Does this make sense to remove carbon buildup? Piston knock - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


Bonneville GXP/ Northstar Powered Cars Discuss your Bonneville GXP and/or any other Northstar powered Olds or Cadillac... Including the 3.5L Twin Cam V6 (Short Star ) 4.0L and 4.6L Northstar V8's. Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 03-19-2008, 10:08 AM   #1
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Default Does this make sense to remove carbon buildup? Piston knock

I copied this from somewhere. It is an interesting read.... let me know if you all agree. thanks

As you know, working with GM'* Powertrain has its advantages. I see many engines and learn what works and what doesn't. I get to see many engines returned by dealers and do many of the inspections.

Occasional full throttle accelerations with a Northstar and any engine 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'* 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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Old 03-19-2008, 10:51 AM   #2
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I did that all the time on my GXP. Although you cant really WOT on it in 2nd gear because it would always shift on me so I slowly brought the RPM'* up in 2nd gear and kept it hovering around 6k iirc then slowly back down and repeat the process. I thought it made sense.
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Old 03-19-2008, 11:16 AM   #3
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Or....just run seafoam through it? I don't see how SO much carbon can build up SO fast that it would become an issue. You shouldn't have to race any engine to keep it within normal operating specs IMO.
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Old 03-19-2008, 11:16 AM   #4
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Get a windex sprayer, rinse it out, and fill it with tap water.

At idle, spray just as much as you can directly into the throttle body to make the engine stumble, but not actually die.

This is an old trick I used to use on my old carb'd vehicles back in the day (not telling when) to keep them from dieseling (run-on after key off). It'* an old-school mechanic'* trick that works very well.

Proof is in how clean the internals of my last engine were from running water injection. This is far simpler and more effective, and doesn't waste gas. It cleans the entire system between the TB and the exhaust manifolds. On an older car, you can do this with the downpipe disconnected from the cat, and watch black chunks get spewed all over the driveway under the car.
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Old 03-19-2008, 11:18 AM   #5
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OOOH, I really could have used that trick with my old Nissan truck. it would always diesel when I'd shut it off. it was amusing, but annoying.

good tip though.
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Old 03-19-2008, 02:03 PM   #6
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Quote:
Originally Posted by willwren
Get a windex sprayer, rinse it out, and fill it with tap water.

At idle, spray just as much as you can directly into the throttle body to make the engine stumble, but not actually die.
Thanks,
Silly question, Where do I get into the throttle body?
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Old 03-19-2008, 02:10 PM   #7
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The water trick works great for keeping old 2-stroke outboards clean, too

You can get into your throttle body by removing the air intake bellows from the front of it; careful, though; you don't want to spray water directly onto your MAF sensor.

I'd suggest removing the MAF screen from the mouth of the TB (big c-clip holds it in), and spraying to the side of the MAF sensor if you're going to use water from a spray bottle.

EDIT: strike that; you're talking about a Northstar; I have no experience there.
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Old 03-19-2008, 03:03 PM   #8
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Thanks, That will get me where I wanted. I will look out for whatever sensor I find there.
I haven't looked into the Northstar TB yet. Lots of power, and definitely louder, but not as much quickness off the line. BUT I STILL LOVE MY CADDY . (Miss my Bonney) :(
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