Hard "THUNK" when letting off accelerator

I love my truck but I'm getting a really hard "THUNK" when abruptly letting off the accelerator. Almost as if the engine is in a bind and then resetting. I'm leaning toward either bad engine mounts, or something transmission related (?clogged torque converter or front differential mounts?).

Originally I thought it was the inherent binding "thunk" these 1st gens have when coming to a stop, and then letting off the brake pedal. But after the rear portion of the drive shaft literally fell off in transit a few months ago, and then having a completely new one installed with new U joints as well, I ruled that out as the culprit since that binding thunk at a stop and brake release went away.

Now my main issue is the hard "THUNK" when letting off the accelerator. If I let off gingerly, it doesn't do it. Any suggestions or experience with this same issue?

 

If you search “axle wrap” and “slip yoke” here you might find that either one of these could be your issue.

How many miles are on your rear leaf springs?

 

Welcome to TacomaWorld
Definitely not the front differential on a
2wd.

 

probably worn-out rear leaf springs ( axel wrap) very common.

 

Thanks for the replies. Truly appreciated \,,/\,,/

249,000 miles on the rear leafs (and slightly sagging now that I have a camper shell, even more-so when fully loaded with camping gear). I thought the same as far as the axle wrap is concerned. The slip-yoke is brand new as well. That was the part of the driveshaft that fell off in transit a few months ago, so I had it replaced including the U-joints front-to-back. I dropped the driveshaft, replaced the carrier bearing and tranny mount myself about two years earlier trying to remedy the thunk (didn't solve shit). I recently serviced the rear diff as well, and it's as clean as can be. No shavings whatsoever, and no leaks so it's not that either.

Been scoping out the Old Man Emu leaf upgrade, but I've heard it rides like a go-cart afterward. One mech suggested airbag suspension for a better ride. Finances are extremely tight atm, so I'll likely go with the least expensive while still sticking to quality parts.

 

I wouldn't rule out the rear driveshaft even if it was just replaced, could be a chance something wasn't torqued right or greased properly...just saying, it's worth looking into.

It does kind of sound like axle wrap, I've never felt it on my manual trans 04, but I had a 1999 auto Prerunner that got axle wrap real bad. Except for me, it was a big THUNK when coming to a stop. Soon as I completely stopped it would feel like I got tapped from behind by another car.

The way it was explained to me was that the stock leaf springs, esp when they get tired, tend to twist a little and release that tension when you come to a stop or let off the throttle, etc. You feel that thunk throughout the entire truck.

 

Willing to bet it's axle-wrap. These leafs are definitely tired... Likely going to purchase a good upgrade set and see if that solves. If not, at least it won't have that fugly sagging ass afterwards.

 

Im at 151k miles and I have narrowed the exact same problem down to axle wrap. Some helper springs removed the issue, but I didnt like the fitment of them so I am just looking to replace the entire leafs on each side. Some new shocks helped a little bit too but as others have mentioned its mostly likely axle wrap and some new springs will likely fix it.

 

Had the same problem. Motor mounts looked fine when I took them out but the driver side one was split about halfway through the rubber when I torqued on it in the bench vice. Replaced the mounts with ones from @Bonzen mounts and my thud is completely gone

 

yeah since his is when letting off the accelerator that's the reason I am skeptical of it being axle wrap....for me, axle wrap happened when coming to a stop for the most part. Broken motor mounts would definitely cause a thunk when letting off the gas, I would imagine. Engine settles back into place more violently

 

With axle wrap, it was a very clear feeling of it happening from the rear, like getting lightly rear ended

 

Agreed, when it was happening to me it was just a noise that sounded like it was coming from beneath me whenever I would let off the accelerator quickly or downshift without rev matching.

It never had a feeling to it, i.e. Suspension unloading.

The only other thing I noticed with it was a surging feeling if I was accelerating quickly and let off the clutch too fast shifting to 2nd.

My best guess is that it was the motor flopping around a bit and not sending all the power down the drive line

 

My bed hits the cab when I'm hard on the throttle.

 

Go simple, check/lube U joints

 

I'm currently working a big contract with a company that provides off road driver training to civilians and military. They have a dozen tacoma trucks 1st, 2nd, 3rd Gen as well as some 4runners - every single truck had bad motor mounts, and some had frame damage from the bad motor mounts. They didn't even know until they started heading "clunks" and then got under the trucks and inspected them. And same thing, they thought the mounts on some of the trucks looked ok until they removed them and noticed the rubber pucks were broke in half.

They are replacing all of them with Solid Offroad mounts, even on the brand new 3rd Gens. Prevention is key!

 

Check all your rubber mounts..trans engine etc

 

Look under the hood with the engine running and rev it a few times...can you reproduce the thunk this way? Observe excessive engine movement?