I have a 2005 Jaguar STR with 75k miles and with the hood open I can hear a hissing that sounds like a vacuum leak. I know it’s very hard to pinpoint a vacuum leak and was wondering if there is a common area for these supercharged 4.2’s leak to give me an idea of where to look. It seems like it is on the passenger side (US). I picked up some carb cleaner and have sprayed around a bit but nothing really made a difference in idle.

Thanks!

 

Check fuel trims to see if it even is a leak.

 

I hooked up my scanner last night on the drive home. It’s my first time doing OBD diagnostics so bear with me. What would be normal for the short term and the long term fuel trims?

Thanks!

 

Just post the numbers that you are getting both at idle and at 2500 rpm.

 

Might be the ufo shaped breather mechanism....they have a hiss all the time...it is more pronounced when the engine cover is off...the corrugated plastic hose can split over time...after you check trims, an easy test is to detach the corrugated hose from the ufo breather and plug the hose with something...a cork or rubber plug works great...then listen again for the hiss...the ufo breather is not a pcv valve as much as a slight restrictor so as not to suck too much oil from the crankcase....there is a high vacuum there at all times because the other end of the hose goes into the throttle body and not the intake manifold as regular pcv systems do....i had the same concern on my 06 str and ruled out leaks by trim values being good and physically checking all vacuum related lines and connections

 

My 2003 had a mystery vacuum leak that I could hear but not pinpoint. It turned out to be the brake booster.

 

Was gone for the weekend but thank you all for the responses. Here are my findings. This is after a 30 minute drive into work. Idle was bouncing from -0.8 to 3.1ish and and 2500 was staying around -1.6. Long term in photos as well. Not sure how to decipher but from a little reading it doesn't seem much for concern?


Idle

2500 RPM

 

Those LTFTs mean no leak.

Which leaves some other cause f the hissing but I can't guess what!

 

Well that's good news, or at least I hope so!

 

Great trims.,.what you are hearing is probably the natural hiss of the ufo breather valve.....

 

After a few days of watching trims and listening to the UFO breather I think that must be what I am hearing. Thanks for the help!

 

deleted

 

That shitty check valve on the booster costs $4.59 at O'Reilly's. What a pain in the butt to put that on but that was what was leaking on mine too.