Avalanche Owners Zone > Problems: Engine

Piston Slap - The Truth

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There has been a number of posts on the issue of piston slap on GM Vortec engines.  Some are based in truth but others are based in hyperbole.  Before this gets completely out of control let me state clearly and without bias what piston slap is and how to define if you have a REAL problem.

What is Piston Slap?  If you look at what makes up a cylinder in an engine you've got the cylinder.  The cylinder has a bore, how wide it is and within that the piston goes up and down.  The piston needs to fit tightly in the bore but it has to be very exact.  To tight and the piston can score the cylinder (scratch the sides) or can even seize (get trapped in the cylinder walls).  To small and you can have oil leaks into the combustion chamber, loss of compression, and piston slap.

Piston slap is when the piston is smaller than the bore and the piston "rattles" within the cylinder.  This is typically on the compression stroke (the up stroke) of the piston in the cylinder.  As the compression increases the air, fuel mixture is squeezed and wants to find a way out and the cylinder moves back and forth.

Is piston slap bad for my engine?  The answer is, that depends.  If you think General Motors is the only company with piston slap issues think again.  Piston slap is an issue with the large block V-10 engines made by Ford, and on current Subaru V-6 models.  This is also a problem on Dodge 3.3 liter V-6 engines and surprise, Johnson outboard motors.

Manufaturers of engines are under increasing pressure from two directions.  First the government is pushing for cleaner and cleaner engines while the public is pushing for more and more horsepower.  Now the two don't go hand in hand and so it takes a lot of tweaking and pushing of technologies to balance this equation.  This is why piston slap is becoming a wider problem on modern engines.

You didn't answer the question above.  Is piston slap a problem on my GM engine?  Maybe.  On 1999 to 2002 model GM vehicles with the 4.8, 5.3, and 6.0 liter Vortec V-8 engines the issue of piston slap seems to appear on normally maintained engines between 12K and 24K miles.  The problem manifests itself as knocking or dieseling at cold start.  In order to know if you have a real problem you need to ask some questions.

*  Is the dieseling or knocking extreme.  Think 1996 Dodge Cummins Turbo Diesel powered truck on cold start.  If you're engine sounds like that - you've got a problem.

*  Blue smoke on start up.  Are you burning oil visibly on start up?  If you've got blue smoke out of your exhaust on start up - you've got a problem.  Do not confuse oil burning on start with the lifter problem that was unique to mid-2001 built Canadian 5.3 liter Vortec engines that require PVC and lifter change out to solve.

*  Does the knocking last longer than 90 seconds after a cold start up?  Do not confuse piston slap with lifter noise.  The way the Avalanche has been designed adjustments are made to the oil system during the first few minutes of a cold start to allow the engine to warm up faster.  This means better oil circulation and it can mean bizzare transmission shifting on a cold start for a few minutes.  If you have diesel sounding knocking for more than 90 seconds - you've got a problem.

What about the 8.1 liter V-8?

Some light dieseling on cold startup of the 8.1 liter V-8 is not unusual but should not last more than 10 to 20 seconds.  With each cylinder having a displacement in excess of one liter, the lack of lubrication upon startup as the oil begins to circulate is going to make things noisy.  It is not piston slap.

Why is the sound there when I first turn the key and then goes away?

Good question!  As the engine warms up the cylinder walls, piston, and seals all expand from the heat.  Simple thermodynamics 101 in progress.  As the engine warms everything seals up and fits right and the engine operates normally.  You've got a problem when this isn't happen.

Sometimes I hear knocking under load, is this piston slap?

No, the Vortec 5.3 liter V-8 can be subject to some valve knock (pre-ignition) under certain specific situations, almost always under load (towing a trailer, steep uphill under highway speeds, etc.).  The knock should go away with a throttle position change or within 10 seconds as the computer automatically adjusts.  If it doesn't the problem is NOT piston slap.  Change the fuel you're using (higher octane won't help).  if you're still having problems you should have your vehicle checked for spark plug problems or dirty injectors.

OK, I've got a problem by your definition above, now what?  If you have one of the three danger signs above it's time to contact your dealer and your regional service representative to reach a resolution.  Remember be persistent but POLITE.  If you're not satisfied with the answer your next point of recourse is push the OnStar button in your Avalanche and talk with a customer service representative.

What is General Motors doing?  General Motors IS doing things for customers that have a serious piston slap problems.  Mitigation has included making adjustments and repairs the solve the problem, providing free extended warranties for 6 years/100,000 miles, replacing the engine, and in extreme cases buy back of the vehicle and/or exchange.

Is any solution better than the other?  Yes, absolutely.  A lot of people have the illusion that if a car company buys back your vehicle you get your money back.  That just isn't true.  The car company can charge you for reasonable use based on mileage, ususally charged at the leased vehicle rate of 12 cents a mile.  Now if you have 5,000 miles on your vehicle that probably isn't bad, but if you have 30,000 miles you should think hard about it.  If you don't plan on owning your vehicle for more than 6 years, the extended warranty is a pretty valid offer.  A replacement engine should also be highly considered if offered.

What about resale value?  Of all the issues on piston slap this is by far the biggest one.  What will happen when you try to trade-in or sell your vehicle, especially in private sale.  I don't have an answer for this one.

So is piston slap bad for my engine and the life of the engine?  If you own a 1999 to 2002 GM product with a Vortec V-8 engine if you don't meet the three danger signs above the official GM position at this time is, "no."  There are many GM vehicles that have over 100K miles on them in this group that are still running strong.  However, if you have one or more of the three danger signs above than you should contact your dealer to have the situation DOCUMENTED at the minimum.

What makes the 2003 Vortec engines different?  Can this be retrofitted on my 1999 to 2002 Vortec?  General Motors according to the current issue of Truck Trend Magazine has done two things to address piston slap in the Vortec engine line.  First, they have changed the inside coatings of the cylinder walls.  Second, they have changed the seal materials on the piston and feel this problem has been completely addressed.  The nature of these changes does not allow them to be retrofitted on a 1999 to 2002 engine.

Well if it's not a problem, why did they make a change?  As I don't work for GM and I'm not privy to that information I can only take two educated guesses.  If I want be sinister than I will say it's because they have a real problem and they are covering up.  If I want to give them the benefit of the doubt than I'll say it's because they want to listen to customer concerns and continue to improve upon their products.  A third option is that this problem may have been ignored under the old guard, but Lutz wouldn't accept it and ordered it be addressed.  If I was a betting man I'll take option three - but it's a matter of how you see the glass.  Half full or half empty.

OK, so in the interest of full disclosure does your Avalanche have piston slap?  Yes, on cold start when it is under 40 to 45 degrees I have some light dieseling that lasts for about 30 seconds.  I am not burning one drop of oil and have 28K miles on my Avalanche.  I get about 15 MPG combined.  I am at this time satisfied with "the company line" on piston slap with my only reservation being on trade-in and resale.

Well - comment away...

Piston slap has just made it's self known on my Av over our last little cold spell. I have just shy of 14k on the clock.

Even with an extended engine warranty, I would be concerned with diminished "saleability" as well as  resale value of any vehicle thus afflicted. I have heard that the fix on the 2003 is a revised design of the piston skirt.

  i have this problem chief.  it does knock after the allotted time.  i took it in and talked to the service manager.  they recorded the problem.  it was actually one of the parts guys who gave me instructions on how to get the 6/100,000 warranty.
  yes, i did get the warranty.  and yes i was polite during the process.  it was very hard though.  i mean with all the info out on this subject these guys, GM, act like the problem does not exist.  the conversation was kinda like, "what can i do for you today?".  "what seems to be your problem?".  "piston slap?".  "do you own an Escalade?".  "this problem only exists in those vehicles?".  crap like that.  
  i think if you are a push-over, not mechanically inclined, or have not research the problem/solution you will be taken advantage of.  i am very fortunate to have gotten that inside scoop.  he obviously knew what he was talking about, since he, parts guy,  owned a vehicle with one of these engines.  he even gave me a copy of the letter they sent explaining he was given the extended coverage.  i felt better after i was given this coverage too.

  i hear what you are saying SHC.  i think if you were going to trade the vehicle in it should be okay.  if they say it is okay for you to continue driving then it should be okay for them to give you full trade in value for it too.  now if you were going the private sale route though... ???
  i guess we will have to wait this one out, and see what the future holds for us.  as of now i plan on holding on to this thing for a long time.

Let me get this straight.  They are aware of the problem but not fixing it?  Is this the trade in issue?  Help me understand.  I also don't understand why you wouldn't have this fixed and maybe a class action would be in order.  Thanks


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