I have two pending codes P0171 and P0174 and I know they mean both banks are lean but its not triggering the engine light yet. But a few weeks ago the engine light came on and only showed P0171. I did some digging and found out its most likely a vacuum leak so I started poking around and ended up taking the throttle body off since it needed to be cleaned anyway, and replaced the PCV line the runs to the back on the intake cause it looked pretty bad and I replaced the EVAP purge hose that connects to the top on the throttle body because it was really bad. Now the computer shows these two codes pending, and I have horrible MPG 10.4 on a 60 mile trip cruising 75. I did some more digging and found out its possible the intake is leaking and some said to spray throttle body cleaner around the intake while running and see if the idle jumps and it did once on the passenger side front but I couldn't get it to do it again so I'm not 100% there's a leak. Than I hooked the scan tool up to it on the way home last night and watched the ST and LT fuel trims, the ST read constantly around -3.3% to +1% LT around +6.5% to 8% while cruising 70MPH over a 50 mile trip. I really don't know what those mean if someone could help me with that. Also the O2 sensor readings were very erratic O2S 11 and O2S 21 which i'm assuming is Bank 1 upstream and Bank 2 upstream constantly bounced from .100 mV to .715 mV but the down stream O2s read consistently at .650 mV. This doesn't seem normal to me, any insight?

 

Your kind of suspended at is point.
Clear the codes and let the system do all the diagnostics from scratch and work from any new results if any.
.************************************************* *****
The OX sensor readings.
Front; continuously swings it's output in response to oxygen level changes in the combustion gas. This is what controls fuel injection for the 'next' cylinder to fire.
It's reading should vary as the ox content varies.
.
Rear; this sensor also detects oxygen level 'after' the front cat and part of the rear cat processes the exhaust gas. Why; it is the indication of cat health and is a monitor only. It has no control of engine functions.
If the cats function has deteriorated to a set lack of ox level, code is set to tell this fact.
It's reading should be relatively stable when viewed by a Scanner..
.

The loss of fuel mileage is evidence of the fuel tables being shifted to their rich end adding fuel trying to recover the air to fuel ratio to protect the motor from damage by overheating and parts expansion.
These tables being out of limits is what sets the codes.
Good luck.

 

Exhaust leaks in front of the front O2's will also cause lean codes.

 

Do you know which engine you have?

 

Sig says 5.4.

 

I disconnected the battery when I took the throttle body off so it would reset itself and it ended up being disconnected for a day as I was waiting on new gaskets. The truck ran alright for a while until I noticed the idle wouldn't drop down below 1000 after I came home so I checked pending codes and they were there. I figured the upstream O2 sensors were suppose to vary like that just not that extreme. Does anyone know what the fuel tables are suppose to be at when cruising?

Yes I have a 5.4 it has 180,000 on it, I have an Edge evolution on it but its set as stock after I had the first code I also have a K&N air filter which I read when over oiled could gum up the MAF and throw this code but I don't put a lot of oil on it and it hasn't been oil since last summer. I also cleaned the MAF when I worked on it last time.

The passenger side manifold had a leak and got fixed a few years ago so I know the sound and I know there's no exhaust leak. I'm starting to lean toward the intake gasket.

 

What sig ? I don't see any sig..

 

It shows up under my first post but not the last one for some reason

This morning I got up bright an early to go ice fishing and on the way to the lake the check engine light came on because of the codes so I cleared it when I parked at the lake and than on the way back into town it came on again. It seems like it only comes on when its really cold, todays high was -1 degrees, but once I cleared the code and the truck warmed up it stay off. Tomorrow I'm going home to do some poking around with a vacuum gauge, do you guys have any other suggestions on where to check vacuum?

 

The board is set to only show the sig in the first post on the page.

 

Nope, I can't see a sig in the first post, def nothing on the engine.

Maybe I turned something off in the control panel?

 

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My signature really doesn't matter

I replaced the vacuum lines to the DPFE sensor, brake booster, and all the rubber lines at the T's for the 4WD over by the battery. The battery was unhooked all day I got everything back together and did a vacuum test with the gauge at one of the T's and I was pulling around 20 PSI and than tested at the PCV valve and I was pulling the same. Test drove and it seemed to be running better and the fuel tables stayed in specs and the codes havent come back yet. We'll see how it does on the trip tomorrow

 

If you want your questions answered it does. It's from your signature we can get quite a bit a relative information. Used to inform us what you have there. Without that info, - your better off using the search bar for answers.

So if that doesn't matter, then so be it.

 

Thanks glc. I'd sure like to know how that got all screwed up. Been here many years and haven't had to screw my options as much as I do now. For awhile there , I wasn't seeing pics as well. Somehow, my options were no longer selected..(?)

 

Like I said before it doesn't really matter, alot of people don't have signitures so all you would have to do is ask the person and they would tell you. Anyway I fixed the codes myself