Motor knocking
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Joined: Sep 1999
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From: Beaufort, SC
Motor knocking
I've had a slight knock spring up on the left side of my 305. It doesn't sound like a valve rattle. This is more of a dull knocking sound. Please don't tell me I have a rod going! This is our only car and I need it to be solid. I'm hoping just an exhaust leak I haven't found yet. This car has really been babied(sp?) so hopefully it's somethind simple.
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Sarge,
Unfortunately, your description of the sound as "a dull knocking" is more common with a rod or main bearing failing or lack of lubrication. A sharp, metallic tick or clicking sound is most generally associated with the valve train. The valve train parts are smaller and lighter, and have more of a linear reciprocating motion. There is also less metal around them to shroud and absorb or muffle the noise.
With a bearing failure, the dull sound is due to the cylinder case absorbing some of the shock, the reciprocating/rotating motion of the rods on the crank that don't change direction immediately, and the higher flow of oil to the mains and rods cushioning the impact to some extent. A rod or main knock will typically be most noticable when you first start the engine after it has cooled. There is little or no residual oil and the oil may not get pumped to the loose bearing immediately. The noise will generally go away or be reduced as soon as the engine oil warms up a bit and starts flowing better. The internal components are also warming and changing dimension as they expand. As the engine heats more, the oil will thin and allow more clearance again, making the sound more apparent after the engine reaches normal temperature. All componenets are again at a relatively equal temperature, and the differences in dimensions will be more "normal". The increased oil clearance of the bearing(s) will allow some excess movement of the parts, creating the sound.
If it is very faint, you may be able to do a couple of things to get by for a while.
Make sure the engine oil is clean, and is of the highest viscosity recommended for your temperature range. The more viscous oils usually have better molecular cohesion and typically have a better film strength as a result. This "tougher" film of oil can help reduce the metal-to-metal interference and cushion the shock better. This is another reason that synthetic lubricants excell. They can have a higher viscosity at operating temperatures but still pump and flow well at lower temperatures for excellent startup lubrication. A 10W40 synthetic flows better cold than a 0W30 mineral oil, and certainly has much better hot protection.
For the assurance of good oil flow, use a good quality oil filter as well. This isn't the time to save two bucks on an oil filter.
Maintain correct oil level.
If the noise is caused by a lack of lubrication flow, you may be doing yourself a favor by switching to synthetic, since it tends to remove varnish and sludge, and can allow full oil flow through partially clogged passages, galleries, and drillings.
Make sure the oil pressure is adequate. Don't rely on the dash gauge for an accurate indication. You may not be able to do much about the pressure without removing the oil pan, but at least you'll have a better handle on the situation instead of guessing.
Keep the engine load as low as possible to prevent further damage. High loading at low RPMs tends to lug the engine and creates extreme pressure on the bearings. Allow the engine to warm before driving, since a cold engine with excessive oil clearainces will compound the problem.
Make sure the noise is not caused by a cracked flexplate or loose or damaged torque converter. You should be able to remove the inspection cover and check that pretty easily.
Good luck.
Unfortunately, your description of the sound as "a dull knocking" is more common with a rod or main bearing failing or lack of lubrication. A sharp, metallic tick or clicking sound is most generally associated with the valve train. The valve train parts are smaller and lighter, and have more of a linear reciprocating motion. There is also less metal around them to shroud and absorb or muffle the noise.
With a bearing failure, the dull sound is due to the cylinder case absorbing some of the shock, the reciprocating/rotating motion of the rods on the crank that don't change direction immediately, and the higher flow of oil to the mains and rods cushioning the impact to some extent. A rod or main knock will typically be most noticable when you first start the engine after it has cooled. There is little or no residual oil and the oil may not get pumped to the loose bearing immediately. The noise will generally go away or be reduced as soon as the engine oil warms up a bit and starts flowing better. The internal components are also warming and changing dimension as they expand. As the engine heats more, the oil will thin and allow more clearance again, making the sound more apparent after the engine reaches normal temperature. All componenets are again at a relatively equal temperature, and the differences in dimensions will be more "normal". The increased oil clearance of the bearing(s) will allow some excess movement of the parts, creating the sound.
If it is very faint, you may be able to do a couple of things to get by for a while.
Make sure the engine oil is clean, and is of the highest viscosity recommended for your temperature range. The more viscous oils usually have better molecular cohesion and typically have a better film strength as a result. This "tougher" film of oil can help reduce the metal-to-metal interference and cushion the shock better. This is another reason that synthetic lubricants excell. They can have a higher viscosity at operating temperatures but still pump and flow well at lower temperatures for excellent startup lubrication. A 10W40 synthetic flows better cold than a 0W30 mineral oil, and certainly has much better hot protection.
For the assurance of good oil flow, use a good quality oil filter as well. This isn't the time to save two bucks on an oil filter.
Maintain correct oil level.
If the noise is caused by a lack of lubrication flow, you may be doing yourself a favor by switching to synthetic, since it tends to remove varnish and sludge, and can allow full oil flow through partially clogged passages, galleries, and drillings.
Make sure the oil pressure is adequate. Don't rely on the dash gauge for an accurate indication. You may not be able to do much about the pressure without removing the oil pan, but at least you'll have a better handle on the situation instead of guessing.
Keep the engine load as low as possible to prevent further damage. High loading at low RPMs tends to lug the engine and creates extreme pressure on the bearings. Allow the engine to warm before driving, since a cold engine with excessive oil clearainces will compound the problem.
Make sure the noise is not caused by a cracked flexplate or loose or damaged torque converter. You should be able to remove the inspection cover and check that pretty easily.
Good luck.
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Joined: Jan 2001
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From: Annandale,NJ
with the old 2.8 i had in an S15 we had a rod knock!!! and I put in 50wt oil and just hoped for the best.....finnaly i went and got a rebuilt and the old engine.....OMG you would not belive the rod movement........I got 1 yr out of it with that rod knock and 50 wt oil. 
Thread Starter
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Joined: Sep 1999
Posts: 436
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From: Beaufort, SC
It's time for an oil change so I'll switch to synthetic. Will 10w40 be good? I finally got rid of my biggest oil leak when I replaced my timing chain, cover, seal, etc. Oil pressure holds steady at 20 pounds ideling and 40 at speed ~ 2000 rpm. Thanks for the advice guys. Hopefully we'll be able to keep her going till we get a second car.
On another note, keep the seven Marines who died today and their families in your prayers. Semper Fi.
On another note, keep the seven Marines who died today and their families in your prayers. Semper Fi.
Last edited by SgtSkip; Jan 9, 2002 at 09:48 PM.
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