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Drew, Super Mazda Tech.
Category: Mazda
Satisfied Customers: 2518
Experience:  19 yrs. experience. Mazda Senior Certified Tech, with Ford, Linc. & Merc. experience. ASE L1 Advanced Certified Master Tech.
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Mazda 6 I: Hi. I have a 2004 Mazda 6 2.3 L 4 cylinder. The

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Hi. I have a 2004 Mazda 6 2.3 L 4 cylinder. The car overheated a month ago, and when I pulled codes, I got a P2187 and a P0303. My symptoms after that point were: rough idle (RPM fluctuate between 500 and 1000), misfire, rapid heating, slow acceleration, and a shakey steering wheel. Paying attention to the engine misfire code, I replaced the spark plugs, spark plug wires, coil pack, air filter, and I cleaned the MAF. After I replaced the coil pack, the RPMs didn't fluctuate like they had when it's parked, and it ran really well for about 4 miles. I was about to test it on the freeway when it started to overheat. It also started to run very poorly (poor acceleration, coughed and sputtered at stoplights, knocking sounds). I came home promptly. Since then, I've done some research on the internet. It seems the most likely problems are either an internal leak, bad thermostat, or bad o2 sensor. I thought it could be a vacuum leak, but it seems that a leak wouldn't cause the overheating. Before I go purchase a compression tester and replacement o2 sensors, I wondered if you might be able to narrow it down.
Welcome to Just Answer. My name is XXXXX XXXXX I will be glad to assist you.

Sounds like you have a couple different concerns here.

For your overheating concern, you need to make sure the radiator cooling fan is coming on when the engine temperature rises above normal.

If the fan does work, and the cooling system is full, then I suspect you have a thermostat that is stuck closed. You can verify this by checking the temperature difference between the upper and lower radiator hose. If the lower hose is significantly cooler then the top radiator hose, then the thermostat is stuck closed. I suggest only replacing with a Mazda factory thermostat. No aftermarket parts.

For your P2187 concern, I suspect that you have a vacuum leak and that contributes to the idle concern. The most common area is a EVAP purge valve that is stuck open or a PCV hose that is rotten off under the intake manifold. You may hear a hissing noise when started cold. That is the sound of a vacuum leak. You can verify this by spraying carb cleaner or brake cleaner under the intake manifold in the area of the PCV valve. If you hear a change in idle when the vacuum leak sucks in the additional fuel supply, then you found the leak.

To test the EVAP purge valve, you can pinch off the hose between the purge valve and the engine or just disconnect it from th e vacuum hose and cap it off. If the engine runs normal, the purge valve is stuck open.

For your P0303 misfire concern. I want you to clear the codes and test drive after resolving the other concerns to see if you still have a misfire.

I also suggest using some throttle body cleaner to clean the throttle plate and throttle body bore to remove all the black carbon/coke build-up.

Drew and 7 other Mazda Specialists are ready to help you
Customer: replied 4 years ago.

Great suggestions. I'll give those a try. Very practical and well-explained. Thank you.

Thanks for the rating.

You can reply back here at any time and I will be glad to help with any additional questions.