When I bought my truck ('00 4.2L manual) the following trouble codes were in the system:

P0136 - O2 Sensor Bank 1 Sensor 2
P0171 - Fuel system lean bank 1
P0174 - Fuel system lean bank 2
P0401 - EGR Flow Insufficient

I cleared the codes and now the CEL is back. Haven't had a chance to pull them again, but I was wondering if there is any one thing that could be causing this?

 

Check the pcv system for a vacuum leak. Most common would be at the elbows and especially the elbow where it attaches to the back of the throttle body/intake plenum. Clean your MAF sensor and change your fuel filter. Reset and go from there.

 

Your asking based on codes you cleared and may not now be the case.
The 136 is seperate.
171/174 are the same but from a different cause.
401 is seperate and from a different cause yet.
In short, 3 different issues.

 

I'm going to pull codes again this weekend and see what I get. I can get O2 sensors from my Expedition and the EGR valve too if its the same between 4.2 and 4.6...

 

Not really. All of those codes can be tied to one cause- possible blockage of the exhaust/catalytic converters. That's what caused my 171/174 codes.

 

Toys,I'm not sure about the 4.2 with the metal plenum, but the plastic plenum ones the PCV is attached to the side of the throttle body. The rubber hose that connects the PCV to the hard plastic pipe burns through right there at the T/B. That's a real easy fix if that is the problem.

I think the metal plenums have the PCV in the rocker cover and there is a rubber hose that is kinda under the intake manifold and that is where it likes to give up.

The 4.6 and 5.4 both enter in the back by the firewall IIRC.

 

My 4.2 has the big metal plenum. I have access to a Windstar with a 3.8 that has the plastic version of that same plenum, however. Would the PCV system be the same?

 

A little update to this ongoing saga. First off, the reason for my sporadic responses was due to only having access to a code reader on the weekends. I stopped by a friend's shop to scan the codes again, same four. He suggested I try the EGR valve first. He rummaged through a pile of parts beside a workbench (he's a big Ford guy) and pulled out a good EGR and said try this. I swapped it over and cleared the codes and the truck ran good and didn't pop a code until after I was driving for 20 minutes or so... I finally bought my own code reader (on sale!) and the lean codes came back. The EGR and O2 sensor codes didn't, so I'm assuming that the EGR was fubared. I'm wondering what would be the next step? Plugs/wires, fuel filter?

 

If your saying the P0171 and P0174 came back then you still should be looking to either clean your MAF sensor or finding a vacuum leak. Change your fuel filter for good measure. They should be changed every 15k or so. Those codes typically indicate an account of unmetered air or lack of fuel.

 

I think that on the metal plenum 4.2's the suspect pcv line is under the manifold, but do-able without alot of removal. I think Raptor had a pic in one of his posts. If that line is leaking (it's common) it will give you both 171 & 174 codes. That is where I'd start my hunt.