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SadZGuy

78 280z very loud screeching at idle

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Hello all, I have a 280z I recently got running but with some problems. Whenever I start the car there is a very loud screeching noise coming from the engine bay. I tried to detect where it is coming from but it is literally so loud that I cant even locate it. 

Since I cant locate it through means of sound localization I decided to do some other troubleshooting. For starters my alternator and water pump are new so I know its not either of those pulley bearings. (no ac on this car so I only have 1 belt). First I took off the fan clutch even though I didn't suspect it, and the noise still happens. After that I just took off the entire belt and the noise still happens..... Usually these kind of sounds come from the belt or pulleys so I checked every pulley for lateral play or grinding noises but everything was solid. So its definitely nothing attached to the belt or the belt itself. After finding this out I'm led to believe its maybe some sort of vacuum leak. Almost like there is a very large amount of air being squeezed through a very tine hole just like a whistle. That being said I inspected the engine bay for any apparent vacuum leaks but it looked clean.  I'm also pretty sure its not a clutch problem as the noise is strong from the engine bay. As you will be able to see in the video, I can rev out of the squeak if I push the throttle enough but then the screech comes back when the engine falls down to idle speed. I'm stuck here and don't have any idea what this is. HELP?!?!?

Thanks.

P.S. sorry for the vertical footage.

https://youtu.be/o9nhWS6Y2Vk

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Has the engine had any work done on it?  Always good to know the history of a car/engine.

Your video was with the belt on.  Should have done one without it, it will be fine for a minute or two.

I'd guess something in the damper pulley area.

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6 minutes ago, Zed Head said:

Has the engine had any work done on it?  Always good to know the history of a car/engine.

Your video was with the belt on.  Should have done one without it, it will be fine for a minute or two.

I'd guess something in the damper pulley area.

The engine is totally stock, here's the video without the belt. Trying to get this solved quickly, and I VERY MUCH appreciate your help.

https://youtu.be/aS0lwGR5rkY

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33 minutes ago, Zed Head said:

Has the engine had any work done on it?  Always good to know the history of a car/engine.

Your video was with the belt on.  Should have done one without it, it will be fine for a minute or two.

I'd guess something in the damper pulley area.

Another thing I noticed from the video is that the second the throttle is pushed down then the noise temporarily stops. Is there some kind of whistle coming from the throttle body when the flap is slightly open and then when its fully open the noise goes away because there is no more small gap/whistle?

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When you open the throttle intake manifold vacuum drops.  Less air would get pulled in through the hole, if it is a gap or hole, due to lower vacuum.

The BCDD is a good place to look.  It's under the throttle body.

The crankcase is under vacuum also, from the PCV system.  So any hoses that are involved with the PV could be leaking.  Really, any hose on the engine, that's not a water hose is a vacuum hose. Get a spray bottle with water in it and spray hoses while it's squeaking.  Should change pitch if it sucks in some water.  Won't hurt the engine.

Edited by Zed Head

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1 hour ago, Zed Head said:

When you open the throttle intake manifold vacuum drops.  Less air would get pulled in through the hole, if it is a gap or hole, due to lower vacuum.

The BCDD is a good place to look.  It's under the throttle body.

The crankcase is under vacuum also, from the PCV system.  So any hoses that are involved with the PV could be leaking.  Really, any hose on the engine, that's not a water hose is a vacuum hose. Get a spray bottle with water in it and spray hoses while it's squeaking.  Should change pitch if it sucks in some water.  Won't hurt the engine.

 

1 hour ago, Dave WM said:

smoke the intake, look for leaks

Currently unable to do those tests right now but I will get to them tomorrow. I'm a little skeptical that it's a vacuum leak because the car idles at 500 rpm and I feel like if it was a vacuum leak the idle would be faster. However, I'm not an expert so I guess the possibility still remains.

 

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3 hours ago, SadZGuy said:

The engine is totally stock,

How about oil pump self-destructing?  Or maybe the distributor shaft is dry.

And, the quote above does not tell anything at all about the history of the engine.  Happy to throw random guesses at you though.

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Not sure why it would do that, but I think it's just the idle air whistling.

What happens if you close off your idle air screw completely? You might have to keep it running by manually holding the throttle open a little bit while testing. The reason I ask is because I've messed around with a couple different throttle body options and one of them would whistle like that if the idle air passage was being worked too hard.

I made the noise go away by opening the throttle plate a little, which allowed me to close down the idle air passage. More air past the butterfly and less through the idle bypass and it quieted down.

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17 hours ago, Captain Obvious said:

Not sure why it would do that, but I think it's just the idle air whistling.

What happens if you close off your idle air screw completely? You might have to keep it running by manually holding the throttle open a little bit while testing. The reason I ask is because I've messed around with a couple different throttle body options and one of them would whistle like that if the idle air passage was being worked too hard.

I made the noise go away by opening the throttle plate a little, which allowed me to close down the idle air passage. More air past the butterfly and less through the idle bypass and it quieted down.

So I closed the air bypass all the way and it started right up and idled.... So this confirms that it a vacuum leak.(vacuum leak is acting as idle air bypass). After finding this out I started looking around and found an injector O ring laying on the intake manifold. So I started it up and used some starting fluid to spray the injector area and the rpms rose like it was being revved. So I guess this is 100% a vacuum leak but I'm still confused as to why it whistles THAT LOUD, and why my rpms are only at 500 during idle. could it be because the injector vacuum leak is not enough air for a steady idle, therefore giving the effect of having the idle air bypass screwed down too tight?

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Alright just an update for anyone who wanders into this forum topic with the same problem. There WAS a vacuum leak at the hose connecting to the aux air reg, I still have literally no possible idea how that leak made such a loud whistle, my ears would actually start to ring if I sat too close to the engine for too long. Anyways, I tightened down the hose and the whistle eventually died out. This then allowed me to set the idle with the screw to 700. So now the car runs like an absolute champ, thanks for the help guys.

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Glad you quieted it down and got it running better. So you didn't mention it, but I'm assuming you also fixed the injector O-ring leak while you were in there?

Yes, vacuum leaks large and small are the cause of many of the problems with the fuel injection system.

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On 10/27/2020 at 8:48 PM, Captain Obvious said:

Glad you quieted it down and got it running better. So you didn't mention it, but I'm assuming you also fixed the injector O-ring leak while you were in there?

Yes, vacuum leaks large and small are the cause of many of the problems with the fuel injection system.

Yep, O-ring is back and now it runs like a top.

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