Rap, tap, tap, tap On acceleration - ClassicOldsmobile.com

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Rap, tap, tap, tap On acceleration

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Old August 13th, 2018, 10:27 AM
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Rap, tap, tap, tap On acceleration

My 72 Cutlass Supreme 350 with 350THM sometimes makes a machine gun like sound when firm, but not hard acceleration from a stop. It is not every time, and not as loud ad actual gunfire, but definitely not normal. I can't recreate it every time. It is NOT at full acceleration, but medium. It stops when I let off the gas and does not continue after that. If I floor it so I get a down shift, it doesn't do it.

Any ideas? Universal joint, or motor mount maybe? If yes, How should I test?
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Old August 13th, 2018, 10:55 AM
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Could your fan be hitting the shroud?
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Old August 13th, 2018, 11:10 AM
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I see no evidence of that. No marks or scratches on the shroud. I use a clutch fan for A/C.

It doesn't happen instantly when pulling out. The car is moving and accelerating then it starts. By reflex, I immediately let off the gas, and it doesn't return when I continue to accelerate.

Could something failing in the transmission cause it maybe?
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Old August 13th, 2018, 11:29 AM
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Check for a cracked flexplate.
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Old August 13th, 2018, 11:30 AM
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Originally Posted by brown7373 View Post
I see no evidence of that. No marks or scratches on the shroud. I use a clutch fan for A/C.

It doesn't happen instantly when pulling out. The car is moving and accelerating then it starts. By reflex, I immediately let off the gas, and it doesn't return when I continue to accelerate.

Could something failing in the transmission cause it maybe?

Sure... the torque converter. They will rattle when the stator or impeller vanes are breaking loose.
Are you sure its not pre-ignition? Do you know that sound?
Check the motor mounts for damage and as suggested. Something could be making contact on the chassis. Trans mount ok? Look for something contacting the driveshaft.
Look for exhaust movement. Bang on the mufflers to see if a baffle came loose.
Look for fan to shroud contact on the entire shroud circumference.Is the fan equidistant from the shroud around its entire circumference?
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Old August 13th, 2018, 12:33 PM
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Is it popping out the carb?
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Old August 13th, 2018, 01:26 PM
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Do yourself and everyone else on here a favor, get your cell phone camera set to take video, have it on the next time you take the car for a spin, and record the sound. Then post it on youtube and provide a link. The sound could be anything from a transmission problem to a loose motor mount to who knows what, and trying to diagnose it by having people on here suggest things based on your description of the sound could be a colossal waste of time. It is very difficult to describe a sound. It is much better if we can hear it. It would also help if you could try to pinpoint the source, if possible. Front end? Rear end? Under hood? Left side? Right side? Underneath?
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Old August 13th, 2018, 01:34 PM
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Despite appearances, my money's still on fan/shroud contact. I've heard that sound before and it closely mimics machine gun fire. Loose motor mount allowing engine to flex? Try having a helper step on the brake and wind the engine slightly (maybe 1200-1500 RPM) against the torque converter (don't stand in front of the car or we could all have another Youtube video to marvel at) and see if that recreates the situation.
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Old August 13th, 2018, 03:17 PM
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First thing to do is to check for a broken motor mount on the driver's side.
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Old August 13th, 2018, 03:47 PM
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Definitely not pre-ignition and not popping through the carb. I am very familiar with that sound. It doesn't do it often (happened 5 or 6 times the past couple weeks) and I can't make it happen, even when I try to recreate it. It sounds like it is in the right front, almost tapping on the floor. The tapping is evenly spaced...it doesn't skip beats.
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Old August 13th, 2018, 06:20 PM
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Point the car away from anything close by including humans. Open the hood, start it, put it in drive with foot on the brake and then throttle it up (like a brake stand) then immediately remove your foot from gas. If its a bad mount you should see and feel the engine jump up then come back down on the cradle. Its likely the fan hitting the shroud on 1-2 upshift.
If it is a bad mount replace all 3 at the same time.
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Old August 13th, 2018, 06:36 PM
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Loose bolt/s torque converter to flywheel.
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Old August 14th, 2018, 12:32 PM
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I am going to check the mounts. It hasn't done it since the one time yesterday,. Someone suggested I record it, but it has only done it a half dozen times in the past month, in spite of driving it nearly everyday, and I am not able to make it do it, it is hard to record the sound.
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Old August 14th, 2018, 06:34 PM
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A rock stuck in behind one of the trim rings that lets loose on some uneven pavement ?
I'm on the bad motor mount causing fan to hit the shroud band wagon thou .
As others mentioned, foot on brake, feather the fun pedal hard to soft romps and see.
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Old August 15th, 2018, 08:34 AM
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Still hasn't done it since Monday, despite numerous attempts to make it happen.
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Old August 15th, 2018, 11:59 AM
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Originally Posted by brown7373 View Post
Still hasn't done it since Monday, despite numerous attempts to make it happen.
Did you try the procedure described in Posts #8 and #11?
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Old August 15th, 2018, 12:45 PM
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Originally Posted by brown7373 View Post
I see no evidence of that. No marks or scratches on the shroud. I use a clutch fan for A/C.

It doesn't happen instantly when pulling out. The car is moving and accelerating then it starts. By reflex, I immediately let off the gas, and it doesn't return when I continue to accelerate.

Could something failing in the transmission cause it maybe?
Just because you don't see any marks or scratches on the shroud doesn't mean it isn't making contact. I used to have a similar problem w/ my car years ago after I re-cored my original radiator and re-installed it w/ a clutch fan upgrade. It only did it when it was cold, and w/ the trans in reverse. These cars are not torsional granite slabs like a lot of newer cars are, especially the convertibles. Things shift and move around with temperature, road contours and engine loading (specifically how much torque is being transferred down the driveshaft to the rear.........and in reaction to the engine & trans. I ended up re-adjusting the shroud to increase the clearance to the fan and it took care of the problem. Since the engine & trans are rigidly bolted together, it is unlikely IMO that an intermittent noise could be coming from the flex-plate / torque converter; there is a lot less opportunity for that interface to shift around.
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Old August 15th, 2018, 02:38 PM
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I checked the converter bolts and they are all tight. I didn't have time to remove the tranny mount, but I put my jack under the tranny pan with a 2 X 6 on the jack, and I can lift the transmission up from the cross member. Am I correct in interpreting that as that mount is shot? I plan on replacing the 2 engine mounts when I change the front seal to the crank. It leaks oil everywhere. Manual says I have to jack up the engine off the mounts, remove oil pan and replace all those gaskets along with the timing chain cover.

I see nothing in the Service Manual about replacing the rear mount. If I remove the 2 bolts to the cross member, will I be able to jack the transmission high enough to get to the bolts to the trans. case?
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Old August 15th, 2018, 03:25 PM
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Yes, you can change the tranny mount by lifting it up from the cross member. I suggest you plan to pull the motor to work on your seals. It is next to impossible to cleanly remove and reiinstall the pan with the motor in the car. Oil will deteriorate the rubber on the motor mounts and tranny mount, so you made a good call to replace all of them.
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Old August 15th, 2018, 07:22 PM
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I have been considering pulling the engine to do it. As you said, very difficult to do with engine in the car, and sometimes it is easier to just pull it. And also very hard to clean all the oil off everything with the engine in the car.
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Old August 20th, 2018, 12:45 PM
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I drove the car Thursday through Sunday with our antique car club on a 500 mile tour, mostly 40 to 50 mph, but 70-75 the last hour, with no problems. I have not changed any of the mounts yet, and didn't do any hard acceleration. Not suggesting it "healed" itself, but it performed fine. No to find time to fix it!
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