'99 3100 Piston Slap

[knockknock]

I have asked this question in other places but was unable to get a direct response, so I just want to see what your opinions are.

1999 Lumina
3100 V6
Oil change every 2500 (5W-30 non-synthetic).
New plugs, wires, idler pulley, fuel injector cleaning.

I have a 1999 Lumina with almost 90K on the clock. I purchased it May 2005 with 76K. Back then, the only noises I would hear it make on startup were the random valve noise and a clacking noise from the pan. The clacking noise starts about a second after ignition (clearance issue?).

However, in November 2005, I noticed a knock I would get when I would accelerate with low-rpms. This noise lasted for about 100 feet, and was gone just like that. I can also replicate the noise by sharply hitting the gas in Park or Neutral. It only occurs under load. Not when the RPMs fall to idle.

The noise went away one morning, and I couldn't replicate it even with revving in P or N. It came back, however, and the car still does it to this day. It is much more noticeable in winter (cold temps), but even now after the car was fully warmed, cooled off for two hours, started it back up after a meeting (got the clacking and valve noise) and then the low-rpm knock came as soon as I backed it out of the lot and drove forward for 25 feet or so.

Once the knocking is gone, so is everything else. You can barely tell the car is running it is so quiet.

The random ticking I figured was valve racket. I thought the clacking when it was started was piston slap. What is this knocking? I know it is nearly impossible to tell without being there, so if there are any specific tests you want me to try I'll be glad to. Or, is the knocking in fact piston slap?

[John Michael]

I don't think I have any good answers for you, but I have a '97 Lumina with the 3100 and it has the EXACT same symptoms. It has 85,000 miles. Now that it is summer I hardly notice it, but when it's cold out it is definitely worse - a deep knock sound under load from stop to about 30mph. After the car is warmed up it runs perfectly quiet. (I also have the valvetrain clatter at start-up but that is normal).

I've also taken very good care of the car, always oil changes 3k miles.

I hope someone else responds to verify if they think this is piston slap, and if so, my question is:
Does this condition get worse fast, or will this motor last for a long time yet?

I had the lower intake manifold leak and I know some coolant got into the oil, but not much and not for long before I fixed it. I originally thought maybe the Dex-cool did some damage to some bearings, but if that were the case then the noise probably wouldn't go away when warm, right?

So I've assumed it's some piston slap starting. I've just accelerated very gently when cold to hopefully minimize further damage.

[prophiseer]

As long as the knock goes away after the motor has warmed up, it should be alright. But if it continues to get worse, or the motor starts knocking at all temps - then you're breakin open the wallet.

Usually you can get rid of the valve racket by changing to 10W-30 oil, or you can try using some kind of half synthetic, half conventional blend 5W-30; the synthetic properties might help to keep the valve components better lubricated after shutdown. Another option is to add some Lucas brand oil stabilizer, but this (IMO) is a last resort, I've found that it tends to thicken the oil a little too much for my favor, and plus thicker oil will hurt your fuel economy.

I don't think you've got to much to worry about, so far though, just be gentle on 'em while they're cold!

[Timothy]

Knocking on heavens door

I would put an oil pressure gauge on it and compare specs, I've seen defective oil filters cause havoc as well as oil pressure relief valves getting hung up this is also a reputable machinist thoughts as well.

[Superdave]

in 2003 they started using pistons with coated skirts to prevent piston slap, it's common in the older gen3 660's

[PCGUY112887]

in 2003 they started using pistons with coated skirts to prevent piston slap, it's common in the older gen3 660's

My moms 03 Buick with the 3100 in it still slaps like every other 660 i've heard, and hers only has 23k miles of pussy driving.

[tractorman]

What ya'll are describing is classic of piston-slap.

I know that GM has dealt with this piston slap issue for years now, and they claim that it isn't a problem. There's a '99 malibu down at the college that started the piston-slap knock at 90K miles. They sold it at around 160K and it was still running fine. Neither of my 3100 engines make the noise, but as long as they have good oil pressure and don't use oil, I wouldn't worry about it.

I've run 15W40 fleet oil in my '94 corsica for about 6 years now, and my '01 lumina has had 15W40 run in it since it was new. Maybe that's why my pistons don't slap. GM's reccomendation of 5W30 is gas-mileage-squeezing nonsense. I'll gladly sacrifice 0.25 MPG if my engine lasts longer.

[Superdave]

My moms 03 Buick with the 3100 in it still slaps like every other 660 i've heard, and hers only has 23k miles of pussy driving.

Must have been late 2003 then, my 2004 Grand Am GT dosen't have any piston slap, i've put about 45K miles of semi abusive driving on it including 2 cross-country trips.

[3400-95-Modified]

I have a 01 3400 and I noticed a coating on the pistons when they came out and I put back in Sealed power replacement pistons that had a Teflon skirt on them, and now at 32k on the motor, it has similar noises, and this thing has only seen synthetic oil its whole life. I've just come to accept it because there is not a knock under load which would mean a bearing is going.

[prophiseer]

It's just usual GM v6 noise, even the 90* motors seem prone to it . . .

my '99 GA 3400 has racked up about 135k miles and only has a slight knock when cold. It's been doing it since I bought the car and the motor had 74k miles on it. Nothing to worry bout yet . . .

[IsaacHayes]

Mine makes noise, but once it warms up, it's quite. 180k which half of that was abusive, and still going strong.

It's the LIM that are the major PITA maintenence on these motors.

[knockknock]

Well thanks for all these replies, it makes my engine seem "normal" now.

The only reason I got so worried about it was since I bought the Lumina last May it never did it until November (oil change at a vioc...didn't have time to do it on myown) and it developed at that time. Sounds really nasty and makes you want to shut it off if outside is ever colder than 40F, and the noise has stayed until this day even when it's 80 outside. It is quieter the warmer it is...annoying in summer, scary in spring, fall, and winter.

And about Lucas, yeah I've tried that recently, tried Slick 50 before it got too cold out, and ValveMedic even. Nothing really helped. Made me believe it wasn't really an internal noise, guess I just have the slap. I've never heard another GM do it so I don't know exactly what it sounded like...just a rythmic "knock-knock-knock" every turn of the crank it seemed every time it sat for more than two hours. thanks again

[IsaacHayes]

Mine will quiten down after it warms up. In summer it's not really noticeable at start even. I tried 10w30 and it took it longer to quiten down, and HP suffered. I switched back to 5w30 and it was better.

Mine never makes a knock knock when I turn it over though....? Mine sounds more like valve train noise and the lifters get oil and hush up..

Only time it's made noise cranking was when it sat for 1-2 months and was dry when it turned over. Scraping sound. Another noise is if you don't crank it long enough and it dies, you will hear a scrape/pffft metallic noise.

So what is the noise everyone experiences? knock knock on crank, or ticking noise unitl it warms up?

[prophiseer]

Mine is defi piston slap . . . it's too low and deep a sound to be any type of valve noise, and it goes away after it warms up. Plus, with my car on a lift, it's louder from underneath, closer to the oil pan.

I do my best to baby the darn thing when she's cold, I don't want to contribute to any premature wear (but the motor runs so great cold ).

[knockknock]

my bad, didn't mean it knocked when cranking over. just every rotation of the engine is about the frequency it does it i think. mine doesn't sound at all like the valve noise (well the idle clacking does which is what i thought gm piston slap was all about), but this noise is in perfect rythym. it's there, then it's gone.

heh, every now and then i snub the engine by not cranking it long enough, but i never noticed any metallic noises. weird.

here are my warm weather engine starting noises

1. random ticking at idle (valve)
2. less random clacking (originally thought piston slap)
3. rythmical (notrandom) knock under engine load heard perfectly inside cabin. if i let the car idle for a minute two then drive the engine "sounds cold" like it used to but i don't get the knock.

by the way,forthose of you who have the knocksimilar to mine, does yours seeem more noticeable theless load is onthe engine (excuse the missed spaces, laptop blows). like first gear, it's more noticeable at low range, less at high, upshifts to second, really noticeable again, less at high, upshifts to 3,noise is gone.

and yeah, mine seems to perform best when cold too, go figure