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Dragonsm

2002 F150 w/ 5.4 P0171 and P0174 codes

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Yesterday morning after letting the truck warm up, on my drive into work, service engine soon light came on...but I didn't notice much of a change to the engine.

The light came on as I was decelerating on a downward incline to a stoplight.

Reset the computer at work disconnecting the battery and figure I'd see if I could set the code off again.....nothing. Thought maybe it was due to the extreme change in cold weather?

Again, this morning the truck popped the light again in the same exact spot as yesterday.

The codes that I grabbed were P0171 and P0174.

Truck has about 155,XXX miles on it....other than that, ask any other questions you may need to know.

Trying to figure out if it is something possibly more simple to check or if it is intake manifold gaskets or what not.

Thanks,

Steve

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Plenum gaskets and rotted rubber hoses (you find all of those when you pull the plenum). Typically the cold will aggravate the leaking gaskets to the point where the light comes on. Wait long enough and it will start to run rough on a really cold morning and then start to smooth out as the engine warms up.

The best way, IMO, is to use a smoke machine on a clod engine. Fill the intake with low pressure smoke and see where it comes out. Leak confirmed, repair done, and confidence that it will be fixed.

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Airjer, I was hoping to get a response from you...and your response was about what I was expecting...(though not hoping for)

I had talked to a relative of mine who is also a mechanic up around the grand forks area and he had given me the same initial thought due to the age of the motor along with the change in the temp.

I did hear back from the local shop. They did hook it up on the smoke machine and thankfully didn't find any leaks.

Ended up being the rear two O2 sensors that didn't work. Upon removal of the one, the threads rusted out so they need to weld in a new insert.

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P0171 and p0174 are lean codes indicating a vacuum leak, replacing the rear 02 sensors will most likely not take care of your issue, as many of those engine styles I have seen the rubber elbows in the PCV system will deteriorate from engine oil and cause a leak, not saying that your local shop didn't do a good job as I have smoked engines before and not find any leaks but ended up finding leaks with a good visual inspection of all components in the intake system

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Appreciate the heads up!

Not sure if this local shop is good or not myself as it is the first time I have used it. Since relocating to SD (with no prior ties), thankfully I haven't ran into many repair issues, but they came recommended from neighbors.

When I saw the 5 or 6 page list of potential causes for the codes, there was no way I was going to attempt with my mechanical (or lack their of) skills especially after what both Airjer and my cousin mentioned to me.

You can bet if I pop a code, it'll be back at the same shop...(though I'll be cordial about it at first)......I do hope I have found an honest shop that will take care of me at a price point better than the dealership!

When it gets a bit warmer this weekend, I'll hop up in the engine bay and take a look at some of the elbows and hoses.

Steve

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I also find it odd that both upstream O2 sensors went bad at the same time. One I can see but than it may only affect one bank. Either of the codes not both of the codes.

The rear two O2 sensors imply that the replaced the downstream sensors. Downstream sensors monitor catalytic efficiency and typically through a P0420 or a P0430 code. I can re call ever seeing a downstream sensor causing lean codes. The upstream sensors are the ones that help control fuel trim. Hopefully a misunderstanding of the repairs they made and not another red flag?

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Well...a little update

I did look at the invoice and it was rear 02 sensors that were replaced.

None the less, the truck won't make it to this weekend to be looked at by an untrained eye. On my way into work this morning, the service engine soon light came back on and I gave them a call as I was driving.

Dropping it back off around noonish today to give them today/tomorrow and at the very most Saturday to get it corrected.

alright....here goes...."you were right" LOL

Thanks again and I'll let you know what I find out.

Steve

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