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Well as the title says I have a high-pitched wheezing sound under the hood. It is constant while parked and RPM has little to do on it. When you accelerate it tends to go away but once the load off the engine is gone it comes back.

When you shut the car down you can feel the engine completely stop (no more mechanical movement) and for approx. 1 Second the sound stays and then dies down to nothing.

This sounds like something hydraulic and I am curious if anyone has experienced this before? I'll be getting the car into our shop later this evening to pull the serp belt and such but is there anything else I should check? I'm actually concerned that this could be a trans problem although the car shifts beautiful and the fluid seems good. Are there relief valves that could be an issue?

Thanks

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....Are there relief valves that could be an issue?
There is a circular manifold pressure relief valve on the RH side of the intake manifold directly inboard of the power steering pump pulley. The gasket is known to leak.

The resulting noise is more like a whistle.

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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May also be a vacuum leak. Maybe a split or cracked hose. Power brake booster diaphrapm leaking might also cause a hissing sound.

Chuck

'17 XT5, '04 Bravada........but still lusting for that '69 Z-28

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Well as the title says I have a high-pitched wheezing sound under the hood. It is constant while parked and RPM has little to do on it. When you accelerate it tends to go away but once the load off the engine is gone it comes back.

When you shut the car down you can feel the engine completely stop (no more mechanical movement) and for approx. 1 Second the sound stays and then dies down to nothing.

This sounds like something hydraulic and I am curious if anyone has experienced this before? I'll be getting the car into our shop later this evening to pull the serp belt and such but is there anything else I should check? I'm actually concerned that this could be a trans problem although the car shifts beautiful and the fluid seems good. Are there relief valves that could be an issue?

Thanks

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OMG!!! I am having the same issue. I took it to a mechanic and he said I would need a new catalytic converter, but, I have read all over the internet and I agree with the gasket leak, the circular manifold pressure relief valve that was mentioned on the other post made. Thanks to all. Hope this helps

Edited by sarakw806
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Well the relief valve issue would make sense. I listened around the other night and it is a deafening sound that makes you want to rip your hair out when leaning over the P/S pump. This relief valve; is it under the pretty magnesium manifold cover?

Do the gaskets on these tend to dry out or break? Just wondering if I should pick up a new one before I start working on this pig.

Thanks for all the responses! I was leaning more to the TC before, because I've heard noises similar to this being created from other cars' TC's but this car is in pretty rock solid condition and the sound is most definitely coming from the top of the motor and not underneath.

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Well the relief valve issue would make sense. I listened around the other night and it is a deafening sound that makes you want to rip your hair out when leaning over the P/S pump. This relief valve; is it under the pretty magnesium manifold cover?

Do the gaskets on these tend to dry out or break? Just wondering if I should pick up a new one before I start working on this pig.

Thanks for all the responses! I was leaning more to the TC before, because I've heard noises similar to this being created from other cars' TC's but this car is in pretty rock solid condition and the sound is most definitely coming from the top of the motor and not underneath.

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I am going to have them a muffler shop install a new catalyst converter about 200.00 in Texas and I think if that part is not releasing the exhaust or whatever, it could be compression some type of pressure then adding to this sound. Good luck I will keep you posted.

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I am going to have them a muffler shop install a new catalyst converter about 200.00 in Texas and I think if that part is not releasing the exhaust or whatever, it could be compression some type of pressure then adding to this sound. Good luck I will keep you posted.

Please do keep us posted because I am not able to follow the logic that leads you to replacing a catalytic converter.

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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I am going to have them a muffler shop install a new catalyst converter about 200.00 in Texas and I think if that part is not releasing the exhaust or whatever, it could be compression some type of pressure then adding to this sound. Good luck I will keep you posted.

If you have a bad catalytic converter... it will set codes.

No need to replace it if it isn't bad...

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I am going to have them a muffler shop install a new catalyst converter about 200.00 in Texas and I think if that part is not releasing the exhaust or whatever, it could be compression some type of pressure then adding to this sound. Good luck I will keep you posted.

Please do keep us posted because I am not able to follow the logic that leads you to replacing a catalytic converter.

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Well, I took it to a repair shop and the man I know him, not a stranger, said that it was the catalytic converter, the code it pulled now is PCM 0420

That is the correct code for a bad converter...

P0420 - Catalyst System Low Efficiency

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Well, I took it to a repair shop and the man I know him, not a stranger, said that it was the catalytic converter, the code it pulled now is PCM 0420

That is the correct code for a bad converter...

P0420 - Catalyst System Low Efficiency

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So that's why I'm taking to get it done. lol Or is there something else I can do?

Not at this point...

The P0420 code says you need one.

Get it replaced and then see if there are any other problems and report back and we will try to help.

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So that's why I'm taking to get it done. lol Or is there something else I can do?

The something else I would do is:

Get on a stretch of Interstate highway

Run the vehicle at the speed limit in 3rd gear (even better would be 2nd gear) for 30 minutes

Your P0420 is set by the PCM based on the activity of your O2 sensors. And if your O2 sensors have been subjected to urban short trip stop and go driving, they "might" benefit from a thorough heat-soak and high velocity exhaust gas experience. And the above procedure will certainly do that.

Plus - If the above did not resolve the code, I would do some serious diagnosis of the O2 sensors before I butchered the exhaust system to replace a catalytic converter.

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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So that's why I'm taking to get it done. lol Or is there something else I can do?

I retract my first answer...about going ahead and replacing the converter.

JimD..in the post above this one is right.

Do as he suggested first, before replacing the converter...

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I am going to have them a muffler shop install a new catalyst converter about 200.00 in Texas and I think if that part is not releasing the exhaust or whatever, it could be compression some type of pressure then adding to this sound. Good luck I will keep you posted.

If you have a bad catalytic converter... it will set codes.

No need to replace it if it isn't bad...

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So that's why I'm taking to get it done. lol Or is there something else I can do?

The something else I would do is:

Get on a stretch of Interstate highway

Run the vehicle at the speed limit in 3rd gear (even better would be 2nd gear) for 30 minutes

Your P0420 is set by the PCM based on the activity of your O2 sensors. And if your O2 sensors have been subjected to urban short trip stop and go driving, they "might" benefit from a thorough heat-soak and high velocity exhaust gas experience. And the above procedure will certainly do that.

Plus - If the above did not resolve the code, I would do some serious diagnosis of the O2 sensors before I butchered the exhaust system to replace a catalytic converter.

I am wondering if I should go the speed limit on 2nd or 3rd or will it matter? Thanks

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So that's why I'm taking to get it done. lol Or is there something else I can do?

The something else I would do is:

Get on a stretch of Interstate highway

Run the vehicle at the speed limit in 3rd gear (even better would be 2nd gear) for 30 minutes

Your P0420 is set by the PCM based on the activity of your O2 sensors. And if your O2 sensors have been subjected to urban short trip stop and go driving, they "might" benefit from a thorough heat-soak and high velocity exhaust gas experience. And the above procedure will certainly do that.

Plus - If the above did not resolve the code, I would do some serious diagnosis of the O2 sensors before I butchered the exhaust system to replace a catalytic converter.

I am wondering if I should go the speed limit on 2nd or 3rd or will it matter? Thanks

If you have freeways around you .. I would probably use 3rd...

Speed limit around here on a lot of the freeways is 70MPH.

In my car...2nd gear at 70 MPH is over 5000RPM...

In my opinion...that is too much RPM for running it 30 minutes.

For a long 30 minutes run... I would use 3rd gear...

If you are running at 50 MPH or so... use 2nd...

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So that's why I'm taking to get it done. lol Or is there something else I can do?

The something else I would do is:

Get on a stretch of Interstate highway

Run the vehicle at the speed limit in 3rd gear (even better would be 2nd gear) for 30 minutes

Your P0420 is set by the PCM based on the activity of your O2 sensors. And if your O2 sensors have been subjected to urban short trip stop and go driving, they "might" benefit from a thorough heat-soak and high velocity exhaust gas experience. And the above procedure will certainly do that.

Plus - If the above did not resolve the code, I would do some serious diagnosis of the O2 sensors before I butchered the exhaust system to replace a catalytic converter.

I am wondering if I should go the speed limit on 2nd or 3rd or will it matter? Thanks

If you have freeways around you .. I would probably use 3rd...

Speed limit around here on a lot of the freeways is 70MPH.

In my car...2nd gear at 70 MPH is over 5000RPM...

In my opinion...that is too much RPM for running it 30 minutes.

For a long 30 minutes run... I would use 3rd gear...

If you are running at 50 MPH or so... use 2nd...

Okay, well then I am getting ready for my test trip, the bad thing is that fuel is 3.75 right now, ouch! but I know it could save me $$$$$$$$ Thanks I will post the findings. In addition, they say my Caddy may have not 2 but, maybe 3 cats, ouch 200.00 a pop

Edited by sarakw806
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....In addition, they say my Caddy may have not 2 but, maybe 3 cats, ouch 200.00 a pop
I have no clue who the "they" is but your car has one catalytic converter. You do have three O2 sneors. Maybe a terminology problem??

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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....In addition, they say my Caddy may have not 2 but, maybe 3 cats, ouch 200.00 a pop
I have no clue who the "they" is but your car has one catalytic converter. You do have three O2 sneors. Maybe a terminology problem??

They are the shop I took it to, said it could have 2 or maybe 3 converters and then it would cost 200.00 each, wow, some dishonest repair shops do target people.

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