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Discussion Starter #1
1996 Toyota Previa with approx. 150K miles.
It runs well for the most part, however when I am on the freeway, and around 50-60mph, I will get a
periodic THUNK when accelerating on slight inclines... sometimes even on level surfaces / lanes.

It can be pretty loud, sounding something like I just ran over a pot hole or good size speed bump.
I am assuming it is some type of transmission issue. It's doesn't seem to happen below 50mph
or at least I do not hear it. On a 30 minute journey down the freeway, I may experience it 2 or 3 times, mainly on an acceleration. It is not a pleasant sound. If I am careful with the gas pedal,
I can somewhat lessen the occurrences.

What is going on inside the transmission that could be causing this, and how extensive of a repair might it be?

What is a reasonable amount or range that this should cost to repair?

Should I go to a regular mechanic, or start with a "transmission place"?

thanks...
 

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do you have a tow hitch installed on your previa?
i get what you are describing and found out it was the muffler hitting the tow hitch.
 

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Discussion Starter #3
Hello,

I do not have a toe hitch installed. It is an Automatic Previa with Turbo.
The "thunk" is pretty dramatic when it happens. The shock runs through the whole van... like hitting a 6 - 8 inch speed bump at 60 mph. It gets your attention.


do you have a tow hitch installed on your previa?
i get what you are describing and found out it was the muffler hitting the tow hitch.
 

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You can check the motor mount yourself. Put the van on ramps or otherwise get it off the ground. Set the emergency brake (provided it works) and have someone else (preferably a mechanically inclined person) apply throttle to try and simulate the engine breaking free from its mount. And watch and see for yourself what happens.

Otherwise, you may want to check the SADD shaft bushings. I have never replaced them. Now mine will probably break tomorrow. But my understanding is that they react unfavorably to torque once they start to go bad. And torque is what the engine is producing in a low speed high throttle position situation such as an interstate on ramp merging approach.

Either way, I feel sure that with the e-brake applied while throttle is applied constantly you will be able to simulate the problem.
 

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Discussion Starter #7
I appreciate the feedback. I never considered the motor mounts, so I will take a look. The odd thing is that it does not do it on the on-ramp (that I can recall at the moment) or regular "street driving". I have to be going at least 50 miles an hour, and then accelerate.

Perhaps there is not enough torque at the lower speeds to
create the condition / thunk. It is annoying and I would like to resolve it. Would be GREAT if it was not the tranny.

I will check out the motor mounts and the bushing.

-sparkkey
 

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Discussion Starter #8
This is a follow up to the "Big Thunk". Long story short, it turns out to be the turbo charger. I took it into a mechanic and after searching for a while, he discovered that if he manually triggered the turbo-charge clutch, BANG! He disabled the relay that engages the turbo-drive clutch and the problem went away. For now I am going to drive it around without turbo, as it is not worth the $$ to replace it, and I was told it may drive fine without it... but with less umph...

If it ends up being a problem, I found a place that will swap out the engine, tranny and turbocharger with imported "lower milage" replacements for around $2500.

fingers crossed

-SP
 

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What did he say was the problem with the supercharger? Is it low on oil? It could be the electromagnetic clutch going out. A post on the yahoo previa group describes how someone who is in the heating and cooling business replaced his easily. Is the belt to the supercharger okay?
 

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Discussion Starter #10
He didn't say specifically what was wrong. He basically said it needed to be replaced and that it would be around $5K... or I could drive it around a while with the supercharger disabled to see how it runs. I chose the latter. I do not really want to put a lot of money into it. If it drives ok without it, I will stick with it. If complications develop I will either do a used engine/tranny/SC swap-out for $2500 - $3000 or move on to another vehicle.
So far it's been driving ok... no engine/warning lights. No whistle sound on accel... but with a little less power... and the THUNK/BANG is gone!... just smooth gear changes as it should be...

-Joe
 
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