over the last few months I have gotten a loud hissing sound coming from the pasenger side. It is RPM dependant.

Im also having a hard time going up hills, although the trans shifts fine and the fluid level is good.
any thoughts?

2008 F250 V10 Lariat, 124K

 

Broken exhaust manifold studs? If it's sucking air in, and yes, it can actually suck air into the exhaust under load, it'll throw the air-fuel ratio off according to the O2 sensor, and run way rich to make up for the extra oxygen.

 

Vacuum leak will indeed cause the hissing... I have had one before... not exhaust manifold though. I also lost some power with the leak.

Good luck tracking it down and let us know what you find.

James

 

Duh, don't know why I didn't think vacuum leak, good call DSN46

 

Thanks Krewat!
I just read my post again and I absolutely was not suggesting that it could not be the exhaust manifold (cause it very well could be).... I was just saying that the manifold was not the problem in my particular case.
Hope you understood what I meant...

James

 

I understood perfectly, no worries

 

cats are plugged

 

Check injectors and for manifolds leaking so replacement cats do not fail also.

 

Sounds like your PCV valve, I had the same exact problem and it was like an $8 fix