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P0304 and P0430 Misfire and Catalytic Converter Issue on 1996 Ford Taurus

Model: 1996 Taurus Fault Code: P0304 Posted: 2007-02-08 18:44

I'm experiencing an intermittent misfire in cylinder #4, followed by a Bank 2 catalytic converter inefficiency (code P0430). The check engine light blinks initially, then stays on as the engine runs roughly for about two minutes before returning to normal. The vehicle starts and runs smoothly otherwise, but the misfire occurs intermittently. I've already replaced the downstream Bank 2 oxygen sensor, cylinder #4 spark plug and spark plug wire. I also swapped the injectors between cylinders #4 and #5, replaced the fuel filter, PCV valve, and vacuum hoses, installed a new fuel pump one year ago, cleaned the MAF sensor, and ran fuel injector cleaner through the tank. I even switched the spark plug wires from cylinder #3 to #4 to test if the misfire was affecting another cylinder. All checks for vacuum leaks have been negative. The vehicle passed smog testing with excellent results after clearing the codes. The sequence of events—misfire first, then the catalytic converter code—suggests that the misfire is triggering the cat inefficiency. I'm now looking for further diagnostic steps to identify the root cause of the cylinder #4 misfire. What additional troubleshooting or testing should I perform to pinpoint this issue?

Related fault codes
P0304
Comments (9)
Anonymous 2007-02-09 08:26

Perform a compression test on all cylinders to assess engine health and rule out mechanical issues.

Anonymous 2007-02-13 11:46

A compression test shows cylinder #4 at 90 psi, cylinder #5 at 100 psi, and cylinder #6 at 110 psi. About five to six years ago, both cylinder heads cracked and were replaced—each time during separate incidents when my wife was over 3 hours away in opposite directions. The engine has 192,000 miles on it, so I suspect ring wear is occurring. It passed smog testing in December 2006, and the technician noted it was a very clean-burning engine.

Anonymous 2007-02-13 14:03

Try adding oil directly into the spark plug hole of cylinder #4 and retest the compression—this can help determine if ring wear or internal sealing issues are present.

Anonymous 2007-02-13 14:10

Yes, after adding oil to the hole, the compression increased to 100–105 psi.

Anonymous 2007-02-13 15:56

Try pouring brake fluid into the spark plug hole of cylinder #4 and let it sit for a full day. This may help loosen carbon buildup or worn rings before concluding engine failure.

Anonymous 2007-02-13 16:24

I'll try that—thank you for the suggestion.

Anonymous 2007-02-13 16:59

This is worth a try. Be sure to remove the spark plugs before cranking the engine to avoid hydro-locking. Afterward, change the oil as a precaution. Carbon buildup can restrict ring movement and cause misfires. Once the engine runs again, consider using a bottle of Seafoam through the intake vacuum and drive aggressively for several miles to burn off carbon deposits. You might be pleasantly surprised by results—though if it doesn’t resolve the issue, you’re only investing time. If there’s any improvement, repeat the process. I’ve used this method multiple times on 3.0L engines in vans when valve guides became sloppy and oil consumption increased. Once, a cylinder went from 70 psi to 110 psi just after adding two bottles of brake fluid over a weekend while cranking with plugs removed. Some older mechanics have even used bleach or coolant through the intake vacuum for cleaning—though I don’t recommend this on newer vehicles equipped with oxygen sensors. Any experienced engine rebuilders can confirm that engines burning coolant are typically very clean internally.

Anonymous 2007-02-21 17:43

I performed the brake fluid and Seafoam treatment on cylinders #4 and #5 over a three-day weekend. Cylinder #5 compression improved from 100 psi to 120 psi, but cylinder #4 remained at 90 psi. After clearing the engine control module, the P0304 misfire code returned quickly. This strongly suggests that ring wear is present in cylinder #4. Thank you for the helpful advice.

Anonymous 2007-02-22 15:54

Good luck with your diagnosis and repair efforts!