I've been tackling a puzzler that I am out of ideas on. The truck is 1996 F150 with the 4.9L i6 220,000 miles. Over the past few months I have replaced the EGR tube, got my smog pump back up and running, and replaced all of my vacuum lines except for the plastic lines on the vacuum panel on the driver's side underneath the EGR valve (those are still in tact and properly routed, just not replaced with new lines). The truck is running well, however I am struggling with a VERY persistent check engine light. In the past few weeks I have cleared my codes 3 times. The first two times, the code P0174 shows pending within no more than 5 miles of driving after clearing the codes. Within another couple miles the light comes on. A few miles after P0174 registers, P0171 shows pending, and then fully registered within another couple miles. Yesterday I replaced my ECT sensor (the plastic part of the old sensor literally disintegrated when I removed it), as well as cleaning out my IAC, and MAF. I cleared the codes and started driving around again. P0174 turned pending after 7ish miles, and turned the light on shortly after. That is the only code in the system right now, p0171 has not shown up.

I checked my fuel pressure and got strong numbers both with the FPR's vacuum line hooked up and without. In both cases, fuel pressure held strong with the truck off. I also put vacuum on the FPR with the truck off and it held vacuum. The truck's idle is normal and does not hang or hesitate. It is pulling 20in of vacuum at idle coming off the manifold. I have sprayed multiple times with carb cleaner. The Secondary Air Injection tube has a minor leak at the number one cylinder, possibly some other cylinders but the only definite reaction from the motor came when I sprayed at the number one cylinder. Even this leak though seems far too minor to set off a lean code within 5-10 miles of driving.

Also, on a few occasions where I've stopped and filled up with gas during a long road trip (3-4hrs), the truck has a VERY rough idle for about 15 seconds. Turns over and fires immediately like normal but it is very rough for about 15 seconds. Pressing on the gas marginally helps even out the idle and then things return to normal and the truck is fine. I talked to a mechanic about this. Given that my fuel pressure is strong (55-60psi without the regulator hooked up, 47-52 with it hooked up) and the codes I've mentioned, he recommended replacing the ECT (the sensor right next to the thermostat housing) and cleaning the IAC. This type of brief, rough idle has only occurred a few times after filling up during a long road trip.

Does anyone have thoughts about what could be triggering specifically the P0174 code and NOT the P0171 code? Even if p0171 does show up in the next day or so, it has been consistently p0174 that appears first and seems closer to the source of the issue. The truck idles strong but does have some occasional hiccups, not full on misfires, but just enough to be noticeable. Also, this may just be me, but the engine feels GREAT when the codes are cleared (very quiet and smooth) and then it feels like performance decreases slightly once the code(s) shows up. Nothing you would probably notice, but just a hair rougher at idle. Even with the codes on, the truck idles well and revs up smoothly. I've been getting 13-16 mpg HWY depending on wind direction, and around 10-12 mpg in town. 13-14 mpg when it's 50/50 mixed.

Sorry for the long post. Just wanted to get all the details in there ahead of time.

Thanks.

 

I have the same truck and I had those lean codes but it was a weak fuel pump. But thats not your issue so i don't know. Someone hear will tho.

 

I had the same situation on mine. I ended up changing all 3 O2 sensors - did not fix the problem. Changed the EGR - still did not fix the problem. Changed the EGR pressure sensor - still did not fix the problem. Changed the fuel pressure regulator - still did not fix the problem. Cleaned the MAF - still did not fix the problem. Replaced all the intake vacuum lines - still did not fix the problem. Then my catalytic converter got stolen and I had to replace it. Amazingly the problem went away.

With 187k miles, it looks like a bad cat was the source of my P0174 problem.

 

Thanks for the responses Ronnie and warren. I will keep those suggestions in mind. If there is a definitive way to check a catalytic converter, let me know. I just drove the truck on a 5 hour road trip one-way (10 hours total on the road) for the weekend. I can confidently say that P0174 is the ONLY code stored. No sign of P0171, not even pending and this has been 650+ miles of driving since I last cleared the codes. Other than P0174, the truck still bogs down badly for the first 15 seconds of idling after fueling up on the road after 2+ hours of driving. It did this both on the way down and back on the trip. Both times, I tried cycling the key on and off three times (just to the "battery on" position to prime the pump) to try and build up pressure in the fuel line, assuming the fuel pump was weak. This did NOTHING to change the rough start/bogging which happens only when I've been on the road for 2+ hours and start the truck at a gas station. The bogging feels like it's running on maybe 3 cylinders initially and it feels like more cylinders gradually start firing correctly as the idle returns to normal.

So again we are left with only a P0174 code and rough hard start when the engine is hot as symptoms. Gas mileage was about 15.7 driving at 65mph with 20mph wind helping me. P0174 only showing Bank 2 as lean along with the good numbers I got from testing the fuel pressure, as well as getting no change in warm-starting from extra priming the pump leads me to believe it is likely not a fuel pump related issue. I'm not completely ruling that out though yet.

I have a tune up kit (the sixlitre tune up) which has new plugs, wires, and coil which I will install soon. HOWEVER, I'd like to figure out this engine code first because, if it is the result of a problem other than routine maintenance, I don't want to mask the problem with a new tune up and then ruin my new plugs while the engine runs lean in one bank.

Thanks again everyone.