NPetts

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!

billschroeder5842

You have a couple of things going on that I can't seem to relate.


My guess is that your AUX fan is coming on at 200 and is the cause of your scream.

NPetts

That was my thought, but I looked up the auxiliary fan operating temp and it shouldn't kick on until 225. It also doesn't do it at idle or revving without load. That's why it's so perplexing!

VikingTrad3r

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?

NPetts

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.

Dt86

use a paper clip and see what code is stored.

billschroeder5842

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.

NPetts

I'm thinking you may be right. I found the lead pipe for the air pump to the cat disconnected. My air pump my be fried 😬

81c3

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...

NPetts

Good idea, I didn't even think of that. I'll check that out to so if it's loose

NPetts

It's an auto, by the way.

NPetts

Is there a possibility that a bad EGR valve could be causing this? Or perhaps a clogged/worn out catalytic converter? Or would a bad A.I.R. pump be a more appropriate answer?

81c3

Remove the serpentine belt and see if it goes away. Obviously you only want to run it for a minimal amount of time... That will let you know if its the pump, or water pump or PS pump... but not wont tell you which of them is making the noise...

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...

biggiefl

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.

NPetts

That's good to know! I'll check it out.

ctmccloskey

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!

NPetts

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).

Randy M

ctmccloskey

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!!

81c3

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!