Q: P0302 misfire, car won;t go past 25 mph

asked by on April 24, 2017

So recently, my 99 Miata was getting a P0304 misfire code and ended up replacing the plugs, wires, coil pack, fuel injectors with no luck from any of that. I later figured out that I had low compression, so I swapped the head with a different one from a local Miata guy. About 2 weeks into the head swap. My car breaks down going up a small hill. The car starts making a weird exhaust note, somewhat like a 4 stroke dirt bike engine. I pull over and the car dies. I am able to start it up and drive it around 25 mph max speed, but will die on idle. My coolant and oil gauge are dead center. The coolant and oil look normal with no odd contaminants. I can't figure out what it can be. I read about fuel, so I replaced the pump and fuel filter. It ran decent for about 20 seconds and it's back to the same problem. Can it be a bad ECU?

Summary: weird engine exhaust noise, very low power, dies at idle, flashing P0302 code. Any help will highly be appreciated.

My car has 190000 miles.
My car has a manual transmission.

I am not sure if the mechanic who replaced your head ran a compression test after the head was replaced but if not I can think of a couple of possibilities both of which lead back to low compression. The first of the two is that it is possible the mechanic did not replace the cylinder head properly resulting in low compression still. The other idea I have is that the low compresson may have been due to the piston rings rather than the head or head gasket. However, you say that the oil does not appear to have any contaminants in it. If the compression rings were faulty you would likely see some fuel in the oil. However, it is also possible that the compression could be so low that the engine is not even able to run long enough to get fuel in the oil. Either way, I would strongly recommend getting a compression test done to determine if your compression is within the manufacturer’s specification. If your compression is within specification, I think you are on the right track to assume a possible computer failure. In this case, I would recommend having the vehicle inspected by a qualified technician who is proficcient in automotive computer diagnostics.

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