P0172 and running erratic revving at idle

Before I begin, this is my first post here so be gentle. I am relatively proficient with the mechanical field but am new to the "quirks" of the Tacoma.

So I just acquired a 2000 tacoma from my little brother. Long story short he was less than responsible and my girlfriend and I were in need of a second vehicle. So the parents opted to teach him a lesson and help me at the same time.

The tacoma has 212k miles on it. When I got it there were probably 100 codes on it. Ok, maybe closer to 30, but its still quite a few. There were misfire codes among a few other issues. It might have something to do with the diesel he tried to put in the tank...

I changed the sparkplugs, seafoamed the engine, drained the tank and refilled with fresh gas (twice), changed oil. I managed to clear out all of the codes with an exception of a pesky p0172. Too rich on upstream O2 sensor.

I pulled the sensor and tested it with a torch and multimeter. Dead O2 sensor (diesel.....) I ordered a new one and hooked it up. Now the O2 sensor is reading right. The engine began to idle rough. I dug around the engine bay and found a hose at the cruise control module that had gotten unplugged. I reseated the hose and the idling got a lot better but its still deviates a bit (up revving then down revving).

When driving on the highway the p0172 stays off. I can drive up to school and around the city without the code kicking on. When I park or stop at a light the code will tend to kick on.

It isn't a guarantee that it will kick on, I tried to test in the garage and left it running for 30 min with no light. Then I went to walmart and it kicked on at the light right off the road. While sitting in the garage the engine rev deviate a bit, it's not noticable on the tachometer but its audible if you're paying attention.

I plan on using a propane torch to go around and try to find leaks tomorrow. But before I spend several hours trying to find it; are there any particular causes of a P0172 that Tacomas are known for?

Any stories, tips, experiences, etc. are greatly appreciated.

 

The computer sets the fuel injector pulse based on all the sensor indications of how much air is coming in, then it fine tunes the mixture based on the O2 sensor. This is called the fuel trim. The computer can only adjust so far in the lean or rich direction based on the O2 sensor. The P0172 code (rich fuel trim) is saying that it can't lean out the mixture enough to get the correct air-fuel ratio. This is most likely because other sensors are indicating more air is coming in than actually is.

The repair manual (at least for my 96) says to check:

• Fuel line pressure
• Injector leak, blockage
• Heated oxygen sensor malfunction
• Mass air flow meter
• Engine coolant temp. sensor

(Check the repair manual for your year to see if it says anything different.)

So, if the MAF is telling the computer that more air is coming in than really is it will throw everything off. You might start by cleaning the MAF sensor with MAF cleaner. If you have an OBD-II reader, you can check that the computer is reading the correct coolant temperature as the engine is running.

These are at least easy things to check first before looking at the fuel line pressure or injectors.

 

I've already cleaned the MAF sensor, i forgot to mention that in my above post. It was of the first things I attacked after I saw the code.

The oxygen sensor heater should be good; I just got a new sensor.

I don't understand how the engine coolant temperature could through a P0172? My OBDII reader isn't that smart. I may be able to get it read at autozone/Oriellys, I can look into that.

I would think that the fuel injector issue would also show up as a misfire? I have not checked the pressure on the fuel line yet, I will do that.