The vehicle in question is a 2000 4runner v6 automatic with about 64k miles. Except for changing the coolant, the service is (now) up to date with the owners manual.

Some time towards the end of last year I began to notice my 4runner pinging/knocking from time to time. Over the winter it got worse, but it was cold and out and I was busy so I just ran higher octane gas until spring.

This spring I replaced the spark plugs and air filter. Little change in the pinging, but it did run much better. Then on a road trip two weeks ago, the check engine light came on. Since I was out of town I pulled the ECU fuses to see if it would reset the light, but it didn't. When I got home I pulled the codes and got P0171 (lean bank #1). I erased the codes and I cleaned the MAF with soap and water as described here. It was like buying a new truck - it had not run that good for a long time.

There was no pinging the first few days, but not the pinging is back. Particlarly after a warm start under moderate acceleration in second gear (automatic).

Any thoughts on where to look next?

A note on gasoline- I've run regular (87 octane) gas in the truck since it was new without trouble. It does appear that I can avoid the pinging now by using 91+ octane gas, but since it ran great on regular gas for 60k miles, I would rather fix what's wrong and have my same old truck back instead of adding 20% to my gas bill.

 

I would pay close attention to the error code. Sounds like you are running lean in one or more of your cylinders. It wouldnt hurt to check the operation of your injectors. You can test them by following the FSM procedure, or have them tested at a reputable shop.

You may also want to focus on the ignition side of things. Coil packs ok? Plug wires ok? Plugs?

You can pull the plugs and note which cylinder they came from. By looking at the condition of the electrode, you can tell which cyl is running lean. Post pics of the electrodes if you are not sure.

 

Here is an update on my pinging problem:

-The day after I started this thread, the weather turned cold and the pinging mostly disappeared. Since then I've noticed increased pinging on hot days and little or no pinging on cooler days. It's clear that the computer is "causing" the pinging because it's capable of running fine after an ECU reset and it's learning to ping after a few days. So where is the bad info coming from?

-The old spark plugs all looked vey consistent when I removed them. I'm still amazed at how good the plugs look after putting in 30k miles

-Can you give me more information about the "FSM" procedure for checking fuel injectors?

 

For a while I got too busy to chase this problem and started using 93 octane to avoid the ping, but on my last long highway trip I cleared code P0171 six times, so when I got home I decided to get serious about a fix.

Now I'm most of the way through a tank of regular gas without any pinging, so it looks like I may have solved the problem. Turned out that my first MAF cleaning was not nearly good enough. This time I got a spary can of electrical parts cleaner and made sure that the inside parts of the MAF was just as clean as the more easily reached outside parts. I put it back together and it's been running great. Of course, I'm holding my breath until my next long highway trip to see if I get another code P0171 and a couple of tanks of regular to see if the pinging returns.