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P0303 code troubleshooting help

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I do have an issue with my 2001 Ford Focus LX,(140K miles) just bought it from a dealer... it was driving fine at first, with no problems, now 3 weeks later - it is driving rough, poor acceleration, shaky motor... etc.

Here's what I did:

I scanned the code and it is a P0303. I looked at the spark plug, it was fouled, carbon, no oil.

I replaced all spark plugs with OEM, also new spark plug wires, and a new coil pack.

I also replaced the fuel filter, it was very dirty. Also a new air filter... drove it and as long as it is not under load, acceleration, it is okay, but still has the hesitation and will eventually throw the same code.

I also did a compression test, they are 180+ on all 4.

I looked at the new plugs and # 3 is black and fouled, others look okay.
There is spark on all plugs.

I am thinking it may be a bad fuel injector on # 3.. other trouble shooting areas, like a vac leak, EGR valve seems they would affect all cylinders.

I have heard of bad valve seats on these cars but can't confirm.

Appreciate any help! THANK YOU![dunno]
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· Premium Member
50,838 Posts
Here's a link to the "which engine" sticky, sounds like you have an "SPI".

With what you've done, including the compression test, the valve seat issue isn't likely in your case.

You've covered the bases on ign. faults, while injectors aren't a common issue it looks like it's time to either swap injector positions to confirm an injector problem or just try one in the hope it's a good guess.

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