New member here...picked up a 98 4Runner several months ago as a replacement for our old beach pig. 206,000 miles on it. Ran excellent for it's intended purpose...ferry the kids and surfboards to the beach and suffer whatever abuse that might comes it's way.
Couple months ago started getting a misfire (randoms)...misfires only occurred after vehicle was at full operating temp...roughly 5 to 6 miles of driving. Changed out temp sensor with no joy and then later found post cat o2 sensor was not bolted into exhaust pipe. Secured O2 and car ran like a dream...for a month or so. Recently started doing the exact same thing again.
Very, very consistent pattern...misfires come on very suddenly after about 6 miles. Will at times correct it self when warm (rare) but generally have to wait till engine cools off. Also running rich when misfires come on....smell is evident. CEL comes on every now and then...random misfires or cylinder specific (varies)
Web search has indicated a collection of possible issues and getting ready to start going after them. Been turning my own wrenches for 40 years but have never come across an issue like this that was so consistently (and suddenly) centered around engine reaching complete warm up (if I put a stop watch to it, it would probably hit near the exact same time each time).
Haven't yet started going at it but wondering if this issue rings a specific bell other than the more general and common misfire problems. Thanks for any possible insight (from an old FJ40 owner...bought the thing in 1974 and sold it last year).

When the CEL comes on random or specific cylinder, is it always the same cylinder? And when it's missing is the CEL steady or does it flash?

I know you said you were going to dig in, but it would help if you told us what you HAVE done. In all my years of working on cars I found people would ask me for help and I'd say "Did you check the secondary wires?" They'd say yes, then I'd look and see cracked insulation all over the place. I'm not saying you are doing that, just help us narrow down what's happening. This maybe much simpler than you think.

Dumb question, maybe, but what plugs are you running?

4Runners like NGK or DENSO, DUAL GROUND plugs.

Any other plug and you'll have problems.

Just a suggestion.

CEL comes only when the misfires are most pronounced....so far only a steady CEL...have yet to see a flashing. No definitive cylinders...one day could be number 3...the next day could 5 and 2.

Digging into the thing tomorrow. As of now the only item replaced was the temp sensor (the misfires after warm up sort of pointed a finger at that...plus it's cheap and easy) and then securing the loose post cat O2 (which caused an instant correction of the issue...for a month anyways). Plan on doing the wires and plugs tomorrow. If possible without removing intake, will also check ohm's on the injectors. Basically with the information I've gathered via web search, culprits could be plugs, wires, O2, injectors, etc. The nature of the misfires though is odd to me...only after reaching operating temp...and coupled with running rich at that point. I'm no stranger to fixing or modifying cars but have never come across something like this which is so very much 'operating temp' specific...wish the damn thing would throw a code that points at something a bit less generic than a simple misfire. Always leery of just throwing a collection of new parts at a car with hopes that one of the items corrects the issue.

Thanks for the heads up...will go with those tomorrow.

Sounds like you've not got the right plugs in there.

That would point to the random misfires.

The O2 sensor is interesting as well. Could be the O2 sensor going out. But that would normally throw a code for the sensor.

I would go with plugs and wires first. See if that fixes it.

I understand the frustration. Once had a vehicle that was locking up the rear brakes after driving 10 miles or so, would never do it on short drives. Turned out the rubber hose from the frame to the rear diff would swell up and close off, not allowing fluid to return. So yeah, weird shit happens.

It would be awesome to change one part at a time and see what finally fixes it. But that's a pain, and I'm sure you just want it fixed. Personally I'd concentrate on verifying(duplicating) the failure, in order to better diagnosis it. Keep us posted, these things are captivating.

I'm very well versed in vehicle frustration....wifey-poo has an Audi.

Plugs and wires seems to have cured it. 15 mile loop with no misfires. Did have the correct plugs and they all showed normal burn patterns...the wires though were toast...two came apart while removing leaving the metal connectors stuck down on top of the plugs. Thanks for the help guys!!

I sometimes forget, but a wise man told me something over 30 yrs. ago that has served me well.


"Check the simple/easy things first"

Congrats on solving your problem, and sorry about the Audi.