2002 Toyota Corolla diagnosis of PO300/PO302

DAVE1310

MEMBER
2002 TOYOTA COROLLA

Hi-I am still trying to diagnose the above mentioned codes (the frustrating PO 300) random mis-fire. The PO302 narrowed this down I thought, but no matter what I do I still come up with the above mentioned codes, and I will list these latter. I think possibly I have narrowed down the diagnosis to a likely vacuum leak, since the CEL does not comes on not at idle but when the car is being driven. The # 2 coil has been replaced with an import direct new coil, MAF cleaned and working well, had the ignition switch check for correct operability, changed all 4 plugs w/NGK Iridium IX, valve does not seem to be bent since car runs and idles smooth after about 2 minutes, before that the car idles very rough in reverse (cold start), and some what in drive. The thermostat has been change as well as the coolant temp. Sensor. Any suggestions, or does this diagnosis have high possibilities of being right on?

3 Replies

ED-WEEZY21

MECHANIC
248 POSTS

I would perform a leak down test and a compression test to make sure the piston rings are still good. I've also had this problem before and my culprit was a valve that had carbon built up on its mating surface causing it to not seal properly. My leak down test failed due to the improper seating of my exhaust valve. Best way to check if its the exhaust valve is the check the exhaust tailpipe to see if anything is being blown out of it while performing the leak down test. I don't know the exact specifications for your vehicle but I like to think anything more than 5% is bad. The valve itself was good but needed a good cleaning. This happens a lot especially when using cheap fuel. If its an intake valve, you may be able to perform a fuel induction cleaning and try to clean the carbon. If its the exhaust valve, the head is going to have to be taken apart and cleaned.

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Saturday, June 20th, 2015 AT 10:44 AM

DAVE1310

MEMBER
63 POSTS

My fault-I did a compression test dry and wet and it came out within the specs-I also have really nothing coming out of my tail pipe,

thanks for the quik and detailed response

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Saturday, June 20th, 2015 AT 11:29 AM

ED-WEEZY21

MECHANIC
248 POSTS

My compression test also came out good but my leak down test did not. If you have a boroscope, try looking at the valves inside the head and make sure they're isn't any carbon deposits on them. Typically a vacuum leak would cause a "Too Lean" code and would affect all of the cylinders not just one. If you have a scan tool that reads live data, you can monitor the fuel trims and make sure they aren't too lean.

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