I just took my 1999 ES300 (240,000miles) to the shop to help diagnose a misfire. My check engine and trac lights have been on for 5 years for the cat. There was no code for the misfire, however, the mechanics used their computer to determine the misfire was cyl. #6. I replaced the coil. No change. I swapped the plug (which is 10,000miles old) with a spare. No change.

The misfire only occurs when under load such as running the a/c and goosing it to avoid an accident. I have replaced nearly all the coils. New fuel filter. The VVT screens were clean. I cleaned the MAF. I am about to just pull the throttle body for a cleaning. Could it be the fuel injector? I ran some techron fuel treatment through it. gah!

 

Well I think the problem lies with the original problem... The catalytic converter, it's probably plugged and not flowing properly. The fact the code is still there and for 5 years, I would fix that first. Dealer will want an arm and a leg for new ones, go junk yard or aftermarket. Good luck.

 

Replace your Oil Control Valves [~100$ aftermarket, and about 15mins to change]. That was the cause of my misfire - NOT the coils, and it was sputtering; BUT the CEL was intermittent in my case.

My VVT screens were clean as well...

 

As for the cat, you can clear the code and just install a spark plug anti fouler drilled out to 1/2" on the down stream O2 sensor.

 

I don't think it would be the oil control valve because it should be throwing 1, 3, 5 or 2, 4, 6, and the random/multiple cylinder miss fire. If it was.

 

It could be the fuel injector for cylinder # 6. I think when the fuel injector causes the misfire, it will cause the CEL light to blink on when the misfire is happening but it will then turn off after the misfire. This was the case with my Camry. But I think the code is stored so you can find it with a scanner.

 

What about a weak cam position sensor. Not all tone wheels are perfectly straight, and a weak cam position sensor will find the low spot for sure.

 

Come to think about it, it takes longer to open the hood than it takes to replace one, if you can find someone in your area to borrow one from you could test drive and see.