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Knock knock?


Rickm6

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Okay, began checking out my 99 STS which only has 46K miles. I've changed oil religously. When running the engine it has a knocking sound like a piston skirt slapping. Not good. Used a mechanics stethoscope and it sounds like its coming from number 6 or 8 cylinder, in that area anyway. Any thoughts or ideas? Is this common on these? Hard to believe a well maintained Northstar could do this with only 46K on it.

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Rickm6,

If you read through the archives, you will find much on this.

I bought my '99 STS with 46k: knocked then, knocks now @ 98k!

Nothing to worrry about...

Probably needs you to go out and FLOG IT!

REGULARLY!

Good luck!

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If it is indeed piston slap aka "cold carbon rap" as you suspect, it is nothing to worry about. Harmless but annoying. I see you have been a member since Sept. 04 so I assume you are familiar with our resident expert Bbobynski (Cadillac Guru). Here is his explanation and cure. As Cadiking said, take her out and whoop her.

"There are many advantages to occasional full throttle accelerations with a Northstar and any engine.

It keeps the carbon cleaned out of the combustion chamber. This is maybe a little more important with the Northstar than some other engines due to the tight squish volumes between the piston and the cylinder head. It's designed this way to promote good in-cylinder mixture motion (good combustion) but it has the down side of providing a ready place for carbon build-up to touch the piston - causing noise. Ever heard of the Northstar "cold carbon rap" problem?? Simply put you'll hear a rythmic, piston slap-like noise when the engine is cold. Very prominent and very annoying. Cause: excessive carbon build up causing the the piston to contact the carbon on the head - causing it to rock in the bore and "slap" Much more evident when the engine is cold and the pistons haven't expanded to full diameter yet. Simplest and easiest "fix" for this: A few good WOT (wide open throttle) accelerations to clear the carbon out. That is all it takes to eliminate the problem and prevent it from re-occurring.

Occasional WOT accelerations also help seat the rings to the ring lands and exercise the rings and keep them mobile and from becoming stuck in carbon in the ring lands. At high RPM and WOT the rings move around on the piston - they actually rotate on the piston and will polish away any carbon and seat themselves to the sides of the ring grooves. This is especially important on the 2000 and later Northstars which had hard anodized top ring lands on the pistons. Very hard and wear resistant - also harder to break-in and seat the rings to the sides of the ring-lands to promote the best possible seal. Many oil consumption complaints on the 2000 and later engines are related, to some extent, with the rings never seating to the sides of the ring-grooves due to lack of load as the engine was babied around forever. Even engines with rings stuck in the ring-grooves due to carbon build up can eventually be freed up with enough high RPM operation.

WOTs warm up the engine thoroughly and clean out the exhaust due to temperature in the exhaust and high flow rates blasting particles, rust and such out of the system.

Frequent WOT operation will not hurt the engine or the transmission. They're designed for that. The healthiest engines that I have seen at high miles are always the ones that are run the hardest. Rings are free on the pistons and sealing; no carbon buildup.

The exercise that I think works best for many things is to select manual 2nd gear on an isolated stretch of expressway. This takes the transmission shifting out of the question if you are worried about hurting it. Start at 55 MPH or so and go to WOT in 2nd gear and hold it until the RPM reaches near the normal shift point - i.e. 6500 for an L37 and 6000 for an LD8. Hold the throttle wide open until the engine reaches, say, 6200 for an STS and then just let completely off the throttle. Leave the transmission in 2nd so that the engine brakes the car and creates some pretty heavy over-run conditions at high vacuum levels. Let it slow until it is about 55 or so and then go to WOT again and repeat. This exercise really loads the rings, allows variable RPM operation at WOT for several seconds continuously, creates heavy over-run which tends to unload the rings and make them move and thus exercise them in the ring grooves and it will blow-out carbon and the exhaust - all without creating a spectical of yourself and attracting the attention of cops. You can do it on most any freeway and stay within the 70-75 MPH range allowable. Once a week like this will keep the engine cleaned out and healthy and is DEFINITELY recommended for the Northstar in particular.

The Northstar engine was designed/developed/validated to be run hard. It was expected that people would use the performance of the engine - which few people seem to do. The biggest single problem that many issues stem from is lack of use at full throttle by the owners. It just doesn't like to be babied around. The rings are low-tension by design for good high RPM operating characteristics and low friction/good power. They work best if "used" and kept free.

In every conversation with owners I have had, once the owner started doing the WOTs and using the power of the engine they report no more carbon rap, better oil economy, no "smoke" when they do light it up (keep the exhaust cleaned out. If you notice a "cloud" at WOT then you are not doing enough WOTs...) etc... A bit of judicious use of the other end of the throttle travel is a GOOD thing."

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Guys I hope you are correct, sure beats a new engine. LOL

My concern is that its doing it when the car is hot as well as when its cold. I'll try the blow out procedure tomorrow. I'll either have a big hole in my block or it will take care of the problem.

Thanks again, you've given me new hope.

Rick

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I had the same knock in my wifes D'Elegance. I went to the local dealer, and there was a new Caddy in there for the same thing. The mechanic took me in the back and showed me what he does.

He removes all the spark plugs, and fill up the cylinders with ACDELCO Combustion Chamber Cleaner #993026.

He then lets it sit for an hour, sucks the cleaner back out....replaces the plugs, and its done.....

So I went home, and tried this, sure enough..NO MORE KNOCK!!!!!

The bottle of cleaner is sold at all Cadillac dealers, or GM dealers I suppose, and it was only $8.00 for the aerosol can, but you can get the one he was using that was a regular bottle that you had to pour in usind a funnel.

I can tell you there will be a big difference, there was in my wifes car!

At this particular dealer there is one mechanic that does this everyday on all Cadillacs that come in for warranty work for a knock! UNDER WARRANTY! so it is an authorized warranty repair!

Big Jay

Life is too short to grow up!

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I had the same knock in my wifes D'Elegance. I went to the local dealer, and there was a new Caddy in there for the same thing. The mechanic took me in the back and showed me what he does.

He removes all the spark plugs, and fill up the cylinders with ACDELCO Combustion Chamber Cleaner #993026.

He then lets it sit for an hour, sucks the cleaner back out....replaces the plugs, and its done.....

So I went home, and tried this, sure enough..NO MORE KNOCK!!!!!

The bottle of cleaner is sold at all Cadillac dealers, or GM dealers I suppose, and it was only $8.00 for the aerosol can, but you can get the one he was using that was a regular bottle that you had to pour in usind a funnel.

I can tell you there will be a big difference, there was in my wifes car!

At this particular dealer there is one mechanic that does this everyday on all Cadillacs that come in for warranty work for a knock! UNDER WARRANTY! so it is an authorized warranty repair!

Big Jay

THIS is good to know! I just need to find a gadget to suck it out with...

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I had the same knock in my wifes D'Elegance. I went to the local dealer, and there was a new Caddy in there for the same thing. The mechanic took me in the back and showed me what he does.

He removes all the spark plugs, and fill up the cylinders with ACDELCO Combustion Chamber Cleaner #993026.

He then lets it sit for an hour, sucks the cleaner back out....replaces the plugs, and its done.....

So I went home, and tried this, sure enough..NO MORE KNOCK!!!!!

The bottle of cleaner is sold at all Cadillac dealers, or GM dealers I suppose, and it was only $8.00 for the aerosol can, but you can get the one he was using that was a regular bottle that you had to pour in usind a funnel.

I can tell you there will be a big difference, there was in my wifes car!

At this particular dealer there is one mechanic that does this everyday on all Cadillacs that come in for warranty work for a knock! UNDER WARRANTY! so it is an authorized warranty repair!

Big Jay

THIS is good to know! I just need to find a gadget to suck it out with...

Scotty..

it looks like a greese gun with a tube on it. It comes down to a point, and can get almost everything. I asked the mechanic to show me what he used, and it was pretty neet. Like I said, he does this all day long, and he will do about 5-7 N* at a time if they are there for the "knock". He told me straight up "get yourself a suction device like they use on rear ends, and do it yourself, for less than $10.00...

And I did!

Awesome results, you should see what comes out!!!

Big Jay :rolleyes:

Life is too short to grow up!

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Are we permitted to blow Bobbynsky's Cover? Shhhhh.

Man I have like over 140 on both my NS's and I wide open them suckers all the time. I use the Penns Oil High Mileage and the Lucas oil addative and I never have any Carbon build up. All I have to say is every chance ya get, a Get on Ramp, showing some 'Tang his car is not all that hot, or running the turnpike when all the SHP's are on shift change. Floor it! I mean until you think the air intake is going to suck in the entire hood of the car! I love to do it on entrance ramps.

You can even do it W?O breaing the law, if the speedlimit is 65 you can do 70 and there is now law saying how fast you get there! LOL It's always cheaper to blow that sucker out Regular! LOl

MC

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  • 2 weeks later...

Okay, I did the "procedure" and it worked great. No more knocking. I can't believe it. My buddy that owns a repair shop told me that he wouldn't do that to any car. He's shocked too.

I really appreciate everyone's thoughts and suggestions. Thanks again!

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I don't get it - why not WOT a couple of times on the way home form work and blow it out. Probably same cost in gas as in the treatment - but less time and a hell of a lot more fun....especially if you prey on Japanese burners (no offense, of course).

Tom R.

1998 Cadillac STS with 102K Miles

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First off, other than Temperature, at what different RPM's does this knock occur. Does it get lout as RPM's increase? Does it get faster as RPM's increase? If it's like my car that stays pretty much the same pitch and speed no matter what RPM's then it's probalby a timing component. I've narrowed mine down to either a faulty chain tentioner, or possibly a slightly chipped tooth on the intermediate sprocket, that drives the two timing chains. Judgeing from wher ethe sound is loudest when I check with my mechanic's stethoscope, I think it's the intermediate sprocket. Mine has also knocked liek it does now, since roughly 45k miles. I run the crap out of my car, and it has never gotten louder, or faster,...or worse in generally. I use less oil than I did then, and get better gas mileage, but the engine still run perfectly.

Unless the knock starts to change pitch or speed, or get's really, really loud I wouldn't worry about it. In general, these N*'s tend to be rathjer loud engines. My friend's parent's 2000 DTS is a little louder than mine, and there's doesn't knock or anything. It's just loud. Run's like a champ though.

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Okay, I did the "procedure" and it worked great. No more knocking. I can't believe it. My buddy that owns a repair shop told me that he wouldn't do that to any car. He's shocked too.

I really appreciate everyone's thoughts and suggestions. Thanks again!

I am so glad we could help another Caddy owner out!

Big Jay

Life is too short to grow up!

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