Strange place for a vacuum leak....

My XJ has had a hissing whistle, sounds exactly like a vacuum leak. After doing some wrenching this weekend, it turns out that by squeezing the electrical plug on the #6 (rearmost) fuel injector, the hiss disappears. WTH is that all about?? There should be zero vacuum to that connection....



I also pulled the plug off, cleaned and re-filled with di-electric grease....no change.:smsoap:

It's probably the fuel injector. It's either cracked (hairline) or at the threads. Since they are mounted in the intake manifold and as you squeeze on the connector, you're probably moving it just enough to close the leak off.

Or the manifold bolts - pressing on injector will push on the manifold at the injector boss, wiggling it just enough to change the pitch of the hiss.

The lower O-ring may also be a source as it's on the vacuum side of the throttle body, and more likely.

happened to me also, it was the intake manifold gasket!


my hissin went away when i pushed down on the throttle body

One way to check is to spray brake cleaner around the gasket. If the engine starts faultering you have a leak.