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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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A quality OBD2 reader will give you the O2 sensor voltages which you can use to determine if they are in their optimal range. They also provide the air-to-fuel ratio so you can see if the car is running rich as you suspect.


Do you not have this sensor? https://www.youtube.com/watch?v=MR3l5YwC4kA

It is called something other than a MAF, but performs a reduced yet similar function. You can clean it to be safe. :)


Since the problem is intermittent, if you see no issues with the O2 sensor or the air sensor try checking the following.

  1. You said the issue only happens after the engine warms up, check the engine's coolant temperature sensor. It is a very simple sensor with 1 or 2 wires. If it is intermittently failing, it may still read cold sometimes causing your engine to use much fuel and cause the rough idle/poor performance.

  2. Vacuum leaks, this could be any number of hoses but on an older car and it wouldn't be surprising. The first step would be to visually inspect all of the hoses for cracking/poor condition. Most hoses can be replaced with the cheap ~$1-2 by the foot hose from an auto parts store as long as you match the diameter. There are some other techniques that can be found online. Youtube finding vacuum leaks.

  3. Check the Idle Air Control Valve (IACV) as it may be dirty and occasionally sticking causing the rough idle. However this would only affect idle and it sounds like you continuously have the engine running rough. This usually can be cleaned with some brake cleaner/carburetor cleaner if it hasn't failed yet for restored performance.

  4. Check the throttle body to see if there is massive carbon build up, the throttle plate could be sticking as the engine warms up causing issues. This can be solved witch cleaning as well by removing the throttle body and any sensor attached to it and cleaning it very carefully with brake cleaner/carburetor cleaner.

  5. If nothing above points you in any kind of diagnostic direction, try pulling your spark plugs, they should be free of oil and in decent condition. https://www.ngksparkplugs.com/about-ngk/faqs/spark-plug-faqs/how-do-i-read-a-spark-plug

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