I've learned a lot reading your posts and following the how to stuff. Sadly I've learned what's not wrong....

Here's the situation:
1997 F-150 Lariat, 5.4L, 4x4 ~158k miles, all interior electronics/features in great working condition, body is in great condition.

In the last 60 days the following has been replaced:
spark plugs and coils
idle air control module
throttle position sensor
both down stream o2 sensors (in the exhaust headers)
EGR Vacuum Solenoid/DPFE sensor (one unit I believe)

This all started with Cylinder #4 misfiring-aka not firing at all. Figured I'd just do all the spark plugs and coils at once- after this replacement it ran perfectly. No idle issue, great gas mileage. Then after about 1,000 miles it's idling very rough and throwing P1131 and P1151 constantly.

I've tested the vacuum system and I don't seem to have any leaks, all the elbows and tubes look good.

The issue is not just the codes- the engine idles rough- in the 500 rpm range. Some times it gets happy and idles at 1500, no gas peddle needed.

Highway speeds are perfectly fine, no power loss but in stop and go traffic, it bucks and chugs.

I've had a cylinder die and it's not a rough idle with a misfire, its almost like it's choking.

I disconnected the EGR/DPFE unit wires and the engine won't start so I know that is working electrically. There is vacuum on the white hose to this unit but none on the green hose. I haven't done the EGR valve yet.

Any ideas on what I should hunt for next? If it throws more codes (I know it will) I'll post them.

Thanks!

 

Just for fun a did a "reboot" but pulling the negative battery wire and touching it to the positive. No changes.

 

Seems I answered this same issue very recently.
YOU are not paying attention to the code descriptions.
This is why your not repairing for the......codes.
They tell you both banks Ox sensors are detecting a lean condition.
It's not the sensors but an excess of 'unaccounted for' air in the intake tract, as evidenced by your 'wandering' idle rpm.
Trust the system to tell you the truth.
Your missing the fault area.
Leave other parts of the control system alone or you can introduce a second problem on top of what you already have..
Good luck.

 

I've over complicated things it would seem (bad habit). I will investigate the intake side and see what I can find.

I'm assuming that I should be hunting for an intake leak before the butterfly valve/TPS then?

 

The TPS has nothing to do with the issue.
The fault is after the throttle body in the common area to all cylinders and before the intake runners to each cylinder..
Both banks OX sensors are detecting it.
All rubber parts are suspect and anything that could let air in.
Gaskets, PCV, power brake hose etc.
Good luck.

 

Excellent. I only used the TPS as a point of positional reference. That helps my search. I'll see what I can find. Thanks for the quick response!

 

You were 100% right! Got some brake cleaner, sprayed it around suspect spots and viola. The elbow on the back of the throttle body, right below the IAC was totally rotted out.

Once replaced she idles like a purring kitten once again.

Thank you so much for setting me straight. Keeping it simple is always best.

Thanks again!