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Engine rod knock????

D

dayco

Guest
I just bought a 2000 a month ago on 7/19/02 and every thing was good, I had to take a trip for 10 days and when I got back and started the car it sounded like it had a rod knock for about two blocks before it went away. It was OK the rest of the day. The next day I started it up and it still had a knock (sound not as loud) but only for a minute. I am about to go out and check it again, I almost hate to turn it on??????? Oil level is full and percent of oil change is at 15%, the car has 23,000 miles on it. Does anybody have any Ideas????? I don't have any documentation that the oil was changed when it should have been and I am woondering about warranty, it is suppose to be good until April 03????? I would appreciate any information or thoughts on this. Thanks.

PM. I did not want to start it up today thinking the worst, like a really bad knock but when I started it there was no knock at all, it ran fine, this is beyond me !!!!!!!
 
V

vettepilot

Guest
If your oil monitor on the DIC says 15%, then the first thing I would do is change the oil, and filter with the correct weight and type. Use Mobil 1, and the recommended viscosity for the temperature in your area. Probably 5W-30. I would also use the AC PF44 oil filter, this will put you back to the factory fill and filter combination, thus eliminating oil or filter problems right from the start. From the way you described the noise, I believe you are hearing piston slap during a cold start-up. Run it a couple of days and see if the noise you are referring to goes away. If it is still there, then perhaps your best bet is to get it to a dealer to see if they think it's normal piston slap at cold start-up.
vettepilot
 
D

dayco

Guest
Engine rod knock

Thanks for the input Vettepilot, I had planned to take it to the local Chev dealer and have them change the oil and filter. I have never heard this noise before, you refer to normal piston slap, I guess I don't know what normal piston slap is, I am sure the engine is not getting lubrication in some part of the engine but since it goes away I would hope it is not damaging anything internal?????? Thanks again Vettepilot.
 
V

vettepilot

Guest
I don't know if I helped or not, but sounds like you are heading in the right direction. That is, letting the experts listen to see what they think. The LS6 engine in my 02 Z06 has some really decernable piston slap when first started, and remains for about 60 seconds... enough time for the piston to begin it's expansion due to the heat of combustion. Once it begins to get warm, the piston slap goes away. Piston slap and rod knock can be confused with one another to an untrained ear. It's a little difficult to describe the diff. I guess the best way I can describe piston slap is; a light dull metallic knocking sound that increases in frequency with engine RPM. A rod knock on the other hand is a heavier dull metallic sound which also increases in frequency and intensity with engine RPM. The greatest difference is usually in the number of cylinders that exhibit the knocking sound. Usually a rod knock is confined to just the cylinder that has the bad rod bearing, whereas piston slap may present itself as several or all cylinders exhibiting the slap. The latter being especially true in the late model high performance engines because of the extra clearance and shape of the pistons in the cylinder bore.
I hope I haven't thoroughly confused you with all this, like I said it's difficult to describe sometimes.

vettepilot
 
J

John L

Guest
I have a 2000 A4 coupe. I purchased the car brand new in February, 2000 with 14 miles on the odometer. It now has 49524. It has knocked at cold start-up for about 30 - 45 seconds every morning that I've used it since I've had it. The car runs strong and perfect and always has. It does not use oil and I have had absolutely no engine or drivetrain problems of any kind. Check out this article for some good info about piston slap in the LS1/LS6:

http://www.idavette.net/hib/02ls6/page5.htm


The information in the article which relates directly to your issue is toward the bottom of the page, beginning about the 24th paragraph down, just below the photos of the pistons.
 

cntrhub

Well-known member
Joined
Jun 17, 2002
Messages
390
Location
North Hollywood, Ca.
Corvette
Miss my '62 & '80 4- Sp. Vettes
Just to be safe, I would dissect the oil filter and pull out the pleats. You want to see bearing material between the folds. They (bits of metal) will be somewhat large and obvious. Drain the oil in a super clean drain pan. What you are looking for, is a metallic-silver hew on the surface of the drained oil. If you do, you have more than a slapping piston. A dealer/shop may not take the time to diagnose your oil in this manor. They almost never use clean the drain pans, or they use a drain barrel on rollers, that causes the oil to disappear immediately to the bottom.
Try this now without draining the oil. Start the engine, (assuming the damage has been done) and take it for a ride. Take at least a 10 mile loop. Now, pull the dip stick and place a drop on a clean piece of white paper. As you look for contamination, angle the paper enough times so the sun's rays can set off any kind of sparkle in the oil drop. If you see any metallic debris floating in that drop of oil, it AIN'T no "piston slap"! I doubt you will find any. Aren't you relieved?
You would think you'd hear more than one piston slap in an 8 cylinder engine? It does sound like a piston. But I would still check for oil contamination. Let's not guess, or assume.
Pistons are lubricated by a "splash" method. Meaning, as the crank begins to spin, and the oil from the big end bearing shoots oil out the side of the rods, it tends to lubricate or splash the piston walls, and inside the piston. This is how the piston is cooled and lubricated.
If a rod was going out, the material has already done enough damage to keep knocking, even when warmed. Cutting open the oil filter will leave a clue as to internal damage.
Remember, the piston changes direction on the up stroke (1st slap) as well as the down stroke (2nd slap).
 
D

dayco

Guest
Engine rod knock

I appreciate the information you folks have given me, I wish I had read your mail before I had the oil and filter changed on Friday. Cntrhub that is a good idea to check for metal in the oil, I will do the dip stick check today. After the oil and filter was changed there was no knock at first start up, which I would assume there would have been if there was a problem. It has been fine everysince the new Mobil 1 was put in??????? I will go back to the place that did my oil change and see if I can retreive my old filter and cut it open.
John L. I read the article and I assume GM does not consider the knock a problem, if I were to test a corvette at a dealers lot and it knocked I sure would not buy it (thats a problem for GM). They also said "engines after start-ups in COLD WEATHER is not a durability concern it's a pleasability issue". My knock was in 75 degree morning temp start up and only occurs after a long down time (oil drained down from engine). I don't think I have a major problem like I thought when it first happened but it sure needs more home work.
Thanks to all you folks for your help.

Dick Day
 
J

John L

Guest
Dick,

I agree with you, I don't think that you have a problem, either. Mine doesn't always do it, either. It is more noticeable on cool mornings (we don't have a whole lot of those down here) or if the car has been sitting for two or three days. It never lasts for more than half a block and it has gotten so that I don't even notice it anymore. I hadn't even thought about it for months until I read your post.

Bye the way, GM did address the problem beginning with the mid 2001 model year production run by coating the newly redesigned, skirted pistons with an anti-friction polymer and increasing the piston-to-bore clearance. I haven't heard of this "anomaly" occurring in cars produced after that. If they've got a knock, it really is serious! Personally, the fact that mine has the piston slap doesn't bother me, because I know what it is, it is of extremely short duration, and only occurs intermittently at cold engine start-up. And frankly, I'd just as soon keep the increased clearance offered by the original design specification.
 

c4vette92

Active member
Joined
Dec 16, 2001
Messages
35
Location
albany ny usa
Corvette
2003 50th
i have a 2000 with 30 k on the car and the engines has the same knock sound for 30 to 45 seconds. it doens't use any oil and the engines runs strong.. you need to get the oil and filter changed. stw.
 

cntrhub

Well-known member
Joined
Jun 17, 2002
Messages
390
Location
North Hollywood, Ca.
Corvette
Miss my '62 & '80 4- Sp. Vettes
You can tell a rod knock because it will literally be, a knocking noise. If a piston is burned at the skirt area, it will sound like a valve tick. Remember that the next time you think you have a lifter that needs replacement or an adjustment. You pull the rocker covers, find compression and push down on the rocker assembly. Nothing is out of the ordinary, and there is no excessive movement...... Burned Piston Skirts for sure!
Production machining falls within a certain tolerance. Say for example, the machinist at the factory comes to work in the morning to begin work. The room is cool. The machine's bore cutting tip and arm are also cold to the touch. The engine block is on the cool side also. First cut is made. Cylinder wall begins to heat up. The tip of the cutting tip and arm begin to expand caused by heat. Tolerances are now compromised. This is why you sometimes hear maybe one piston begin to slap and not the whole bank or more pistons. It's all in the machine work being done at certain stages. Ohhhhh..... those %*#@ Variables!
To make a long story short, I noticed my sponsor make a cut to one of his racing cylinders. After the cut, he would turn the machine off, walk away, and do something else. Then he would go back to take a measurement of the cylinder's wall. I'd asked why he didn't just take a measurement right after the cut? He said he wanted to get a correct measurement...."Cold."
 
D

dayco

Guest
Engine Rod Knock

I want to thank all you folks again, I was unable to retreive the oil filter but I did do the drop of oil on a white paper and looked for mini chips, I did not find any and the car has not had a knock since I changed the oil. I am thinking I don't have a problem and I will go forward with that in mind. This car sure is great I feel like I am flying an F-18 among a bunch of cargo planes when I am on the street with it, but of course I don't drive that way (just an analogy). Hope I never hear that noise again.

Dick Day
 

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