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P0304 Fault on 1999 Ford Ranger 4-Cylinder: Diagnosis and Solutions

Model: 99 Ford Ranger 4-cylinder Fault Code: P0304 Posted: 2006-09-19 10:19

I recently replaced all four spark plugs, ignition wires, and coils. I also cleaned the intake manifold and fuel system components, yet the P0304 misfire code persists. The engine runs smoothly at idle but noticeably bogs down under load. Despite confirming proper injector voltage and resistance, the issue remains unresolved. I'm seeking advice on potential root causes—especially given that the vehicle has a 4-cylinder configuration with eight spark plugs. Any insights into ignition modules, oxygen sensors, or vacuum leaks would be greatly appreciated.

Related fault codes
P0304
Comments (8)
Anonymous 2006-09-19 12:46

Ignition modules were a common issue on 1990s Ford Ranger 4-cylinder models and can cause intermittent misfires.

Anonymous 2006-09-19 13:44

I've already replaced both ignition modules. The engine runs fine under normal conditions but occasionally bogs during cruising or when under load. Given that the vehicle runs too smoothly to suggest an internal engine problem, I suspect either faulty oxygen sensors or a clogged catalytic converter. All injectors are receiving correct voltage and resistance.

Anonymous 2006-09-19 14:21

Did you replace both ignition coils or just the coils? Is this a 4-cylinder engine with eight spark plugs? Have you performed a compression test to rule out mechanical issues?

Anonymous 2006-09-19 15:13

It's an 8-plug 4-cylinder setup—only the ignition coils were replaced. I recently swapped both oxygen sensors, but the problem persists. A vacuum leak seems unlikely since it only affects cylinder #4. The issue appeared suddenly and has no clear trigger.

Anonymous 2006-09-19 15:37

The symptoms still point strongly to a faulty ignition module—unless one of the fuel injectors is failing. Try using a digital multimeter (DVOM) to test injector resistance or swap injectors between cylinders for testing. A compression test remains a good diagnostic step. Vacuum leaks can also trigger misfire codes. From personal experience with 1990s Ford Rangers, ignition modules were known to cause erratic behavior—sometimes resulting in the engine running on only two cylinders. Connector corrosion at the module was another common failure point. Good luck and please keep us updated! Jeff

Anonymous 2006-09-19 15:44

I've already replaced the #4 injector with no improvement. All injectors receive full 12 volts, and the wiring harnesses show correct resistance. I'm now looking into the ignition module—where is it located? It's hard to find for sale online. If you're referring to the coils, I replaced them again today with no change.

Anonymous 2006-09-19 17:46

In older models, ignition modules were mounted on the intake manifold and coils bolted directly to them. The design may have changed in later units. Try checking a Chilton's manual or downloading a diagram from AutoZone. Have you done a compression test yet?

Anonymous 2006-09-19 19:41

Compression tests showed consistent readings across all cylinders, and the no-idle lights are fully illuminated. Tomorrow morning I plan to remove the intake manifold to inspect for vacuum leaks and consider replacing the IAT (intake air temperature) sensor.