11-17-10, 09:01 PM
96 sts. 125k miles. had heads off several yrs ago. start car and motor is quiet for 5 seconds or so than a little rattle starts. maybe valve train? quiets down after 2-3 minutes. but why does the motor not rattle for the first 5 seconds? i have started car with hood open and listened to motor. are the timing chain tensioners extended at startup based on when motor shutdown and was warm and had full oil pressure and than actually retract since the car has low pressure when cold? maybe because a cold motor sends most of oil thru bypass, i really have no oil pressure at startup?
11-18-10, 09:32 AM
The oil pump relieves excess pressure (cold oil) through a pressure control plunger and spring while the oil filter internal bypass prevents cold oil from blowing out the media, but at no time does the engine not receive oil flow or pressure: oil always flows to the engine - only the filter media is bypassed until the oil warms sufficiently.
The chain tensioners are hydraulic but they retract very little at shutdown, so if the rattle is in fact chain oriented, it would most likely be a slipper (guide) or worn nylon tensioner shoe.
Belt tensioner or idler? A/C compressor clutch bearing? Water pump drive? Harmonic balancer pulley wobble? What oil, filter, last change?
11-18-10, 09:21 PM
did oil in aug. about 1500 miles. 10w-30, delco filter. had motor apart for HG repair and guides looked fine. almost no wear. new idler pulley and tensioner arm too. has different p/s pump and alt. in the last year. strange. engine purrs at start and than 5-10 seconds in, it starts to tick/clatter. goes away while motor is warming. did heads 3 yrs ago. i think rattle has been present since than. not sure if it did it before repair. cleaned piston tops and combustion chambers also when heads were off.
Occasional Full-Throttle Acceleration Is Good For Your Engine (check this discussion for references): (back to the top)
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...
Sorry but i copied and pasted this from the Cadillac Technical Archive but i believe it will help u understand better.. Hope this helps!!!!