juha261

I have a 2005 MT with around 82k miles on it. I bought it used so I am not sure what was done to the car before I got it. Once I got it I changed the plugs, wires and coils and also change the transmission, differential and brake fluids. Recently the car has started hesitating/bucking. This only happens once the car is warm, about 10 minutes of driving, and under load. The car acts normal under partial throttle. The only code the car is throwing is P0420 but I believe this to be from the cat being gutted. Doing some more testing I found the fuel going very lean when the bucking happens. The odd part is I can turn the car off then right back on and the problem will go away for a little bit. Any thoughts or suggestions on what the problem might be? Any help would be greatly appreciated.

RIWWP

iTrader: (2)

Fuel pump. Its overheating, reducing fuel flow. Shutting off the engine lets it cool down some

juha261

Thanks for the quick reply. Shutting off the engine for 5 seconds is enough to cool the pump down? I was guessing it was a sensor or wire with how it acted.

RIWWP

iTrader: (2)

There are other things it can be, but the symptoms seem right. Pretty much everything else that can cause that while under load can't be "reset" by shutting off the engine.

The fuel pump is cooled by the gasoline, and will generally cool quickly. If I am right, your fuel pump isn't so far gone that it's just shutting down from the heat, but you are pretty close to that point.

A few other things it could be (somewhat not tied to restarting the engine)
- ESS needs to be wiped off, then the ESS profile reset
- MAF fouled
- O2 sensor failure
- Intake valving acting up
- Spitting oil into the intake where it's momentarily messing with the combustion (doesn't really fit the lean AFRs, but possible)

juha261

I have disconnected the battery and did the 20 stomp reset. I believe that reset the ESS, let me know if I am wrong. The MAF was my first guess so I replaced that, sorry forgot to mention that. I will check the front O2 sensor according to the FSM tonight. That was my second guess and I have a spare at home. As far as the intake valve I assume there is a checking procedure in the FSM. I will check that. I did see some dried oil spots in the intake. I was just assuming I needed a catch can. Thought of this as a separate issue. I was going to take care of that once I got the car running smoothly again. Is the oil something I should look more into? Thanks again.

RIWWP

iTrader: (2)

You are doing all the right things... which just reinforces my first guess of a fuel pump failure.

Be wary of going full throttle with it, a sudden fuel pump shut off can indeed destroy your engine from the resulting lean spike.

juha261

I have looked a little for a fuel pump. I have seen Airtex and Carter listed online. Are these acceptable replacements or should I get one from Mazda?