Rough Idle.... - Page 2 - 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 07-02-2007, 02:47 PM   #11
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Uh, I don't think that should be necessary. Or even advisable on a frequent basis.
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Old 07-02-2007, 03:07 PM   #12
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Well from everything I have read on other Cadillac Forums it has been recommended. Something about the LD8 need the high rpms it almost begs for it. Took it out for a 3 km run got her up to 6k let off repeated 3 times stopped and got out listened to her and you could barley hear her running.
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Old 07-02-2007, 03:45 PM   #13
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Is that a "clean out the carbon" type of run? lol

If it works great. Although you mention some internal work needing to be done?
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Old 07-02-2007, 03:53 PM   #14
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Quote:
Originally Posted by ReallyAGXP
Well from everything I have read on other Cadillac Forums it has been recommended.
Yikes.
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Old 07-02-2007, 03:54 PM   #15
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Yeah but it has yet to be diagnosed and the SA was hesitant to start pulling things off to find out the problem. Being a throw away block he is afraid to get into it untill the rep clears.

and yeah a clean out the carbon run is what that is.. running to 6k them letting off lets the rings move freely thus cleaning the carbon off of them. this method is supposed to help with any oil consumption as well, I'll throw a link up when I find it.
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Old 07-02-2007, 04:02 PM   #16
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Heres a long read just one I have found

Quote:
As mentioned, GM top engine cleaner is the alternative method. More expense, more work and less fun. The Northstar was designed to be run long and hard. I forget the number but during testing they were run for hundreds of hours on end at WOT on a dyno at the factory. You won't hurt it. Take her out and whoop her! Here is a quote from our old friend and guru.

"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'* 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."
Except with the New trans it will shift as soon as you hit 6200 RPM.
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Old 07-02-2007, 05:19 PM   #17
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Ya, I've heard that as well for the 4.0. I don't really buy into it.

Before we all met up last weekend PeterG got a good shot of carbon out-blast from my car and I didn't need no high RPM runs. It just takes a good pedal-to-floor push and that was enough.
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Old 07-03-2007, 10:01 PM   #18
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Quote:
Quote:
As mentioned, GM top engine cleaner is the alternative method. More expense, more work and less fun. The Northstar was designed to be run long and hard
if its desingned to be run long and hard why is it a cadilac engine .funny i think
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Old 07-03-2007, 11:24 PM   #19
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It wasn't designed to be that way. Early failures dictate that as a preventive measure. Unaccounted for and unpredicted.
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Old 07-04-2007, 12:47 AM   #20
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The valve clearance is so tight on the 4.6, the valve clatter can even happen with just a few days of easy driving. Or bad fuel/low octane fuel. The condition your having Rob is called cold carbon knock. It has plagued the L37/LD8 since it'* inception The standard fix as I've read on many Caddy forums is to out it in 2nd gear and nail 6000 rpm. ( The Italian Tuneup) I myself do two things routinely. Disconnect the battery and let the engine do it'* learn procedure over again and just run WOT'* thru the gears a couple times and the engine returns to purring like new. A good NOX run really cleans it out, but I haven't been buying any since they pulled my License. Internal problems with the engines are next to nil and when they do happen, it'* usually catastrophic.
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