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hello everyone, my Pontiac Aztek with 3.4 liter V6 engine has been recently rebuilt but now has valvetrain tap on cold start up that lasts 3 to 5 minutes before going away. Will this cause any damage to the car or is this normal? valves and lifters have been replaced during rebuild. No noise when cold prior to rebuild, engine originally had rod knock. Many thanks for letting me know.
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so I used a mechanic stethoscope to try to isolate the sound and found that there is both a valve train noise and a deeper noise, maybe piston slap. The valve train noise last about 1 to 2 minutes whereas the piston slap lasts about 3 to 5 minutes. No noises when the engine is warm. I cannot explain the deeper noise other than to say that it sounds like a deeper metallic knock that may correspond to people’s common complaint of piston slap in GM engines. However, it is frustrating that this engine has already been rebuilt and it did not have this noise previously.
Giving the rebuilder the benefit of the doubt, probably he followed the GM OEM specifications. however, the specifications likely allow for piston slap. I am sure there many GM engines that are within OEM specs that also have piston slap.
One of the valve lifters may be a little slow to pump up. They are very precise devices and it doesn’t take much dirt or misassembly to get one to hang up. I would not worry about it. Our 1997 Odyssey wagon has had a mild tick for the first three minutes for 140,000 miles and it hasn’t caused any problem.
OP, so far, everyone suggested to stick it to rebuilder. I know it’s tough, but if it didn’t do it before and it does now – well, it’s on him.
Btw, I never realized that rocker roller is called tappet. I always knew that tappet is a bucket type metal cap sitting atop of the valve spring, between spring and camshaft.
But anyhow.. OP, what type of rebuilt was it? Did they replace pistons and rings? Which is quite normal for a complete rebuilt. As what you describe starts sounding like infamous GM engine piston slap.
Tons of literature and videos on this online. I know it from personal experience. Thing is, GM had crappy bore quality in some of their engines, resulting in slightly oval cylinder bore shape. As the result, on cold start, piston lower skirt will move slightly inside that oval, causing audible “slap” on the cylinder bore. As engine warms up and metal expands, that gap closes and noise goes away. Unfortunately, both piston and cylinder mutually “adjust” to each other. Replacing piston in such engine without re-boring it right will cause trouble.it was supposedly a complete rebuild including pistons and rings. I’m not sure if they rebored the cylinders although that’s what was written in the invoice. I did approach the rebuilder regarding this issue but encountered some resistance especially given that the problem is absent after 5 minutes after startup. I’m trying to gather more information/knowledge to make my case more forceful.
Thought so. Now it all goes down to how well they did the rebore – if it was done at all. It is tempting to simply slap new pistons and rings into honed OLD bore, as this works just fine, I had it done myself several times. Problem is, piston slap on GM engines is due to bore being OVAL in shape, so you set something ROUND in there – it’ll move.
My truck started having slap at about 60K miles and I sold it at 157K miles. Without any major defect or power loss resulting. But indeed it was extremely annoying, esp winter time, to listen to the noise. Also, you have to sit and wait for yey long, till it goes away.
GM issued a 6 page letter on piston slap, calling it normal. That’s on GM.
I think, you’ll have tough time with rebuilder taking responsibility.Btw, find out if you have oil lifters or tappets on your engine. If it’s a pushrod engine, you have oil lifters. If new, they should pick up oil pressure boom, right away. Also, oil lifters noise goes away if you revv engine a little bit. If you do not have lifters, and have tappets – well,gaps ain’t right then.
I uploaded two videos onto youtube of the noise; don’t know what everyone would say is the rhythmic noise esp with revving. Is it the piston slap? Lasts about 3-4 minutes, gone after warmed up.
Startup: https://www.youtube.com/watch?v=8U9zoGBZwl4
Warmed up: https://www.youtube.com/watch?v=VZnXd5FRlSIPlease let me know, many thanks!
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