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

asked by on

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.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. P0302 misfire, car won;t go past 25 mph

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Fuel pressure issues, hesitaion and occasionally wouldn't start

An erratic fuel pump may be one of a few different problems. You need to check to see if the fuel pump is getting power when the problem is happening. If it is, then the ground in the right rear...

Q: Passenger window off track

If the window has come off the track, then there may be a problem with the window regulator. Many times the cable will snap, and the window will fall down into the door. I would recommend having a professional technician,...

Q: Steering wheel vibration and shaking

You probably should have changed the struts in pair at least because now you have an original strut on one side and a replacement strut on the other. The two might be slightly different in construction even if they seem...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.