I've got a 2001 Tahoe with 53,000 mostly-highway miles. Lately, I notice on startup in the morning (outside air temp ranging from 50 - 65) fairly noisy valve tappets. After a few minutes of low-speed driving, it quiets down. Is this normal, or should I get the valve clearance checked? What's the likelihood of one or more faulty tappets?
Thanks.
Sure it's valve noise? A noise like that could also be piston slap, it seems to happen on some of the newer-design V8 engines..
Sounds more like the dreaded piston slap.
Oh you beat me to it. :-)
Okay, so what does the "dreaded piston slap" mean to me?
Piston slap occurs when the piston is forced rapidly against the side of the engine cylinder wall. The more clearance between the piston and cylinder wall, the louder the knock. Controlling piston slap is a complex process. Too little clearance between the piston and the cylinder wall and the parts will score and fail. Too much clearance and you get a knock. It doesn't help that usually the piston and cylinder are made of different materials and have different expansion rates.
Several features are used in piston design to reduce slap. To keep the piston close to the cylinder yet allow room for expansion, the piston skirt (the part that slides against the cylinder) is tapered - it is bigger at the bottom than at the top. The top of the piston expands more, where the extra clearance is, because of higher heat at the top of the piston. The bottom always remains close to the cylinder.
Pistons are also made oval shaped. The large part of the piston is close to the cylinder, while there is clearance on the smaller sides. As the piston expands, heat is transferred into the smaller sides, so the piston becomes more round. Thus, the large sides of the piston always stay close to the cylinder and piston slap is avoided.
There are several other piston features to counter piston slap, such as offsetting the piston pin position, but I think you get the idea. Too much clearance between the piston and the cylinder and we hear that Knock, knock, knock.
In the past, the sound of piston slap meant trouble. Worn cylinders, damaged piston skirts, or cracked pistons were common causes, and all meant expensive repairs. Now things have changed.
Engine designs have changed to make them more compact, lighter, with less internal friction, and higher revving. To do all this, piston design had to change, and some of the major changes are shorter piston skirts and straight piston skirts. The short, straight skirts allow the piston to rock more in the cylinder, and we hear it as piston slap.
Wow. Thanks for the detailed response - I do appreciate it. Soooo, should I be doing anything with my 5.3?
Alan
You may want to use a thicker grade engine oil or check with GM if they have revised piston kits for the 5.3L (new gudgeon pins and rings etc.). Generally I would not worry too much about it unless it starts to consume an unacceptable amount of oil and or the piston slap still occurs when the engine is at its normal temperature. If your local dealer says "its normal for it to happen" Ask them if they do have a revised piston kit.
It still does.
My 2001 Malibu needed a new engine at 46,000 kms due to piston slap.
Brad
To add, if they say it's normal, get that in writing. Be friendly, but try to get them to assure you that if they are wrong and it breaks, they should be responsible. GW
Dan--- wrote:
Normally, that doesn't happen with piston slap. The 3.1L engines seem to be a special case, there seems to be a piston-scuffing issue involved with them..
"AlanRab" wrote
It the noise is gone by the time the engine warms up, it's totally normal. The dealership will probably do nothing for the problem as there have been bulletins released about this noise, and about the fact that GM will not be fixing this problem unless it's really bad. There are parts for the problem, but I have yet to hear one of these engines that are bad enough to warrant replacing the pistons. We have gotten to the point in the dealership that I work at where we will have a foreman do a cold start check and then basically let the customer know that nothing will be done for this noise.
It's just a fact of life with short skirt pistons. And it's not a particularly new problem, it's been around for years on different engines.
There were some other problems that could cause a "lifter" type noise, specifically an o-ring that seals the oil pump pickup to the front cover, but that problem would have showed up at an early mileage.
Ian
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