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Can Opeler
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3,462 Posts
Discussion Starter #1
So I was driving my 1970 Opel GT the other day and suddenly a piercing high pitched squeal came out from under the hood. It sounded like slipping belts to me so I replaced my belts and tightened up and lubed them. Unfortunately it still squealed. The sound seems to be coming from the passenger side of the engine near the thermostat, but below the carb. I removed my carb and heat shield and replaced the old gaskets and turned her back on... guess what it still squealed.
Enough backstory, here are the dynamics on the squeal:
1. It seems to not be effected by rpm.
2. When the throttle is depressed the sound gets less intense and at about half throttle it seems to disappear regardless of RPM
3. It seems to be effected by temperature, when the engine has been off for over 24hrs the squeal will not be heard upon start-up or the first about 10 minutes of driving.
4. The pitch of the squeal doesn't seem to change very much to any stimuli it just stays the same or gets quieter...
So does anybody have any idea what's happening to Mira, (my Opel) I personally think it is an intake leak at gasket to the engine.
 

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Über Genius
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9,354 Posts
So I was driving my 1970 Opel GT the other day and suddenly a piercing high pitched squeal came out from under the hood. It sounded like slipping belts to me so I replaced my belts and tightened up and lubed them. Unfortunately it still squealed. The sound seems to be coming from the passenger side of the engine near the thermostat, but below the carb. I removed my carb and heat shield and replaced the old gaskets and turned her back on... guess what it still squealed.
Enough backstory, here are the dynamics on the squeal:
1. It seems to not be effected by rpm.
2. When the throttle is depressed the sound gets less intense and at about half throttle it seems to disappear regardless of RPM
3. It seems to be effected by temperature, when the engine has been off for over 24hrs the squeal will not be heard upon start-up or the first about 10 minutes of driving.
4. The pitch of the squeal doesn't seem to change very much to any stimuli it just stays the same or gets quieter...
So does anybody have any idea what's happening to Mira, (my Opel) I personally think it is an intake leak at gasket to the engine.
Vacuum leak on the hose that goes to the power brake unit.
There should be a "valve" inline on the hose. I think it's plastic.
 

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tomking
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2,483 Posts
I would go to the alternator. I thought my high pitched squeal was a bearing on alternator. But it turned out to be the fan on alternator was dragging against the alt housing. Sounds like alternator to me.
 

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Just Some Dude in Jersey
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15,120 Posts
I had that high pitched whistle-like sound happen due to a vacuum leak a while back. It would rise nand fall with engine speed and disappear as the rpms went up. Like yours, it would disappear when the throttle was open because your vacuum drops to almost nothing when you hit the gas.

Try spraying some carb cleaner around the carb, intake/exhaust manifolds, and hoses while the engine is running and making the sound to isolate where it's coming from. You can also use a length of rubber as a makeshift stethascope to zero in on where the sound is coming from.
 

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Über Genius
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9,354 Posts
If the OP is accurate, the pitch doesn't change with RPMs, only the volume. That rules out bearings or alternator fan drag.
Pitch is dependent on a wave. A whistle isn't. Whistles retain their pitch no matter what volume they are given.
A whistle indicates an air issue. Since the engine can't really whistle with air pressure, we can safely assume it's a vacuum issue.
 

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Can Opeler
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3,462 Posts
Discussion Starter #7
I know it's not the water pump, or alternator. I need to check my vacuum to the brake booster. Where does the vacuum tube connect for the brake booster? Also can a vacuum leak be loud enough to hear going 60 down the highway with the windows up and the radio on? This squeal is incredibly loud.
Thank for help so far. You guys are all very helpful.


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Just Some Dude in Jersey
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15,120 Posts
Well, actually, when I heard you say "high pitched squeal" I immediately thought about the noise that the engine that my GTX car had when I first got it. It started making a really high pitched squeal after running for about 10 minutes. I later gave away/sold the engine and the new owner discovered nuked cam bearings. The likely scenario is that the PO who regasketed the engine hadn't primed the oil pump and no oil was getting to the head.

This seems unlikely, based on what you've said, but who knows........
 

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Can Opeler
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3,462 Posts
Discussion Starter #9
Ok, it's not the brake booster vacuum line (the big line with the valve on it attached to the intake) the squeal is coming from the other side of the carburetor. My Opel also as the ARA unit on it, but no belt on it if that helps anything


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Can Opeler
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3,462 Posts
Discussion Starter #10
Here's an update. I was wrong the pitch actually goes lower when I accelerate (open throttle) but if I shift down and raise rpms it has no effect


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Can Opeler
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3,462 Posts
Discussion Starter #12

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Can Opeler
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3,462 Posts
Discussion Starter #13 (Edited)
Video of the squeal

Here's a link to the sound of the squeal. Opel Gt Squeal - YouTube
Please remember it does this without the belts on, so it isn't the water pump, alternator, AC, Fan, etc...
 

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Registered
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11,747 Posts
Kyler
Once I left a small piece of the old gasket on the intake.
Then put a new gasket over it...

The sucker whistled under high vacuum..gone with low vacuum ..throttle wide open.
 

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Can Opeler
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3,462 Posts
Discussion Starter #17

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Can Opeler
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3,462 Posts
Discussion Starter #18
Brake out the can of starter fluid and find the vac. leak, just hope the intake manifold isn't cracked :ugh:
This is the interesting part, I've drenched that whole area with WD-40 and starter fluid. There's no effect.


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Bikini Inspector
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5,771 Posts
take carb off and try again, if noise goes away it is somewhere in the carb, either carb gasket, or throttle bushings are leaking.

get out the 15mm box end wrench and try to torque down the intake bolts a little bit.

yes I mean where intake bolts up to engine. are you using this gaskets or rtv silicone for gaskets? I do not have much experience with using a heatshield but be careful with how good the seal is.

Also there is the T coming off intake where brake booster hose hooks up, there should also be another small tube for hooking up to the valve cover. there might also be another fitting on opposite side of intake for vacuum retard on distributor. look for any of these to be not hooked up.

these fittings are dependent on year. might all be on the T or 2 and 1. In your video at like the 7 second mark, you show the gap between carb and valve cover. the black hose you see should be going to the orifice fitting on the T, in the same frame, you can see a cutout in the heat shield, this is usually for the other vacuum line. this will be where other fitting sometimes is on the intake.
 

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Über Genius
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9,354 Posts
OK, so let's narrow this down a little.

It's NOT a leak between the intake and head.

So it's either a cracked intake manifold or a leaky carb gasket or a defect in the vacuum hose routing or a bad carb.

What on the carb could do this? How about the shaft that the butterfly rides on?
OK, not likely.

What IS likely is that the carb mating system is leaking.

You should have two gaskets and a heat shield between the carb and the intake.

It IS likely that it's leaking there.

Now, how do we know this? Easy. The whistle is on EVERY piston stroke which indicates it's connected to a central system. The only central system that makes sense is where the carb mates to the manifold.

You will be doing yourself a favor to check each of the four carb mounting nuts to see that they are snugged down. And possibly give each one a little extra tightening to see if the results change at all.
 
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