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I rev the engine and something thumps!

So I got back from my drive today and came up the driveway in neutral just for funnzies, but while in neutral if I rev the engine up quickly, around 4k RPM's there is a serious thump.  The whole car gives a pretty good shake when it happens.  When I bring the RPM's up smoothly I don't notice it so I am not sure what it could be.  I thought it may be a clutch engaging, but what would engage at that speed that wasn't in already.  I also thought it might be a clutch disengaging, but that would result in a serious thump, just a smooth wined-down.  The car is a 2003 dodge durango with 160,000 miles on it.  It is an automatic which can somehow kill itself when trying to start going, and it is the coolest, most durable car ever.

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lemonie4 years ago
Don't do this; the transmission doesn't have unlimited durability.

L
jj.inc (author)  lemonie4 years ago
What does this have to do with transmission durability, it just leaves the input free spinning.
lemonie jj.inc4 years ago
If your transmission isn't taking a big-stress when the whole car gives a pretty good shake - what is?

L
jj.inc (author)  lemonie4 years ago
It isn't in gear, and it isn't moving forward or backward, it just kind of shudders, like if you shake a desk.
lemonie jj.inc4 years ago
You haven't answered my question, which I can rephrase as "I say you'll break the transmission if you keep doing this; if you don't think so, what is it that you will be breaking instead?"

L
jj.inc (author)  lemonie4 years ago
Nothing, what is your impression of what is happening, the car shakes when I rev it, the only thing it is stressing are the engine mounts, and not to a large extent
lemonie jj.inc4 years ago
Nothing, are you going to keep playing with the car like this, or be cautious and stop?

L
jj.inc (author)  lemonie4 years ago
I'm not just playing around anymore, I am trying to find out what is wrong. After what this car has been through I am pretty sure a little bump isn't going to hurt anything. I feel that you are misinterpret this question, like you do on all of my others, and you just comment to comment. This isn't a little baby car, it is a heavy duty off road SUV.
lemonie jj.inc4 years ago
OK, what is wrong is the way you are driving it.
If you want to figure out what is happening you'll need to understand how the transmission works (I don't know).
And, "serious thump" is not the same as "a little bump" - which is it?
I don't misinterpret your questions; I try to understand better so that I get a correct interpretation.

L
jj.inc (author)  lemonie4 years ago
I bluntly state everything and you always overthink it to the smallest detail. But anyway I am pretty sure it isn't the transmission. When I say serious thump, which it is, I just meat that the whole car shakes, which is serious relative to say someone lightly knocking on a door. You overthought serious thump apparently so I scaled it down to little bump, because the car isn't flying forwards and backwards by multiple feet, it just kinda shakes.
I am not even driving the thing when I do this, if you haven't figured that out yet, I just rolled up the driveway in neutral which isn't hard on anything, and then I felt like reving the engine, and to my surprise the whole car shook, then it did it again, and again, so I resorted to instructibles.
IT ISN'T THE TRANSMISSION
lemonie jj.inc4 years ago
Lurching due to the forces generated by an engine "spinning-up" I would expect, but shaking does not fit a straightforward rev-up unless your engine was "sick" (but it isn't because you'd know if it was).
Saying that it isn't your transmission in CAPS is no argument without saying what is the problem, and I am aware that no one knows.
I suggest that you don't do this again and leave it as one of life's mysteries, OR go find a "car-mechanic" and ask them.

L

(You're good conversation; I do think excessively)
jj.inc (author)  lemonie4 years ago
Its always fun to debate with you, and in the end someone always learns something, thanks
Not true, in neutral some parts of the transmission are still moving if the wheels are moving. Of more concern is the durability of your engine when you do things like that.
Personally I would check the motor mounts. Also since it's an automatic you could be having clutch problems.

If you sit on a flat spot with the engine running,the tranny in N, and your foot off the break, can you recreate the sound without the truck moving? When the noise happens is it accompanied by a shake or some other physical occurrence?
jj.inc (author)  bwrussell4 years ago
Lol, its not a truck, but anyway yes it happens on flat ground while not moving, and yes it does shake
Anything bigger than my MINI is a truck to me. Anyways 90% confidence it's one or more of the motor mounts.
jj.inc (author)  bwrussell4 years ago
Why would it just have a sudden thump like that though, the mount that is visible looks ok, but I can't see the other one.
Someone else already suggested the proper method for checking motor mounts, you just need two people to see if the engine jumps when it does it. They can also try to hear if there is a specific place the sound comes from. What does the sound, well, sound like? Is it metal-on-metal, rubbing, knocking, grinding?

It's possible that it is thumping all the time but it's just small and unnoticeable until the engine is shaking harder. Have you been brave enough to keep building the revs after the thump? Does it keep happening or just the one time? If you go past the thump point does it happen again when the revs go back down past that point?
jj.inc (author)  bwrussell4 years ago
It is just one single thump in the same spot every time. It basically sounds like someone threw a cat into my fan so it is a low forgiving thump. I had my brother watch for me, but he didn't notice anything and he cant rev it up fast enough so I will try to find someone else.
+1. Engine mounts sounds like a good place to look to me.
Re-design4 years ago
It may be the rev. limiter. Your engine is limited as to max rpm in neutral. When the rev limiter kicks in it will shake the car.
jj.inc (author)  Re-design4 years ago
But wouldn't a rev limiter limit the rpm's the redline is much higher than 4k, and the engine continues to speed up after the nock.
A rev limiter limits it to 3K -4K in neutral. If it revs higher than that in neutral then you don't have a ref limiter or it is not working right.
blkhawk4 years ago
Check the motor mounts. Probably need replacing. One way to know is revving the engine and watch if the engine suddenly lifts or moves. You may need someone to rev the engine while you watch.
jj.inc (author)  blkhawk4 years ago
Ok, I will check