1997 Protege 1500 Carboned Up

[sch72]

My wife's 1997 Protege SE 1500 gets horribly carbon fouled after a few short trips. 92000 km on the clock, front O2 Sensor replaced (and coated with carbon again!), timing and drive belts replaced and valve timing is dead on.
Body is good, everything but the engine works fine.

Code P1170 keeps coming back.

Oddly enough the thing runs great, accelerates well and goes like a bomb until the four plugs get carbon fouled.

I was able to download a 1996 shop manual and also have the Haynes manual.

My best guess is that there is a fuel system problem and any help would be appreciated. Getting that back seat out to access the fuel pump also apparently requires some sort of trick.

[91Protege]

Does it use any oil? I donno if these things have an EGR valve but you might want to ck that.

[sch72]

Thank you for the reply.

Its never had an oil consumption problem but is obviously running way too rich.

The shop manual procedure for a P1170 code is Heated oxygen sensor malfunction, open or short in wiring from the O2 sensor to the pcm, Intake air system, fuel system, ignition system malfunction.

So far I have replaced the front O2 sensor, checked the wiring to the PCM and inspected the mass airflow sensor wires.

The wires on the mass airflow sensor are still shiny - I had a K & N air filter but went back to paper when I found Mazda does not recommend K & N due to the possibility of oil contaminating the air flow sensor.

Yes there is an egr valve - If it was stuck shut would that result in a over rich mixture when the engine is in open loop mode?

I an also wondering if a plastic bag or something could have been sucked in to the air intake.

[91Protege]

There should be 2 o2's if your obd2 as you are, there is 1 infront of the cat and 1 after the cat, but the one in front reads the mixture and the one behind cks to see if the cat is working. The oil from the K&N can gum up the mass air if you have too much oil in it, but your not getting a mass air code. If the EGR was stuck shut ud get a code for that. You have to see what the voltages are at the 1st O2's plug. If you can look up what each wire is suppoed to be that will help alot.

[sch72]

The problem was the fuel pressure regulator. The front oxygen sensor was coated with carbon so of course failed. A new fuel pressure regulator with new efi hose and efi clamps solved the problem.

Well the other stuff i.e. timing belt, plugs, ignition components, etc were just about due anyway. FWIW a change to 5-20 synthetic oil also improved performance and cold weather starting.