Loud whistling from the engine

I've got a 2001 Cherokee Sport 4WD...

Can somebody help me figure this out? I have a high-pitched whistling sound coming from the engine that appears when the engine warms up. It goes away when I'm pushing on the accelerator to speed up but comes back when I let up off the pedal. I also have an error code (I can't remember the code number) that tells me that one of my cylinders is experiencing a too-rich condition. I was told that the problem was a vacuum leak somewhere on or near the intake manifold that is causing the rich condition to one of the cylinders. Can anyone tell me of a likely place to start looking or just rip the whole manifold off and replace all the seals? How does one find a vacuum leak?

Get yourself a can or two of starting fluid (there are other fulids that will work but this is my preference). Start the engine and let it get to temp. Now start spraying fluid around the mounting flanges of the intake. If your engine wants to cut out when you hit a particular spot, then you found your leak. Keep spraying around all suspect areas. Any drop in RPM will indicate a leak.

Hope this helps.

I'll give that a try! Thanks! It makes sense... is there a common reason for this? Meaning is it usually the manifold, or an injector, or some seal that commonly fails on these Cherokee's?

It's really going to depend on where you find the problem. Just as an example, lets say you found the leak by a manifold bolt. It COULD have been that at the factory that bolt was tightened on the loose side of the torque setting. So, with expansion and contraction of the metal over the years, that spot became the weak link in the gasket chain. Eventually, the gasket deteriorated allowing a gasket leak. I could list lots and lots of "supposes", but that will give you an idea.

Before you run out to the store, note that I find that carburettor cleaner works better (and is safer, I think) than starting fluid - ether can be nasty....

On inline sizes, it it actually fairly common for manifold bolts to work loose over time - especially when both manifolds are on the same side of the head. Aluminum expands twice as fast as iron/steel when heated, and this will contribute to a "turning" action on the bolt. Eventually, this will loosen the bolt until it's backed out far enough to cause a leak, and backed out far enough that it can't be pushed anymore (usually half a turn to a full turn.)

It's also possible that you've got a leak somewhere else - like a throttle body bolt loosened by vibration, or a Nylon manifold line has cracked. So, when you do the "fog test," make sure you do it EVERYWHERE there may be a vacuum leak - mounting surface of the intake, base of the throttle body, ALL Nylon vacuum lines, ...

I have found that, by retorquing the manifold bolts once a year, there will usually be one or two loose (more than that if the OEM bolts are still in place.) When I replace the gasket, I get new Belleville spring washers (those little "dishes" under the bolt heads) and new silicon bronze bolts (which are better for this application than steel) and secure them with LocTite #272. The most I've had to tighten anything since I've replaced a couple gaskets has been about 1/6-turn. It was almost a full turn before, with the steel bolts.

5-90