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My '98 Camry (with the four-cylinder engine) usually starts, idles and drives like a dream. About a year ago I had the entire exhaust system replaced (including cat) due to corrosion. Since then it's had an intermittent problem.

Most of the time, it's fine - but every once in a while (sometimes a few times a day, sometimes several weeks between problems) it all of a sudden runs really rough - it's very noticeable at highway speeds. When it's doing this if I stop at a light it idles at a very low speed, and comes close to dying - in fact, sometimes it does die. There's a definite sulfurous "rotten egg" odor from the tailpipe when this happens. It sometimes just stops having this problem, but it usually continues until I turn the engine off. When I turn it back on a few hours later, it's always OK again. This only happens when the engine is warm.

This seems to be a "too much fuel" situation (the odor tells me this), but I'm not 100% on that. I've run some Techron through my car since this has been happening, but it still happens. My check engine light is not on. I'd start by cleaning the mass airflow sensor except my car doesn't have one (no, really!).

Any ideas?

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Given that this started when you replaced the exhaust, the likely culprit here seems to be that one of the o2 sensors starts to fail when it gets hot and your engine starts to use too much fuel. My second guess was going to be your MAS, but as you say, you don't have one.

Have a car shop scan for and OBD II code and see if there is a code being thrown, the check engine light doesn't always come on even if there is a code to be read. See if you can check for the code when you witness the rough idle, when the engine is warm.

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