I know these codes have been posted 100's of times because I think I've read them all, but I need some help!

2002 F-150 4.2L. About a year ago, it threw the P0171 and 0174 codes up. After replacing all the simple stuff, I bit the bullet and replaced the lower intake gasket as well as the upper intake gaskets. Buttoned it all up, reset the codes and we were back in business. No issues, truck ran good. About 6-8 months later, the check engine light came back on, I check it and it was that pesky set of codes again, 0171 and 0174.

It still ran fine, but the codes were there, so I just drove it for a while (it's a company truck that doesn't get driven that much).

A couple of weeks ago while driving it, it acted very sluggish. You could mash the accelerator and it would just very slowly run up the speed. It never hesitated or missed, it just got very very sluggish. Checked the codes, no additional codes.

Then it began to stumble when you mashed the gas.

I put new plugs (old one's looked great, other than a huge gap), new wires, new coil pack and started it. Pushed the gas pedal, it just dies.

I look for obvious vacuum leaks and replace a couple of elbows for the PCV and it seems to run a little better. Enough so that I can drive it, but still not running right.

I ordered all the intake gaskets and new isolator bolts and spend Saturday pulling that apart. I didn't do the lower intake because the upper intake gaskets looked so good, I just don't believe this is a intake gasket problem. None the less, I installed the new gaskets on the plenum and upper intake, new isolator bolts, replaced a couple pieces of vacuum hose that looked cracked, started it back up, it ran as smooth as silk.

Convinced I had it fixed, I went for a test drive. Idling, it is as smooth as you could ask for. Pull up to a stop sign, wait, step on the gas and it chokes. Just stumbles and you can hear it almost backfiring and pushing air back through the system.

Okay, now I'm stumped.

I've put new PCV, new EGR, new vacuum lines in several places, new upper intake gaskets, new plenum gaskets, new isolator bolts, and even new o-rings on the injectors, and I'm still getting the 0171 and 0174 codes.

With the diagnostics on, I can see voltage varying on all O2 sensors, so I assume they are working (none of them are reading 0 volts). It doesn't throw any other code up, other than the 0171 and 0174.

At this point I feel like I'm replaced every single thing ever mentioned in any 0171 and 0174 thread on the internet.

Any suggestions on where to go next?

1 more point, if you very slowly push the gas, it runs fine, but any pressure on it, it bogs down and just stumbles.

 

The codes tell you both banks have been running lean.
This was detected by the OX sensors as to much oxygen in the exhaust gas.
From this detection the computer shifts the long term fuel trim tables rich trying to account for the lean condition.
When those tables are shifted 25% it sets the codes after the fact, to tell you of the condition.
Causes are unaccounted for air getting into the intake.
A drop in fuel injected causing a lean condition from an opposite condition.
Possible EGR leak polluting the intake mixture that can cause leanness by that action.
You have 3 possible causes to check out
Check fuel pressure.
Check EGR operation.
Check for other air/vacuum leaks.
Clean the mass air meter with proper cleaner.
Be sure the TPS is working because it adds fuel for acceleration by the computer detecting it's' rate of movement'.
God luck.

 

Thanks for the reply.

I have replaced the EGR valve as well as cleaning the Mass Air Flow Sensor with the proper Mass Air Flow Sensor cleaner.

How would I check to see if the TPS is working?

I'm literally running out of parts to replace A buddy suggested that it might be the cam sensor not advancing the timing correctly, but I thought when that happened, it threw a very specific code for that, which I'm not seeing.

It's just always the same code, 0171, 0174 nothing new ever comes up.

 

That is usually a significant vacuum leak.
With both at the same time it usually means either the same fault has occurred on both sides of the engine, but more likely it means the fault is somewhere upstream of the intake manifold.

 

The cam sensor in your v6 is only a reference for fuel injection timing and is constant and does not change. There is no advance property in it.
You still have an air leak at some point.
It is 99% of the time the cause of the codes and shifted fuel tables.
Good luck.

 

Thanks guys, any input into tracking such a leak down?

I've replaced the rubber on both ends of the PCV pipe on the passenger side, I've replaced a couple small pieces of tubing used to connect T's up on the firewall on the passenger side (dry rotted), I've replaced all the gaskets and isolator bolts on on the plenum, and inspected all the rubber hoses going into that, as well as checking the hoses coming off of that and going to the brake booster. I don't see anything.

I'm stumped on where to look since the one's in front of my face all seem good. Is there a schematic of the vacuum system somewhere? Maybe there's something obvious I'm missing (most likely).

The fact that all the gaskets looked great when we pulled the plenum off, and putting new gaskets on didn't solve the issue, leads me to believe the leak isn't in that part of the system.

 

Smoke machine!


Also, did you check the dip stick for proper seating?
And the dip stick tube where it enters the block?
And the fresh air side of the PCV system?


What is the fuel pressure when it falls on its face?
Weak fuel pressure can cause a lean condition.

 

I haven't checked those things, but will, except for the fuel pressure, I don't have a way to do that. For that, I'll have to take it somewhere to get it tested.

One characteristic that I'll mention in case it helps, is I drove the truck 60 miles to a job, left there, it was fine. Getting on the on ramp for the interstate, it seemed like it was missing and very sluggish. I just babied the throttle and once it got up to speed it was fine. Drove it 60 miles back. Then drove it about 15 miles away to pick up some materials, it ran fine. Went in did the paper work and pulled around to the loading dock, all was good. Went to start it from there, it started, but immediately struggled to run. I limped to the edge of the parking lot and it cut off. I couldn't get it to restart. Sat there for about 5 minutes, tried it again, it started, it ran a little rough, but it would go, then up to speed, it ran okay. I made it back to the shop and it cut off while backing up to the loading dock.

I put new plugs, wires, and coil pack in, and it started right up. I thought for sure I had fixed it. Pulled out of the lot and it began sputtering.

It's almost like a backfire. You can hear it really sucking air. Not sure if it's sucking air or that's some back pressure coming back through the system or what.

Not sure if that helps any, but I thought I'd add that to the equation.

Are there any obvious places to look for the vacuum leak? All the little hoses that hook into that unit on the passenger side of the plenum are hooked up, the PCV lines are hooked up and good, the brake booster line seems good. Not sure what else to check.

 

Some auto parts stores have a borrow program. They may have a fuel pump pressure tester.


Also, when you have your scanner diagnostics on, look to see if it has a PID for fuel pressure.

 

check the cats if they are plugged.....