: Strange Knocking. '98 STS



speedyman_2
11-26-04, 04:35 PM
Well, I had my oil changed today cuz I thought I was a few quarts low. Since the the car was telling me to check the oil level. I thought that was the cause of this knocking. It only starting doing this last night. Anyways, oil didn't do it. It only happens under acceleration or reving. When the engine decelerates back to idle you don't here it. Only a little at about 1,000rpm. The loudest is when it's passing through 2,000 and 3,000+ rpm. At first I thought it was a really bad exhaust leak, but that's definately not it. Almost sounds like the knock my mustang has from it's solid lifters. But, my caddy is OHC!! I took the oil cap off and saw the cam and everything moving and could tell the sound wasn't coming from there. Seems to be coming from the side of the engine where the pulleys are at. That's the closest I can tell. Anyone got any ideas??

dkozloski
11-26-04, 05:10 PM
The most obvious answer is that you did damage to a rod bearing by running it low on oil. I have seen a rod bearing fail when the engine oil pressure warning light just blinked on for an instant under hard accelleration. Let's hope this is something else. A rod knock is going to get a lot worse quickly.

zonie77
11-26-04, 06:46 PM
I just got an Eldo that had the idler bearing for the serpentine belt destroyed. The bearing rollers were gone! Check for wobble on the idler and the belt tensioner. Also, inspect the belt. You may have lost some of the rubber from the inside of the belt.

speedyman_2
11-26-04, 07:50 PM
:crying2: This is something way worse!!! I drove it again and was headed to the store with a friend. All of a sudden it got loud. Really loud. Now it knocks loud at every RPM!! I also noticed that the crank pulley is not spinning smoothly!!! Looks like it's shaking while it's spinning. Yet the engine itself is running smoothly. And nothin is leaking. HELP ME!! I'm not driving it anymore just to be safe. Would the rod bearing cause this?? Oh yeah, what's this gonna cost??

Ranger
11-26-04, 08:29 PM
You did not damage it by running "low" on oil. The check oil level light comes on at about 5 qts., long before a critical level is reached. As I recall Bbob saying, there is an "elastomeric" bond on the crankshaft pully. That may have failed causing the damper to have excessive runout and maybe contact the block once per revolution. Take a good look at that pully. I bet that is all that needs to be replaced.

speedyman_2
11-26-04, 08:56 PM
Ok, How can I tell if it's the pulley?? I was thinking that earlier. But, I'm not sure how to tell. Everything looks normal from up top. Will I be able to tell from looking under the car??

Spyder
11-26-04, 11:20 PM
Is it a regular knock or does it get all erratic, even when engine speed is constant? Rods knocking, in my limited experience, will be more of a knockknock knock KNOCK knock knockknock...if you get my idea... :):):) My tensioner was doing something like what you describe...the idler pulley was loose and a motor mount was loose...tightened them up and the noise went away...it used to happen to me all the time when I would do high speed runs or after a couple of WOT's, but I locktited the top motor mount bolt in place and its never happened since...at least not until I spun a rod bearing and had a rod knocking. :'(

zonie77
11-28-04, 12:34 PM
You should take off the belt (make sure you have a diagram of it's routing) and check the idlers, tensioner, belt, and crankshaft pulley. You can see the crankshaft pulley from the bottom of the wheelwell or jack up the car and look up from the bottom.

If you can see wobble it's probably bad. Confirm by close inspection of the rubber bonding ring.

an01sts
11-29-04, 12:40 AM
I cannot say about the knocking, but 2 quarts low is major critical. One quart is the minimum amount of oil that the engine should operate at. Anything less is bad news, and you really shouldn't operate it one quart low as the normal operating level. Ever since oil lights/idiot lights were invented, a luminated oil light--just like a temperature light--means a critical condition. (You ought to be thankful that you have a half chance with Cadillac's "check oil" light: Anyone motorhead who has been around knows that many GM's will lock up from lack of oil, without even a flicker from the oil light.)

I have been driving sine 1974, and I have always been able to tell when an engines is a quart low, just by listening to it. That's the way I have always checked my oil because sure enough, the stick shows add a quart.

I'm sorry to contridict, but it isn't sound mechanical advice to suggest that one should operate an engine at an oil level less than the manafacturer's specifications.

Ranger
11-29-04, 06:53 PM
an01sts,
You are absolutely wrong. The Northstar was designed for 7 1/2 quarts to prevent the pump from running dry with a 1g lateral force at WOT. When was the last time you did that? Have you ever done that? The check oil level doesn't even come on til you are down to 5-5 1/2 qts. ("manufaturers specifications"). If that were a problem, they would have the light come on much sooner. Infact Bbob has said that they have run them on the dyno's with as little as 3 qts. at WOT for hours on end (though I would never go or recommend that low) .

dkozloski
11-29-04, 08:11 PM
I have seen aircraft engines that were run out of oil and failed that had less than 1 cup of the nastiest stuff you ever saw left total in the engine and the outside was dry. You have to ask yourself, "Do I call the National Bureau of Standards and report a new standard for dumb?" I also had a Chevrolet Z28 302 engine with a full sump fail under hard accelleration when the oil pickup was uncovered and the oil pressure dropped for just an instant. I added a baffle to the rear of the pan for a cure.