390 fe makes wierd noise on acceleration - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-18-2019, 06:13 PM
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390 fe makes wierd noise on acceleration

390 fe, hei dist with msd 6a, comp cams CCA-CL33-224-3, 2105 edel intake, holley 670 street avenger, fmx transmission. It was running fine, has been for a while. I noticed this weird noise that happens under hard acceleration, it came out of nowhere. It sounds like a whoosh of air and happens in a couple inconsistent pulses and doesnt stop till I start to cruise. It only comes in in first gear, or only after 2,000 rpm, it literally sounds like compressed air, and when it happens I can feel a slight loss in the pedal. I've checked what I can, all fluids, no more leaks than usual, the belts are tight, this and that. I can rev it in Idle and nothing will happen, revs fine, and it starts up and cruises just fine too. I figure its an engine sound, I can hear it coming from in front of me, possibly from the tranny. I've never heard this from an engine before, and my 390 was tuned great and running like a dream prior, besides being slow and pretty bad mpg. Any input is appreciated, thanks.

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Old 08-18-2019, 07:31 PM
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Can you make it happen when you step on the brake pedal ?

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Old 08-19-2019, 02:05 PM
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Originally Posted by Timtbss308 View Post
Can you make it happen when you step on the brake pedal ?

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I just tried standing on the brakes to see, it sits on the converter just below 2 grand and it doesn't spin the tires at all, just grunts against them. Thats how the car has always acted, so no difference, but the second I let off the brake I hear a couple pulses of that compressed air sound. My only guess is that it's the transmission slipping, it still goes into gear and shifts fine, though. It's an fmx with 55,000 miles, it was mated with a stock 302 for most all of those miles, but I've been harder on it once I put in the 390, so maybe it's giving up. I hope that's not the case, but maybe it's time for a c6.
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Old 08-19-2019, 02:29 PM
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Is your brake pedal hard as a rock with little to no assist? Usually when someone says they hear the woosh of air it can mean the diaphragm in the brake booster has ruptured.

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Old 08-19-2019, 02:34 PM
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Or the check valve is bad.
Try it with the booster plugged off.
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Old 08-19-2019, 03:21 PM
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Quote:
Originally Posted by Timtbss308 View Post
Is your brake pedal hard as a rock with little to no assist? Usually when someone says they hear the woosh of air it can mean the diaphragm in the brake booster has ruptured.

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The brakes are feeling normal and the fluid is good, plus they’re manual brakes so no booster.
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