Odd noise during acceleration
Thread Starter
Intermediate
Joined: Oct 2017
Posts: 27
Likes: 0
Odd noise during acceleration
The issue is with my 1986 Corvette. When I give it 50% throttle or more, under load, and only when coolant temp is above 200 degrees there is a loud scratching noise coming from the engine and a dip in power (not total power loss, just a noticeable decrease). Once I release the throttle and reapply the throttle, it does not come back. Furthermore, once the coolant temp drops even a degree below 200, it doesn't do it again. I thought it may be a belt issue, so I've replaced the belt and idler pulley. It still does it but is now throwing a check engine light when it happens. The noise itself it hard to describe. It sounds like a scratch or an electrical noise.
Thank you for taking the time to read this. I hope someone knows what this could be!
Thank you for taking the time to read this. I hope someone knows what this could be!
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,218
Likes: 1,174
From: Southlake, TX
The issue is with my 1986 Corvette. When I give it 50% throttle or more, under load, and only when coolant temp is above 200 degrees there is a loud scratching noise coming from the engine and a dip in power (not total power loss, just a noticeable decrease). Once I release the throttle and reapply the throttle, it does not come back. Furthermore, once the coolant temp drops even a degree below 200, it doesn't do it again. I thought it may be a belt issue, so I've replaced the belt and idler pulley. It still does it but is now throwing a check engine light when it happens. The noise itself it hard to describe. It sounds like a scratch or an electrical noise.
Thank you for taking the time to read this. I hope someone knows what this could be!
Thank you for taking the time to read this. I hope someone knows what this could be!
My guess is that your AUX fan is coming on at 200 and is the cause of your scream.
Oil Producer
Joined: Apr 2014
Posts: 9,372
Likes: 2,736
where is the scratching sound coming from? under hood? under the seats? rear?
edit: i see you said engine so i assume u mean under the hood.
this is a connundrum.
how loud is it?
and it only does it in gear?
edit: i see you said engine so i assume u mean under the hood.
this is a connundrum.
how loud is it?
and it only does it in gear?
Last edited by VikingTrad3r; Oct 14, 2017 at 09:57 PM.
Thread Starter
Intermediate
Joined: Oct 2017
Posts: 27
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Yeah, it's from under the hood. It sounds like the middle, its equally as loud from both driver and passenger seats. And yes, it only does it when in gear and under 50%+ throttle at 200 degree coolant temp. The only thing I can think of would be done sensor somewhere has gone bad, accounting for the check engine light. Like a engine temp sensor or knock sensor, something like that.
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,218
Likes: 1,174
From: Southlake, TX
Yeah, it's from under the hood. It sounds like the middle, its equally as loud from both driver and passenger seats. And yes, it only does it when in gear and under 50%+ throttle at 200 degree coolant temp. The only thing I can think of would be done sensor somewhere has gone bad, accounting for the check engine light. Like a engine temp sensor or knock sensor, something like that.
With this situation, I'm thinking you may have a bearing going bad on once of your spinning accessories; PS, ALT, AC, Air pump....etc. I think the 200 is a coincidence and not trigger.
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Le Mans Master
Joined: Dec 2005
Posts: 8,578
Likes: 963
From: Communist Colorado
Stick or automatic? One of the first thoughts I had was that it could be the pilot bearing/bushing for the trans input shaft or the input shaft bearing in the tranny if its a stick.... otherwise if an automatic, the torque converter bolts could be slightly loose... that will make some strange sounds as well...
Last edited by 81c3; Oct 16, 2017 at 04:21 PM.
Le Mans Master
Joined: Dec 2005
Posts: 8,578
Likes: 963
From: Communist Colorado
Cats that go bad can make noises but pretty much right inside the cat itself because the broken element pieces float around in the body of the cat...
I would say no on the EGR valve...
Last edited by 81c3; Oct 23, 2017 at 06:12 PM.
Racer
Joined: Aug 2017
Posts: 435
Likes: 22
From: Ellenton Florida
I had a Cadillac that would do the same thing but power loss kept getting worse when warm. Turned out the cat was clogged and would really clog once hot and hence the restriction under power. Puzzled many Cadillac mechanics with that one considering it was under warranty.
Safety Car
Joined: May 2001
Posts: 4,759
Likes: 1,647
From: Fairfax Virginia
A Bad motor mount?
Could it be from a broken motor mount? The part where the noise starts under loads and is intermittent is what makes me think this way.
I have seen motor mounts break and under load things started to rub where they normally wouldn't. In one case the serpentine belt was rubbing another accessory in the engine compartment. If this is the problem then you should spot some ground particulate nearby.
They are fairly easy to spot (with the hood open) as when the car is in reverse the engine will rise up off the broken mount, just be real gentle when testing it.
This is just a thought, I am probably shooting off in some other direction but it is worth checking.
Good Luck!
I have seen motor mounts break and under load things started to rub where they normally wouldn't. In one case the serpentine belt was rubbing another accessory in the engine compartment. If this is the problem then you should spot some ground particulate nearby.
They are fairly easy to spot (with the hood open) as when the car is in reverse the engine will rise up off the broken mount, just be real gentle when testing it.
This is just a thought, I am probably shooting off in some other direction but it is worth checking.
Good Luck!
Thread Starter
Intermediate
Joined: Oct 2017
Posts: 27
Likes: 0
Thank you for the suggestion! I checked out the motor mounts and they appear to be just fine. I actually think I have solved this with everyone's help. The noise has recently gotten MUCH worse with the cold weather; now I can hear it at idle. But, since I can hear it at idle, I've narrowed down the problem to a bad A.I.R. pump! I confirmed it with a stethoscope. My next step is to replace or delete it. I'm leaning towards replacing it just in case I want to move to somewhere with vehicle inspections (we don't have them here in Oklahoma, as of now).
Safety Car
Joined: May 2001
Posts: 4,759
Likes: 1,647
From: Fairfax Virginia
Congratulations!!
Thank you for telling us the outcome of the problem you were experiencing!
We ALL learn when you post with the actual problem identified!
Enjoy that beautiful car now that you have solved a challenging issue!
It is good to know that COLD weather is good for something!!
We ALL learn when you post with the actual problem identified!
Enjoy that beautiful car now that you have solved a challenging issue!
It is good to know that COLD weather is good for something!!
Last edited by ctmccloskey; Oct 27, 2017 at 12:49 PM. Reason: added a thought
This helps when an OP posts up the solution. many years from now, some poor guy wondering what that noise is, might get the answer right here! 