First off a little about my car, its a 78 auto, the only emissions I have that I know of are mufflers (which I am pretty sure are stock). It does not idle well, usually ~1500 in P/N (about average, sometimes lower), around ~1000 - ~1500 (can go a little lower than 1000) for any drive gear and these are not solid, P/N is decently solid however. I beleive the idling is related to my carb, and I am in no way a pro in these matters.

The thumping sound comes from the rear. The sound increases in speed the faster I am going, the slower I go the sound is slower but seems to be a little louder or at least more distinct. It only happens when my foot is OFF the gas, and I'm going ~10 - ~35mph. Over that speed (~40+) I can't feel (can't really feel it to much as it is) or hear it, either due to the engine noise or wind, but I have tried to test to see if its there. I have determined it to be between 10-35ish, as far as I can tell. The engine RPM with my foot off the gas is ~1500 or so. I am fairly certain that it is strictly limited to my foot off the gas. When my foot is off the gas on the highway it stays around 3500, so this could mean its RPM related. I have tried putting the car into N while it was happening and the sound continued. As I've said I don't know too much yet, but I am learning =). I am hopefully thinking it is the U-Joint, or maybe debris on the driveshaft. If anyone could help me out it would be greatly appreciated. If theres anything you want me to try just let me know and I'll do it tomorrow morning.

Thank You

 

Your car has six u-joints. Have all of them checked and let us know what the result is.

 

Its either that or a dead cat

 

Let the Biatch out of the trunk

 

U-joints

 

Turn your subs down
Doof,,,, Doof ,,,,,Doof,,,,Doof






Really, check the U joints mate.
Also check your tyres for bulges and delamination.

 

My 80 exhibited almost the same symptoms you are describing. Mine went from thump, thump to a sort of metallic squeaking that came on about 25 mph or so and got quieter the faster I went. It is in the shop right now getting the u-joints checked out and probably replaced. This is something I was going to do myself but just don't have time during the "silly" season. I would bet it is the u-joints in your case as well, just as others have said.

 

If not the u-joints check to see if the front mounting bolt for the diff is tight and that the front mounting bushings for the diff are not worn out.

I had a similar thumping, and after replacing 2 bad u-joints I had less but still some thumping... found the the diff mounting bushings were completly worn out. Replaced them and now its all good.

 

Thanks, U-Joints will be checked out thursday when I get my tires aligned. Hopefully something will be wrong with them, never thought I would hope something would be wrong ^^

I will also get you guys pics since you seem to love them so much ^^

 

I had the U-Joints checked and they said they were fine. I had a friend who isn't to bad with cars come with me to see if maybe he could tell at least where it be from. Strange results, normally I would hear it 95% of the time I'm going to a stop/take my foot of the gas. I took him for a ride around town, max speed limit is 40. Heard it maybe a total of 20 seconds, longest time it happened for was maybe 10 highest. This was also about at least a half hour ride. My friend weighs approximately 170lbs, I weigh approximately 155lbs. Any help?

Also tires are pretty new.

 

I am just glad my 78 isn't the only one that has a rough/bumpy idle! Let me know if/how you get it working properly. You might also want to adjust the timing, that's what my husband and his father did on mine. It helped a little!