mystery thump

[uncle-vinny]

Did this ever happen to anyone? This is weird. I have 1988 Astro 4.3 auto. with 115,000 miles on it, all stock all original.I have had it 1 1/2 years. Sometimes when I stop at a light and I am just sitting there in drive at a dead stop I feel a thump. It is about 10 to 15 seconds after I have come to a complete stop. I feel a thump like I was rear ended. The van actually feels like it is trying to move forward. The first time it happened I looked in the mirror to see who rear ended me but no one was there. Anyone ever have this happen or hear of it happening?

[potskie]

My 93 did that for awhile. Then I did some brake work and it stopped. It appeared one of the caliper pistons was sticking and the thump was it popping out and slamming into the rotor.

[canadavan]

I have the exact same issue with my van and I haven't been able to figure it out.

[SafariRob]

My 89 used to do this until I installed a rebuilt transmission.
I didn't do the transmission swap because of that, though. I replaced the transmission because the original one was slipping when it shifted and was getting hot during normal usage (it would peg the transmission temperature gauge with city driving).
But with the new transmission came an end to the 'bump' at stop lights. I just figured that this was caused by a belated shift back into 1st after the van stopped.

[1996astro]

Had the same condition on my 96 2wd as long as I owned it. I also believe it was a downshift. Never bothered me that much guess I got used to it.

[uncle-vinny]

It only bothers me if it means something is about to die.

[midnighthunder89]

That has happened to me. It is the Transmission, well it was for me at least, I had it rebuilt that problem is fixed just lots more that I have to deal with.

[zook94]

My '93 doesn't particularly thump, but it definiitely jumps at stoplights like I have been "nudged" from behind. After careful inspection, I found that it was the air conditioning compressor, releasing its clutch from the engine, causing a surge in the drivetrain. Possibly an after affect from swapping in the V8?

Just what is happening with mine, your dilemma may be different.

[97TurboAstro]

A local top-notch tranny guy told me that it's the tail of the tranny.

I had taken a 96 Astro tranny in for a bench rebuild. When I went to pick it up, I noticed that the tail was packed with grease. I asked why. He asked me if I'd ever stopped and a few seconds later it felt like someone bumped into me. He said it was the driveshaft popping back and forth. Packing the tail with grease, will avoid the problem.

[rlsllc]

A local top-notch tranny guy told me that it's the tail of the tranny.

I had taken a 96 Astro tranny in for a bench rebuild. When I went to pick it up, I noticed that the tail was packed with grease. I asked why. He asked me if I'd ever stopped and a few seconds later it felt like someone bumped into me. He said it was the driveshaft popping back and forth. Packing the tail with grease, will avoid the problem.

Be careful that the grease that you use in compatable with the trans fluid. I don't know much about it, but I'll bet you could mess something up if you use the wrong thing.

You can replace the cylinder like bushing that the yoke rides in in the tail shaft, it can cause this when worn. So can a groove in the yoke.

I've heard that out of adjustment rear brakes and/or bad shocks can aggravate this condition because of the nose-dive thing.

Most of my rides bump, shake, and lurch from old age or high miles. I don't mind.

[1996astro]

Most of my rides bump, shake, and lurch from old age or high miles. I don't mind.

Right On!
As long as its not from doing this... http://youtube.com/watch?v=Xuow49t-Bjw

[Water Boy]

A local top-notch tranny guy told me that it's the tail of the tranny.

I had taken a 96 Astro tranny in for a bench rebuild. When I went to pick it up, I noticed that the tail was packed with grease. I asked why. He asked me if I'd ever stopped and a few seconds later it felt like someone bumped into me. He said it was the driveshaft popping back and forth. Packing the tail with grease, will avoid the problem.

Yea, the splines get dry and the yoke will bind on the splines when the angles and torque and the moon and the stars all line up just right.
There have been various fixes for it.
Some times the yokes have a small hole for a tidbit a trans fluid to lube the splines.
I thought there was a special "grease" for that application.
I would guess that any non-dino oil that was "wetted" onto the splines would last awhile.

My 90 Astro never did it but my 99 dually did it all the time.

[canadavan]

This is interesting.
http://answers.yahoo.com/question/index ... 123AA8D2jv