Supercharger whine—Only there's no supercharger

Jeepskate98

Jeepskate98

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South Carolina
So my TJ makes a whine/ whistle between 1800 and 2400 rpm while driving... It sounds similar to a supercharger only quieter. Its possible the sound continues after 2400 rpm and I just can’t hear it over the engine roar, and only it occurs during engine load/ driving, not when parked/ in neutral. It’s noticeable while in first, second, and third gear; after third gear I believe the wind noise drowns it out. Someone told me it could be the exhaust manifold gasket?
 
Good question! It idles at around 1000. The odd thing is that once you put it in gear and start moving, it likes to stay at about 1800; for example, I often coast to a stop in neutral, but even though it’s out of gear, the rpms will stay at 1800 until the Jeep has stopped completely. Not sure if that’s normal or not...
 
Also when you slow down to first gear( like when you’re in a parking lot) , the brakes have to be applied to “force” it to idle along at say, 1200 rpms, otherwise it will won’t easily fall below 1800.
 
The high idle also indicates a vacuum leak.

There are only a few vacuum connections to the intake manifold. In park, I think (searching my limited brainal storage...) that you should be able to pull any one of these off and plug it with your finger to see if the idle goes down. If it does (consistently), then you have the base of the problem vacuum connection tree to follow out.
 
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I had a leak in mine that I only found because the vacuum-operated AC/heater controls were acting flaky. It's the small plastic line that runs from the intake, to the firewall, and across to the passenger side, running through a "T" that branches off into the AC freon lines passthrough, and on to dive down behind the ECM. It goes down from there and ends up plugging into a vacuum reservoir mounted in the space behind the wheel well. Mine had a rather nasty bite taken out of it below the ECM and I couldn't see it at all without disconnecting and pulling it up.
Admittedly, it did not seem to make a big dif in the idle because it is such a small gauge tube, but it sure as hell F'ed up the AC controls.
 
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Update: so I drove though town today and was able to hear the engine a little clearer as the sound was reflected by the storefronts. The whine continues through every gear if you are pressing the gas pedal down and the rpms are 1800 or more. Let off and the whine stops. Any ideas?
 
Jeepskate98 said:
Update: so I drove though town today and was able to hear the engine a little clearer as the sound was reflected by the storefronts. The whine continues through every gear if you are pressing the gas pedal down and the rpms are 1800 or more. Let off and the whine stops. Any ideas?
Click to expand...

Drive train contacting the tub when the engine is loaded?
 
Jeepskate98 said:
Update: so I drove though town today and was able to hear the engine a little clearer as the sound was reflected by the storefronts. The whine continues through every gear if you are pressing the gas pedal down and the rpms are 1800 or more. Let off and the whine stops. Any ideas?
Click to expand...
Do you get the whine if the vehicle us parked in neutral and the rpm is brought to 1800? If so, get a helper, park the Jeep, set the parking brake, put the transmission in neutral, have the helper bring the rpm up while you look and listen for the sound source under the hood. That should point you toward a solution or at least rule out any engine bay concerns.
 
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1997JeepTj actually it only seems to whine while it moving. The rpms have be 1800 or more and you have to be pressing the accelerator; let off the accelerator and it stops. Having said that I think you have a good idea: I’ll have a friend hold the pedal down while it’s parked and see if I can hear anything faintly under the hood. I’ll post with the results as soon as I get a chance...
 
Jjvw I will also check during this test for any body contact. Thanks for the ideas guys I’ll keep you posted... Right about now is when i could sure use a GoPro!
 
Is your accessory drive belt worn? Or not perfectly aligned? That can do it. Also if the clutch (brain freeze. 🤯Apologize for terms) front nose bearing or trans input bearing is getting loose they can whine too. Or bearing/gear wear on a certain rail in trans.
 
Update: so I did in fact find a vacuum leak...
7C81B8A1-A3B4-4CBB-8C50-99C2966422F6.jpeg
 
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I promptly replaced the vacuum tubing, which did make the Jeep run better... but it still makes an audible whistle under the above conditions. Come to find out my exhaust manifold gasket is leaking!!!😡
 
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Jeepskate98 said:
I promptly replaced the vacuum tubing, which did make the Jeep run better... but it still makes an audible whistle under the above conditions. Come to find out my exhaust manifold gasket is leaking!!!😡
Click to expand...
The pre-2000 ('97 - '99) exhaust manifolds were a bit of a bee-atch because they were one piece and tended to break because of it. Make sure it's the gasket and not the manifold itself. If it is the manifold, I have heard that the way to go is to replace with one that has accordian-type expansion joints in it like this one (just an example).
https://www.extremeterrain.com/jeep...afALJ6XQjsgnBpViQQYaAm2jEALw_wcB&gclsrc=aw.ds
 
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