Whits end...

New member, thanks for any help! 98 runner... New O2 sensors a month ago, k/m filter, new cat ( self installation, purchased at O'Reilly's 25,000 miles ago, new alternator 5000 miles ago. Two days ago traveling up a steep grade in 100+ degree weather my runner suddenly down -shifted and made a horrible rattling noise. I let off on the gas a bit and it shifted up, and rattling disappeared. No change to performance. Following day, just as hot outside, took the family swimming in the high country. Vehicle kept wanting to downshift, loud rattling, but now accompanied by a whistle noise. Became harder and harder to keep up with the flow of traffic 55-65 mph. On the return trip down hill (20-30 miles) I could hardly maintain any sort of highway speed- even on level straightaways- and even had trouble accelerating going downhill. Vehicle starts up easily, idles just fine at about 720 rpm, and drives fine... on level ground in an area like a parking lot. As soon as I attempt to accelerate there is a very loud rattling and high pitched whistle, it instantly downshifts, and I lose almost all power. I can punch the pedal to the floor and there is no increase in power... All this happened within 60-75 miles (two days). Replaced the MAF, no overheating, no CEL. However code reader does say running rich on bank #2. Any help at all would be so very appreciated! Dead in the water!

 

Sounds like there are 2 problems. First I'd check the transmission dipstick, then I'd change the transmission fluid regardless of level and keep an eye out for large chunks of metal. If you don't know when it was done last assume it needs to be done.

The whistling and running rich sounds like an air leak. Check all the intake for rips, especially around the MAF. Rich means too much fuel, so if air is leaking by the maf it might be making it tweak out. Also check the vacuum tubes, that can cause all sorts of problems.

Another problem could be that aftermarket O2 sensor. It's been a while since I looked at this but the O2 sensor can affect fuel ratio because on some vehicles it is used by the ECU to validate if the amount of fuel it just injected was too rich or lean. I could be wrong, the 4runner is old and it might only use it to check that the cat is doing its job, I could also be mixing it up with the A/F sensor.

 

Make sure ALL your sensors are from TOYOTA or TOYOTA suppliers. No exceptions.

To, me I would suspect the CAT. Aftermarket cats are notorious for failing and can plug up and rattle. To me it sounds like you have a plugged cat. Aftermarket ones seem to last only 6mos to a year.

 

Have you ever heard, or do you understand what "ping" or "knock" or "detonation" is?
Are you familiar with what it sounds like when the valves are rattling?
Are you familiar with detonation?
Climbing a hill in 100+ degree weather would be a perfect place for knock, detonation, from crappy gas and a hot engine under load like climbing a grade in high temps. Road temps are high, under body of the car is hot, fuel line is warm, perfect place for the fuel to boil or vaporize before it gets thru the injector which will cause the fuel to burn before the piston comes to top dead center. This makes an awful rattling noise, ping, detonation, knock as its called. The knock sensors in the engine will then pick up the ping, and the ECU would pull power to try to stop the detonation.

Is your knock a rattling in the engine? An actual knock cause you have knocked the rod bearings out from detonation? A rattle from exhaust? Can you pinpoint things a little better?