Whistling Noise Coming from Engine

edgregorio

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Hello,

I have a 1990 Jeep Wrangler YJ with EFI that has 112,000 miles on it. For the past month, I am hearing a tea-kettle whistle sound coming from the engine. It sounds like it is coming from the exhaust manifold or EFI unit. Anyone else experience something similar with their Jeep? How do I go about diagnosing the source of the whistle noise?

Thanks for your help.

Ed
 
Exhaust manifold leak that is possibly accompanied by a restricted cat converter.
 
Hello,

I have a 1990 Jeep Wrangler YJ with EFI that has 112,000 miles on it. For the past month, I am hearing a tea-kettle whistle sound coming from the engine. It sounds like it is coming from the exhaust manifold or EFI unit. Anyone else experience something similar with their Jeep? How do I go about diagnosing the source of the whistle noise?

Thanks for your help.

Ed
Just about every 4.0 and 2.5 Jeep i've been around has whistled to one extent or another. I'd assume vacuum leaks.
 
I have a 1990 Jeep with EFI and experienced the exact same issue on my 4.2L in-line 6. In my case, the whistle began during colder ambient temperatures but grew worse over time (months). Also, it only occurred at or near idle when the intake manifold pressue is at its lowest (greatest vacuum). I won't trouble you with all the troubleshooting I did, but will go to the root cause and fix. The exhaust and intake manifolds are VERY close together, even sharing the same hold-down studs/nuts, resulting in high temp variation on the intake manifold. I suspect that if you examine the rearmost attach bolts/nuts, you will find them loose. In my case, the most aft bolt was finger loose. This releases the intake manifold hold-down force, allowing air to get sucked into the manifold past the intake gasket, happily whistling as it does so. Throttle to idle, a-whistling we go.

IF (a big IF) you catch it early enough, you might be able to re-torque those rear manifold attach bolts and save the gasket. In my case, both the intake and exhaust gaskets had already experienced blow-by damage, so tightening did not solve the problem, at least, not for long.

The permanent fix was to remove the intake and exhaust manifolds. Resurface the intake flanges nice and flat. I opted to replace the exhaust manifold. Get new gaskets, reassemble and carefully torque all the bolts in the correct order and torque value. While you have it apart, pay close attention to the cylinder head side at the manifold attachments and make sure that that surface has not been compromised.

A couple other things to consider...

1.) This condition results in the rear cylinder(s) running lean, something you might notice when examining the spark plug(s).

2.) VERY IMPORTANT! When the job is done and you are whistle free, you need to periodically get a long ratchet extension and tighten/re-torque those rearmost manifold retention bolts/nuts. How often depends on your vehicle usage. With newly resurfaced or new parts, the heat/cooling cycle allows those particular bolts/nuts to "work" loose. I checked mine every week or two and found them to be initially loosening, but not leaking. After a couple months, the bolts tended to remain tight... the seating and stretching had "set", if you will. But still after a couple years now, I still periodically go in and check the torque on ALL the manifold bolts.
 
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Something I've mentioned before here and found interesting about when I had both intake and exhaust manifolds off my '92 was there was no OEM gasket material for the exhaust. Straight metal to metal.

The new gasket covered both intake and exhaust ports. To properly fit the two manifolds the factory made the exhaust flange very slightly thicker than the intake so the bolts and washers would evenly apply pressure to the top of both flanges. Simply using a flat washer may clamp hard on the exhaust flange but the intake flange on the other side of the bolt may not be as tight. There are special cupped washers or football washers to clamp evenly on both flanges. There's also extra thick gaskets to help make up the difference and that's what I used.

Without them I've read about several intake leaks. It can come down to the gaskets and washers and who did the installation.

1w.jpg
 
Flyer58 - Excellent and important point. Those "cupped" washers are essential and they only work if torqued to the proper value. Over-torquing to the point of flattening these things can actually cause intake leaks (or break/strip the studs!). Great photo, by the way.
 
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