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annoying thumping sound at firewall idling in gear

teejay

teejay

New member
02 GP ,I had all of the engine mounts replaced and had a new GM cat converter put on about a year ago and ever since then at an idle in gear it has gradually developed a thumping noise which seems to be between the engine and firewall. Might be a coincidence that its done this since then but not sure? If I hold my left foot on the brake and just touch the gas with my right it stops. It sounds like something is hitting the firewall but nothing is. I have jacked the car up and tried finding it from the bottomside while running but the noise is not loud enough to be heard outside of the car. At times it seems loud enough to be heard from underneath but the closer I get to the area that I think its coming from I can no longer hear it because of engine noise.
Is it possible to turn the idle up a tad on these 3.8s or will that screw everything up? Also, what is the idle speed suspose to be on this car? Thanks
 
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You can up the idle, but with a tuner only. There is no need to do it on a stock car. IIRC the in-gear hot idle is somewhere around 700.

Sounds like the downpipe may be hitting the sway bar. Give that area a check and see what the clearance is.
 
I have checked that for clearance and have not seen any signs of it making contact with the sway bar. Its almost like maybe something inside of the new cat is making the noise and transferring elsewhere or in the muffler? I really didnt want to have to cut that new cat off and put a test pipe in cause its such a pain to get good welds on it when your lying on your back. I am really starting to believe that it is something exhaust related.
 
Maybe get it in the air and start banging around on the exhaust system at random points and see if you hear it rattling against something. Could just be a hanger or muffler slightly out of place and hitting on suspension or something.
 
I guess I'll get the ole pry bar and hammer out and start moving things and see if it stops.
 
I found it but I don't know how to fix it. Right where the exhaust pipe bolts up to the manifold its knocking inside that cone around the pipe, looks like theres a spring in it or something, not sure.I guess I'll pull it down and see what it looks like.
 


Sounds like where the DP bolts to the rear manifold. Maybe try tightening the 2 bolts up and see if it fixes the rattle.
 
Yes, it is where the DP bolts to the rear manifold and they are tight and nothing leaks.I think I know what is making the DP rattle, the engine vibrates at an idle just enough to make the inside of the car rubble and make the plastic rattle.It's not a miss, I dont think, it runs perfect otherwise.With my foot on the brake and the other on the throttle it never really stops vibrating till around 1100 rpms and then it slowly smooths out. I have sprayed the entire top of the engine with carb cleaner to see if it has a vacuum leak but have not found anything.The plugs and wires are all new.I cant think of what else would cause a vibration like that. Could there be a vacuum leak elsewhere or do you think its something else?
 
engine mounts shot? does the engine move a lot when you sift it from park to drive, then to reverse? it should move, but not much.

you can also either use the e brake if it works, out the car in drive, then go look at the engine, (or have some one hold the brake in gear while you watch the engine) see if its twisting in the bay, same for reverse, you can tell by the dog bones, (and you will see it turn in the bay) they will push to one side
 
I agree with scotty, sounds like a mount is shot somewhere. As a side note, what brand/type of plugs did you install?
 


It has AC Delco plugs in it, I always use what came from the factory in it. As I mentioned in the first post the mounts are a year old but theres a strange story to that.The shop that replaced them told me that the enigine was not sitting correctly in the engine bay and that the new mounts would fix that.After they changed the mounts they told me that it was still not sitting at factory specs so they had to do some shimming? I bought this car new and it has never been wrecked so I don't know if thats normal to have to shim mounts in these GPs or not? They did'nt seen to know either. But recently I noticed that the engine mount is oil soaked again just like before it was changed so I guess its bad again? As far as engine movement I think it moves to much and I can feel engine movement while accelerating and decelerating.

I dont know if them shimming the mounts made them fail prematurely or not?
 
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could be the mount went bad, never heard of shimming a mount before. maybe its time you did the work and then you will know whats going on, motor mounts like a 30 minute job first time out, basic tools are needed. and its easy to change really. i did both motor and trans mounts in less then a hr my first time out.


you may also want to check for oil leaks, as a oil leak from the v/c can leak to the mount and the oil on it will break down the rubber in the mount and kill it.

then flip the dog bone bushing on the rad mount, that firms things up nice too.
 
The valve cover gaskets are not leaking but I know the oil pan gasket is, and they also changed that a year ago.I used to do all the work myself and even have a shop but I have severe back problems and I am very limited on what I can do. It really drives me nuts to have someone do that kinda work for me cause you never know what your gonna get,everybodys in a hurry to do the work as fast as they can and the quality of work has gone by the wayside, but thats how it goes.
 
sorry to hear that. and yes its hard to find a shop you can trust.


ive done all my own work as well, most of my life, the few times i was stuck and went to a shop i got f 'ed on the work.

then i found a shop in town that was ran by a guy who built race cars from the ground up, he was not cheap, but is work was solid.
 
If you can, try and clean off the entire area down there of oil and see if you can pinpoint the leak. This will tell you if the pan is still leaking or if the mount just gave up again on it's own.
 


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