Hello I am new here and I would like to introduce myself. My name is Robert and I am a DIY'er at least trying to. So here's my problem, I bought a used 2006 f-150 with the flex fuel 5.4l . The engine has a rough idle and burns oil only on start up. I had to take the cats off because they were clogged from the oil. Now whenever I'd take the oil cap off the engine would stall immediately. Now my cousin has the same engine as me and we tried it on his truck and his truck continued to run perfectly fine. I am at a loss because I have checked everything i could possibly reach and see. Hoses are fine and no issues when I drive, everything is as smooth as it could be. No power loss as far as I could tell so I don't think piston rings, however, I am kind of stuck on the fact that it could be valve seals gone bad. No CEL either so any and all suggestions or help would be great appreciated.

 

Check the PCV intake hose to see if it is clogged or kinked.

 

I've checked and replaced the valve cover because the pcv is all together with it. It still sucks extreme amounts of air through the oil cap. But yeah that was my first thought also, but I've tried and nothing. The hose is clean, Its not clogged or cracked or anything.

 

I’m not sure how it can be sucking that much air. I mean even if a valve stem seal was gone, that oil fill cap is huge, and for you to notice it sucking air thru there boggles my mind. How many miles on the truck?

 

The truck has 191k on it. Has high miles but still I’ve never worked on an engine from which sucks atmospheric air through the oil cap and dip stick. It is a flex fuel engine but i don’t think it has a sealed system like bmw’s. Also to note there was oil all in the h-pipe and the intake manifold also had a lot of oil and gas mixture inside of it probably around a quart and a half. My first thought was piston rings but the truck does not smoke with the cats off or from the oil cap and runs and drives very smooth. Just has a really rough idle and suction from oil cap. Only time it misfires is when the cats are put on. Only because they are clogged. Any thoughts on this? Essentially opening the oil cap Osbaldo cause a large vacuum leak causing the engine to stall immediatly.

 

Do a compression test, and a cylinder bleed down test.
It's cheap and easy and it will tell you the condition of each cylinder.

 

Okay I’ll try it and see what I get back. Thank you all for all the responses. If you have anymore ideas of what it could be just throw it out there.

 

I made a mistake. The air intake hose is on the other valve cover. That's the one that may be restricted.


Also ran across this:

 

The hose on the other side is not clogged or cracked but I can hear it sucking air through it. It’s really loud and the heated pcv sucks in a lot of air but if I remember correctly when I took it out I didn’t see a valve or anything inside of it. I thought that’s how it was supposed to be. But that could actually be the problem though I’ll check it out and see. Thank you

 

Hook up a vacuum gauge to the intake and get a reading and note if the needle is stable or fluctuating.

You can then look up Vacuum readings and what they mean on the internet.