01 XJ 4.0L Whistle

Ed Bacon, Jr. did pass the time by typing:

With the 4.0 that's going to be 99% throttle body whistle and not the brake booster. However, check the hose between the bosster and the intake rail to see if it's cracked. Might be flexing under acceleration. Clean your TB.

With the TB closed your at max vac for idle.

That could also be a crack in the intake plastic or even a shrinking gasket under the TB. With the engine running spray some TB cleaner around the TB's base and puff some by the booster and it's hose. if the engine stammers you found a leak.

On my 93 GC with the 4.0L engine I also had a loud whistle from that area and it was the short rubber hose (4") between the plastic pipe coming from the PCV valve and the intake manifold. The hose on mine was pretty soft and it would collapse under vacuum and when cold. The whistle would go away if I moved the hose even slightly. I don't know why it only happened in cold weather, I would expect the hose to be stiffer due to the cold weather. I replaced it with a new hose and haven't had the whistle since. If I recall I had to find the right type hose since fuel line type of hose was too stiff. It took a few trips to parts stores find the correct vacuum type hose.

Walt B

Ed Bac> I have an `01 XJ 4.0L that whistles LOUD in the vicinity of the brake

Walt Barie did pass the time by typing:

Actually it's a CCV, not PCV and the whole assembly is only about

12$ from the dealer. I've got the plastic hardline here, on mine the rubber adapter is what gave way.