Help diagnose my possible catalytic converter issue

[GTOchris]

I'm the 3rd owner, I've had my 2005 for 2.5 years and it just hit 90K.

I've felt the response/ performance has dropped off in the last day, I think I lost 1-2MPG over the past few tanks. Since buying it I've always had a low RPM rattle underneath and I've tried to trace it to a heat shield.
Last night we got the rotten egg smell + sulfur, I noticed even the exhaust note/ sound isn't the same (very muffled).

I found a receipt that the catalytic converters were changed under warranty around 30K for the original owner. (why would it have a problem only 60K later???)

I have no service engine light (yet).

Yesterday (before the smell), I changed the oil and always use full synthetic (could this be a problem?), I washed the under body from sand (was on a beach recently) could my hose action have damaged anything?

Where do I start?

 

[earthrealm]

WITHOUT CODES, IT WOULD BE DIFFICULT TO DIAGNOse, you could hook it up to a scanner and see if it has pending codes,

u might have bad gas in tank, poor plugs etc

 

[Ralph Blake]

Using synthetic oil could not cause this problem,also washing the underside won't cause it. Have you tried running your 4Runner out on the highway at high speed, to really heat the cat up? That smell comes from unburnt junk inside the cat.

 

[verman84]

I was getting noticeable performance loss and changing my spark plugs made all the difference in the world. Acceleration and response felt much better.

 

[GTOchris]

Thanks all,
I drove it quite a bit around town today and no smell... I cleaned the MAF sensor and put in a bottle of Sea Foam in the gas.
I changed the plugs about 10K ago.

I ran everything by my normal mechanic who I trust for the jobs over my head, he mentioned the changeover from summer- winter blend gas could be to blame.

Next tank I get I'll do #91 and see if that helps things out. The response seems to have returned, maybe it was just tired last night