts. For the first 20 minutes or so, it ran and shifted perfectly (I was driving it this day). Then, while accelerating onto the interstate, it started “bucking” so I backed off the pedal a little and all seemed well, except the VSC and Trac light were on. Once off the interstate and in stop and go, I noticed the engine was idling rough and at low speeds when I'd let off the gas (say 25 mph, letting off to slow to stop) you could hear a sort of (somewhat slow rhythmic) clanking sound from the engine compartment and sometimes at idle, as well.

OK, so since then, I've driven it a good bit, and know the actual symptoms better (and they stay the same, not worse, not better)...
-Starting cold, it initially seems smooth. Then as rpm slows, the roughness appears.
-For a short period after a battery disconnect, that clanking sound as mentioned above will happen at idle. It turns out to be the butterfly valve clanking open-shut-open-shut). After awhile, this clears, and unless the battery is disconnected again, it doesn't do it anymore, except sometimes when decelerating you can hear it).
-Once engine is warm, it always idles rough (unless a/c compressor on, then smooths somewhat).
-Warm starts go like this... First try: engine always pops off quickly, then immediately quits. Second try: engine pops off quickly and keeps running (happens this way most of the time, occasionally first try will keep running, second try always keeps running). (Cold starts always keep running).
-VSC and Trac lights stay on, always (as well as check engine, but I don't think it is related, as it has been for years).
-When slowly giving gas from idle (like pulling out of stop sign), it's slightly rough at first, but once it has maybe 1200 or so rpm, it smooths out.
-ANY TIME the engine hits 3,000 rpm when accelerating, it bucks fairly heavily. Letting off gas slightly (to under 3k) smooths it out.
-I'm not sure whether to add that I think the exhaust smells a little rich when idles, as I'm not sure how it smelled before this, so I can't really compare it.

I did go by the auto parts store and have them tell me the codes it's giving. Basically, it says all of the oxygen sensors and the mass airflow sensor, but they said those codes had been there for a long time. I don't think it's the mass airflow sensor, as I ran it briefly with it disconnected and there was no change.

Anyway, sorry for the length, but I wanted to be thorough. I really want to help them, but I'm not sure what the problem is. I feel it's associated with idle air mixture, but I don't know what. I'm not sure how that explains the bucking at 3,000, but I wonder if that's something to do with the traction control being confused (because it feels sort of like a rev limiter). So, any thoughts would be REALLY appreciated. It's great to join, and hopefully I can help some people out, too. Thanks!

 

It sounds sort of like a Mass Air Flow Sensor to me. We had a Buick (in the 90s) that ran pretty much that way and a new MAF sensor cured it.....