View Full Version : 97 Lower end thump/knock between 2 and 3k

09-30-04, 06:59 PM
Hello all,
I have a quick question, I am the proud new owner of a '97 STS and notice that if I am sitting still with the car in park or neutral when I rev the engine to about 2k and then feather the gas I get a knock/thump from the lower end. I work at a Jag dealer as a tech and had a few other people listen to it, I got comments ranging from hmmm interesting, to your too F***ing picky and paranoid. This may be the case as I have extrodinarily bad luck with my own personal cars. It has 97,xxx on the clock and was extremely well maintained by the original owner, however when I bought it she was hitting on 6 cyls as it had a burned out coil and I don't know how long the lady drove it like that. I bought an aftermarket warranty and am waiting for an inspector to come out and look at the car. I am thinking about putting some motor honey in it before he comes, however if you aren't specifically listening for the noise its hard to notice. Any thoughts would be greatly appreciated.

09-30-04, 07:55 PM
Hmm... no ideas on your "thumping"... I had a GTP that would actually thump quite severely if you revved it to 6000rpm in park/neutral... It was the tranny, but never did figure out what it was. Also, something to watch out for - because you don't know how long it was driven on 3 coilpacks, the extra fuel being dumped into your exhaust could have overheated the cats, causing the substrate to melt. Easiest check would be to use a vacuum gauge at idle and snap the throttle to 3,000 or so rpm. If the vacuum reading starts steadily dropping, it is probably plugging up your exhaust. Normally the vacuum should drop just as you open the throttle a bit, but then go above the idle vacuum. Good luck,

oh - idea! Smell your oil at the dipstick. If it smells like fuel, it might have been thinned out due to the blowby from the faulty coilpacks, and causing your knock.

09-30-04, 08:22 PM
oil was changed by previous owner right before I purchased it, so it smells as new. The vehicle has flawless performance and you can't feel it running at idle in drive it's so smooth. I am reasonbly sure the cats have not melted as they have not "blued" or drooped. I can only liken this noise to perhaps a wrist pin slap or maybe a rod knock type noise. I have read through tons of these posts and i'm assuming that due to the absence of posts about lower end problems that they are virtually bullet proof. I am extremely confident this car was never ran hard or neglected on oil changes. The aftermarket warranty I am buying covers all gaskets, so that's a plus if it becomes necessary to timecert it, but I just wish that little noise wasn't there for some piece of mind. My co-worker/friend has listened to it a couple times and is certain it is of no concern, I wish I felt the same... :(

09-30-04, 10:56 PM
<shrug> Don't know what to tell you without being able to hear it myself. If your warranty covers everything, maybe it's time to start flogging the crap out of it, and using stuff like 0 weight oil :shhh:

09-30-04, 11:04 PM
If you read the posts about the carbon rap then you know to take it out and give it some WOT accels to make sure the carbon is cleaned out of the combustion chambers.... Excessive carbon build up could be the source of the noise which would be harmless and easy to remedy with some WOT....

10-01-04, 10:22 AM
Suggest for you to go to a NAPA store and get you some seafoam injection cleaner.Comes in a white and red can.Will clean all the carbon inside the motor,and at the same time it will clean all you intake.Stuff will work wondrers.Try it you will like it,and will more than likely get rid of it.Let me know.

10-02-04, 10:38 AM
Thanks for everyone's input on this. I just noticed after my car sat for about a week that there was a very small puddle of coolant underneath (water pump no big deal). As far as the noise being carbon build up that would make perfect sense as I have no idea how long it wasn't lighting off the fuel being dumped into two cylinders and the car I doubt was ever floored in its whole life. The top end engine cleaner stuff is a good idea, I have two different kinds at work B&G and Wynn's I could try them both and see. I figure I'll wait untill the warranty starts on it before I start flogging it "just in case". Thanks everyone for your input. I am mostly a Mopar guy (other than my IROC-Z, had to get it for nostalgia reasons), but the 300C was a little out of my price range, so far I have no regrets with this car and think I made a good choice. :thumbsup: