Cold start engine rap - does anything help?
Thread Starter
Drifting
Joined: Nov 2007
Posts: 1,616
Likes: 11
From: Newport, RI
Cold start engine rap - does anything help?
Just bought a 2004 C5 coupe with auto and 15K miles. It has a very noticeable rapping when cold. I probably shouldn't have bought the car, but I test drove it when warm and there was no rapping. The engine really has to warm up before the rapping goes away. I'm disappointed the seller never mentioned this.
I'm assuming it's a piston which is a loose fit to its' bore, and I know there are issues about GM piston fitment. I also realize that alum pistons expand/contract at different rates than steel blocks. I've owned plenty of other Chevy cars which made the cold engine noise, but this one seems kinda bad.
Are you guys using any special oils or additives which reduces this rapping?
thanks
I'm assuming it's a piston which is a loose fit to its' bore, and I know there are issues about GM piston fitment. I also realize that alum pistons expand/contract at different rates than steel blocks. I've owned plenty of other Chevy cars which made the cold engine noise, but this one seems kinda bad.
Are you guys using any special oils or additives which reduces this rapping?
thanks
Last edited by Chevy-SS; Dec 11, 2008 at 04:09 PM.
Instructor
Joined: Oct 2008
Posts: 137
Likes: 1
From: Wisconsin
Yep...good ol' piston slap. Nothing I've heard helps, and it will get worse over time. The amount of time depends on how hard you drive the car and number of miles (obviously). As you know, quite common. Just keep the oil level full and change as recommended. With only 15K miles, I wouldn't worry for quite a while.
Instructor
Joined: Apr 2007
Posts: 218
Likes: 0
From: JOHNSTON RHODE ISLAND
hey dave,sorry i did not mention it.reason is that when i heard it for the first time,i was like WTF. but after posting here on the forum,i was not alone and it was a issue from gm.your car has seen no abuse at all so the noise is unavoidible from gm.there's both a 06 and 07 vette's on my street with the same knock's and there both under 10k.keep the oil clean and full and warm that baby up before you go.in time with 100k on the motor and it will still run like it does,and sound too!!lol....al
Le Mans Master
Joined: Feb 2002
Posts: 6,937
Likes: 21
From: Waldorf MD
Le Mans Master
Joined: Sep 2007
Posts: 7,604
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From: Belleville Il
The most simple solution I use (it is kind of cold here this tome of year) is to punch up the oil temp on the DIC and take it easy ,,not revving very high or not lugging at lower speeds. when the temp gets to the 100 degree I relax the limits a bit and start driving in a more normal fashion. This lets the car warm without wasting gas(its actually a little better for the motor as well) 
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Le Mans Master
Joined: Feb 2002
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From: Waldorf MD
I would use a good flowing oil filter, like WIX or NAPA Gold (same filter). When I switched from DELCO Premium UPF-44 and Mobil 1 (both filters "filter too good" at the expense of oil flow), my cold oil pressure increased 10-15 psi. Since the C5 oil pressure sensor is AFTER the oil filter, this means MORE oil flow.
Le Mans Master
Joined: Oct 2004
Posts: 5,841
Likes: 19
From: CLT, North Carolina
I'm sure it's annoying as all get out but it will probably not get worse except maybe very very gradually over the course of piling up miles and it doesn't hurt a thing other than your pride. It is difficult to design a quiet piston that still makes good power because in order to do that they must have tight piston to bore clearances and tight clearances soak up power like a sponge. Factory skirt clearance spec is .0007" - .0021" with cast pistons. Clearance set toward the tighter side of this range will be quieter.
One thing's for sure... the only way to make it quieter is to pull all the pistons out and carefully measure all eight bores for size and straightness and all eight pistons for skirt diameter to see what you have. Maybe yours has a couple running with .003, who knows. IMO this is a lot of effort where the only results will be quieter operation. The noise is harmless.
The piston slap website referenced above is somewhat reaching and overstating and alarmist in my view.
One thing's for sure... the only way to make it quieter is to pull all the pistons out and carefully measure all eight bores for size and straightness and all eight pistons for skirt diameter to see what you have. Maybe yours has a couple running with .003, who knows. IMO this is a lot of effort where the only results will be quieter operation. The noise is harmless.
The piston slap website referenced above is somewhat reaching and overstating and alarmist in my view.
Thread Starter
Drifting
Joined: Nov 2007
Posts: 1,616
Likes: 11
From: Newport, RI
I would use a good flowing oil filter, like WIX or NAPA Gold (same filter). When I switched from DELCO Premium UPF-44 and Mobil 1 (both filters "filter too good" at the expense of oil flow), my cold oil pressure increased 10-15 psi. Since the C5 oil pressure sensor is AFTER the oil filter, this means MORE oil flow.
-
Thread Starter
Drifting
Joined: Nov 2007
Posts: 1,616
Likes: 11
From: Newport, RI
I'm sure it's annoying as all get out but it will probably not get worse except maybe very very gradually over the course of piling up miles and it doesn't hurt a thing other than your pride. It is difficult to design a quiet piston that still makes good power because in order to do that they must have tight piston to bore clearances and tight clearances soak up power like a sponge. Factory skirt clearance spec is .0007" - .0021" with cast pistons. Clearance set toward the tighter side of this range will be quieter.
One thing's for sure... the only way to make it quieter is to pull all the pistons out and carefully measure all eight bores for size and straightness and all eight pistons for skirt diameter to see what you have. Maybe yours has a couple running with .003, who knows. IMO this is a lot of effort where the only results will be quieter operation. The noise is harmless.
The piston slap website referenced above is somewhat reaching and overstating and alarmist in my view.
One thing's for sure... the only way to make it quieter is to pull all the pistons out and carefully measure all eight bores for size and straightness and all eight pistons for skirt diameter to see what you have. Maybe yours has a couple running with .003, who knows. IMO this is a lot of effort where the only results will be quieter operation. The noise is harmless.
The piston slap website referenced above is somewhat reaching and overstating and alarmist in my view.
Yeah, I agree that the slap doesn't mean impending engine destruction (at this point, anyway). It just sucks listening to it.
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Race Director
Joined: Jul 2007
Posts: 11,467
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From: 972 and 405
I have just under 70k on my engine and the ambient morning temps here in North Texas are around 30-40 degrees. What should this "rap" sound like, not sure if I am hearing this sound.
-Alex
-Alex
Racer
Joined: Nov 2008
Posts: 378
Likes: 0
It's normal, it won't get worse with mileage and will not affect performance. The vast majority of 2001-2004 LS1's have this. GM calls it "cold start noise", for the rest of us in the real world it is piston slap coming from #7.
The only harm this causes is starting your car in front of people when it's cold and knowing they hear it!!! There are no additives etc. to stop or even quiet it down. Enjoy your car, the noise is normal.
The only harm this causes is starting your car in front of people when it's cold and knowing they hear it!!! There are no additives etc. to stop or even quiet it down. Enjoy your car, the noise is normal.
CF Community Team
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From: Cape Cod, Mass.
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I'm on my second C5 and they both have piston slap when cold. My C6 also had it. Most race cars are built like this and as a result make more horsepower. If they are too tight in the bore when cold there will be too much friction when warm.
Tech Contributor
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From: Somers, CT and Clermont, FL
St. Jude Donor '09-'10-'11-'12-'13-'14-'15-'16
Tech Contributor
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From: Somers, CT and Clermont, FL
St. Jude Donor '09-'10-'11-'12-'13-'14-'15-'16
Melting Slicks
Joined: Aug 2003
Posts: 2,786
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From: Iowa
when cold mine idles quietly, when I start to move I can hear a very mild knock, it goes away within 30-60 seconds of driving down a 25mph road in which you'll hit parked car if you go over 15. it doesn't bother me at all, I doubt you could even hear it from the outside.
Tech Contributor
Joined: Oct 1999
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From: Charlotte, NC (formerly Endicott, NY)
All Corvette LS engines have a cold start piston slap. My 97 would make quite a bit of noise until it got warm (made the noise from the day I took delivery with 4 miles on the car). The 03Z doesn't make as much noise.
Bill
Bill
Last edited by Bill Dearborn; Dec 13, 2008 at 04:49 PM.