2000 XJ Sport, 4.0 auto tranny.

I was test driving it yesterday and noticed that there is a high pitched poise coming from the engine. The moment you give it throttle it fades out for a second or two then comes back. The CEL is one but the owner says it was for an o2 sensor that hasn't cleared yet. Until I put a reader to it myself I won't know. But could the noise be a vacuum leak? And if it is, could it be throttle related since it goes away for a second then comes back when you push the throttle?

 

Could you describe the noise a bit more? A high pitched noise can come from multiple areas, but the type of noise can help narrow the search.

For example, a high pitched squeal could be something like a loose belt, or maybe a spun bearing

A high pitched whistle could be another story. Did the idle seem high, or did it experience RPM surges while you drove it?

 

More like a whistle really. Idle did seem a little high when I first started it up. But nothing super ridiculous.

 

A great way to seek out a vacuum leak under the hood is to get a can of ether and spray areas where vacuum plays a big role. If the engine suddenly surges, you've found the leak. You can also use water, but I've found this method less reliable. Id be looking at vacuum connections for cracks and looseness as well.


Another thing worth checking out may be if the air filter housing is completely sealed. I had a light whistle on mine when I picked it up, found that, and solved it just by closing it properly.

 

yeah, its pretty noticeable and sounds close to the firewall. it goes away for a second when you give it gas but comes right back. I've done the ether thing also, but I've never heard one make that noise though.

 

Do you hear it when you crank the engine over before it starts running?

 

I fixed a friend's bravada (blazer) once that had a vacuum leak in the HVAC controls. More or less what you described, it was a high-pitched hiss that would drop out at higher rpm.

 

Crash Course has a point there. There are a few vacuum lines tied into the HVAC system. if one of those hoses fell off, or there is a leak in the casing or lines, that could cause the same symptom

 

didn't hear it when i first started it up. after about 5 mins into driving it i noticed it.

 

Also the actual valve that redirects the vacuum when you move the lever... Which was the issue with that Olds. The plastc clip holding the two halves of the valve together had broken. I haven't had to take apart an XJ HVAC control yet, but there's always that possibility as well.

 

I have a 2000 xj sport 4.0l manual transmission that does the same thing with the whistle/high pitch noise.

 

Another thing that works to locate a vacuum leak is a propane torch.
Turning it ( un-lit of course !) on and passing it over suspected leak areas will usually make the idle speed pick up a bit, if there is a leak.......

Also, WD40 works too....