P0171, P0174 ,05 TRD Off-road :(

First post on TW!

Greetings from Texas! I picked up my first taco about a year ago, and have had nothing but awesome experiences with this truck, but lately she is running rough and I can't seem to shake it. Getting p0171 & p0174 with tha occasional VERY rough idle and extreme hesitation. Have read several threads alluding to issues with a vacuum leak or MAF sensor, but cannot source a vacuum leak of any kind. Last time I got this code, I pulled MAF sensor and it looked like it was dripped in oil/mud. I cleaned it and problem went away. Now MAF looks clean but problem has returned. Truck has 108k. Any help would be appreciated!

Side note: assuming PO slacked on maintenance, what all needs to be addressed at 108k? Thanks mucho!

 

Low fuel pressure (clogged filter, damaged fuel lines, bad pump), contaminated or faulty injectors...

At 108k when were the spark plugs last changed? For the 4.0 V6 it should be every 30k miles with the stock plugs; if you have the 4-banger they were due at 100k...

 

You think fuel pressure even if problem clears up after 2500rpm? And I have never done plugs, but PO claimed to "take it to the shop" regularly for maintenance. Who knows what that could mean.

Also don't these tacos only have an in-tank filter? :/

 

Lean means either too much air or too little fuel, or something is making the ECM think the mix is lean when it really isn't (a bad O2 sensor for instance), however as you have said that the engine runs poorly with a rough idle and hesitation it probably really is running lean--that leaves too much air or too little gasoline. You indicated no vacuum leaks could be found and the MAF appears clean.

It could be the MAF is under reporting the amount of air entering the engine, causing the ECM to not inject enough fuel for the actual amount of air; or it could be the ECM is being properly informed and is trying but the fuel pressure is low or injectors are sticking/clogged.

Monitoring the fuel pressure would prove/disprove that possibility--monitoring MAF output voltage would determine if it is reporting air flow properly. Check the battery voltage too, if it is dropping below 12 V with the engine running it could cause lean operation (sticky injectors + low voltage = not enough fuel). The battery could be bad (shorted cells are common) or there could be a charging system fault. With modern EFI systems proper supply voltage is important.

I believe the fuel filter is on the pump, in the tank, doesn't mean it cannot be dirty--trapping crap in the fuel is its job...

 

Did you fix it?