How to resolve P0304 for 1999 Toyota Corolla?

Question

I have a 1999 Corolla CE with 183,00 miles on it. Until recently, the engine ran just fine, although there was some loss of power climbing hills. For several months before the check engine light came on yesterday, the engine began to buck and miss at low speeds (below 45 mph) whenever I tried to accelerate or go up a hill. The symptom went away by downshifting (stick shift) one gear below what used to be normal, but there was still a power loss. My fuel economy has dropped from around 36 mpg to about 24 mpg.

Before the check engine light came on, I replaced all the plugs but the problem continued. One mechanic said the clutch plate was worn down and slipping (bought used at 107,000 so I don't know how old the clutch disc is). The dealer said the air filter was dirty (It wasn't!) and my PCV valve was plugged but I hadn't cleaned or replaced it yet. Yesterday, first the check engine light began to flicker and a few miles later, it came on steadily. I haven't checked the resistance of the spark plug leads and I'm not sure if I have individual cylinder coils or a main coil.

Does the P0304 code mean the problem is only with cylinder 4? If so, isn't unlikely that I have a problem with the fuel pump or filter, the air intake system, EGR or any other component that affects the entire engine? Should I try an injector cleaning additive first? Could I have a compression leak in #4?

Most important, if I continue to drive, what will I be doing to my oxygen sensors and catalytic converter with unburned fuel being swept into the exhaust? I want to cure the problem, but should I not drive very far before I do?

Many thanks for a response to these questions.

Answer 1

Hi there,

This code indicates a misfire on cylinder 4. Here is a list of possible causes.

P0304 TOYOTA - Cylinder 4 Misfire Detected

Read more: https://www.autocodes.com/p0304_toyota.html