Cylinder misfire P300?

DTC P0300 is a general misfire code. A specific misfire code would be P030x where x is the cylinder number. So if cyl 3 has the misfire you would expect to see DTC P0303.

A misfire might be caused by a bad spark plug, a bad spark igniter (which sits on top of the plug) or possibly a bad fuel injector providing fuel to the cylinder. Less commonly, a wiring harness (or connector) fault or a bad engine ECU might cause the problem.

30 mpg is pretty bad fuel economy for a Prius so it is reasonable to continue to try to figure out the root cause of the problem. Not to mention that the unburnt fuel is damaging the catalytic converter and diluting your engine oil, which will lead to very expensive repairs to the cc and the engine itself, in the future.

 

Replacing the gas tank, are you serious? I suppose if that is the only option in town, you are stuck, but definitely go somewhere else if possible.

The most common problem is a dirty throttle air intake. The throttle bore gets really dirty because the PCV hose connects above the plate. This creates turbulence, which may cause poor economy, and can cause the plate to stick shut.

Ask nephew mechanic to pull the air cleaner off, clean the MAF sensor with CRC MAF cleaner, and clean the throttle bore and plate with throttle body or carb cleaner.

 

I had a vehicle one time that caused me to chase a misfire diag code. Ended up being a head gasket was rolled and no compression. Do a compression test!!! I replaced injectors, plugs, wires, etc. and everyone over looked doing a compression test. Knew right away and learning more looking hind sight.

 

+1 on Seilerts advice. That will probably fix it. It is simple and cheap to try.

 

For those of you following along at home, this is an excellent example of how you can't simply plug a scan tool into a car and have it tell you what component is faulty.

If this were a Chevy Cavalier, and it presented with Too Lean, then I would immediately think fuel pump. This is probably why nugie has had the fuel tank replaced 3 times.

So it is possible that the fuel pump went bad, but in the vast majority of cases, the pump last the life of the car, so if it keeps going bad, then there is a faulty pressure regulator. A fuel pump problem based on that should fail within the warranty period, however (1 year, unlimited miles).

But consider all the facts, and infer the possible problems. nugie's most recent post has some key information. There are codes for lean burn, misfire, and poor fuel economy. To me, there are only two possible causes: the throttle bore is so dirty that the throttle plate is stuck open and/or the motor that controls it is miscalibrated; or, the upstream oxygen sensor is faulty, causing the fuel trims to go very rich, which leads to fouled plugs, misfiring, and poor fuel economy. I lean towards the latter, but it could possibly be even some other cause, like a problem with the coolant temperature sensor. At least, hopefully, the local garage will have enough to go on here to help nugie out.

 

I would look into the stupid cat exhaust carbon diverter thing that rusts and the vacuum can't move it. You should work that with a hand brake vacuum pump and feel it move with your fingers. Then I'd check the OBD-II readings for the O2 sensors. Change the plus maybe and clean throttle. Also check the voltages on the throttle valve motor there is a M+ and M-, they must be opposed to each other, like one looks like a 90% duty cycle and when it goes to +12V, the other signal looks like 10% duty and goes to 0V.

 

Interesting. Where was the gas leaking to? Did you smell gasoline fumes? Was this a fire hazard?