2000 f150 5.4. 198k miles.

I used to get the 171/174 codes in the winter time. right now the truck runs rough at idle, does ok with consistent speed on the highway and then shudders when idling again. I've searched for vacuum leaks up and down but can't find any. I've not used a professional smoke machine though.

I continue to get p1131 and p1151 o2 sensor codes even when the system is warmed up. My o2 sensors are original from the factory.

It feels like a vacuum leak, and I was wondering if the IAT sensor could be the culprit. I have NO iat sensor codes in the computer.

Here's what I've done so far---changed the MAF sensor, pcv tube (the whole tube with the valve as well), changed the EVAP hose that connects to the top of the throttle body. It was just worn out but not really leaking badly.

I've done a home smoke test and that swisher sweet tasted horrible. I put a latex glove over the throttle body to keep air from escaping and then blew smoke into one of the other vacuum tubes, but I didn't see any smoke escaping from other vacuum connections or the intake itself.

The question I have is what about the IAT. The reason I ask it is that according to 1aauto.com, this part should read approximately 5-10 degrees below the temp reading for the coolant sensor. I have a scanner that works in real time and checked it yesterday on the way home from work. The outside temp was about 42 degrees, and the coolant temp was anywhere from 185-195 degrees. This defies the way it's supposed to work from what I've read.

any ideas or should I just continue to look for vacuum leaks?

 

It sounds like you have done a great job of chasing down a possible vacuum leak and done the low cost things (like changing the PCV and cleaning the MAF sensor. There are two other likely possible items you might consider; both low cost.

Clean the EGR valve. the ruff idle is an indication that this could be stuck. When you take off the EGR, make sure you check the tube from the manifold to the EGR and make sure it is not blocked. I do this by running a wire through it.

You could have an injector problem. Running a bottle of fuel injector cleaner through your next tank of gas might help this. After you have done that, I would change the fuel filter.

It could also be a slew of other problems like bad connections to the Oxy sensors, exhaust leaks, leaks between MAF and engine, bad fuel injectors. However, it think these are less likely and I really think the most likely problem is your EGR.

 

Yea that was your engine temp, -not the IAT, - rather your CHT. The CHT reported 185 -195*