Hi Blazer Enthusiasts,

I am hopeful that someone can help me help my mechanic figure out what's up on my 1999 Chevy 4.3l Here's the issue:

It has an intermittent knock at startup that lasts for about 5-10 seconds. This has been going on for about a year. Last week, I started it up and the knock was much louder and didn't quit. It was so loud (unlike anything before) that I thought if I drove it the would blow up... I shut it down and then started it again and it was the same. Scared to drive it and towed it to the shop!

Mechanic (been in business 30 years) checked it out and said that he thought the only thing he thought it could be was the lifters, so we replaced 12 lifters and the gasket set. Was great for the first 4-5 start ups, and now it is knocking again for 5-10 seconds at start up intermittently. Started it about 12 times today and it knocked on about 8 of those start ups.

I've read that knocking is 'normal'... however, it doesn't do it all the time, so have a hard time accepting this as normal. Especially after $800 and no real progress. Confused!

Thanks in advance for advice you might provide so that I can talk with confidence to the mechanic.

 

The 4.3L & 5.3L engines are both very well known for piston slap on cold start up. Usually most noticeable when ambient temps are around 40F or below. The noise is more noticeable when increasing RPM slightly, and quiets down when RPM is decreasing, and at idle. It can last from a very few seconds up to a minute or more. The colder it is, the longer it lasts. It's caused by a lack of lubrication to the piston skirts. When the piston reaches bottom dead center, (BDC) the skirts extend beyond the lower end of the cylinder walls. As the piston starts to travel upward, it "wobbles" slightly until the skirts "align" the piston in the cylinder. The use of a full synthetic oil such as Mobil1, helps keep it in check. The root cause is the design of the lubrication system. My 1998 has done it since 30K, it now has 225K miles, and it still does it.

 

Thank you Captain Hook!

I bought it used a little over a year ago after wrecking my 2000 Blazer. There was some sludge in the oil the first time we changed it and have been babying it ever since. She's got 149,000 miles on her right now. I heard that it wasn't good to change to synthetic this late in the game as it would mess up the gaskets, but we just changed those too, so I will take your advice and go synthetic and see how it all plays out.

Thanks again!

 

Does that description sound like what you're experiencing? Switching to synthetic oil causing leaks is a myth. If anything, it will stop seal leaks when it cleans the sludge from behind seals. The sooner you start using it, the better. Use a quality filter: Purolator Pure1, Mobil1, Wix premium, NAPA Gold.

 

I have only had issues with the knocking right at start up... never while driving. When it started knocking super loud the other day, I didn't even want to try to drive it for fear of blowing it up, because it sounded that bad... and the temps outside was not cold at all. When driving from a cold start in the mornings it has been a little rough with some sputtering... that seems to be resolved now with the new lifters... and sometimes when accelerating you could tell there was a little miss or something... which doesn't seem to have been corrected with the new lifters.

It's sitting at the garage door of the shop that worked on it two days ago, so tomorrow, we'll see what they say and if they even mention the piston slap. I'll keep you posted!

 

Lower intake manifold gaskets are another very common failure on this engine. It can be caused by several things, but a lack of proper maintenance to the cooling system is the main cause though. They can leak oil externally, and they can also leak coolant both externally and internally. When the leak is internal, the coolant contaminates the oil and thins it out, which will cause lifter noise. If that's the case, replacing your lifters temporarily repaired a symptom, but not the root cause, and it will happen again. If leaky intakes are the root cause, changing the oil & filter after the gasket replacement usually takes care of the lifter noise. Depending on how much coolant is in the oil, it might take a couple of oil changes to get it all out. When oil is contaminated with coolant, main bearings, rod bearings, and camshaft bearings will all be destroyed, which also causes a knock before the engine seizes up.


EDIT: Your night time lows, (40F) have been cold enough to hear piston slap, (daughter used to live in Foley).

 

Well, I took it back to the mechanic and left it for the day and of course, No Knock, No Rattle, No Noise at all... Typical! He said that I just needed to let those lifters settle in and get some good oil to them and see what happens... and nothing has happened since. So time will tell... in the meantime, I am changing the oil pump and going to synthetic with a premium oil filter. Thanks for the advice! I really appreciate it!

Nice talking to someone who knows where Alabama's Beaches are located!

 

It has to sit overnight, (10-12 hours) for piston slap to be noticeable, (unless it's real bad). It only occurs after the oil runs off the lower cylinder walls. As soon as they gets a good coating of oil, the noise quits. It relies totally on splash, nothing gets "squirted" on the walls.


Mid 70's Ford 2.3L had horrible piston slap. Their answer was a redesigned connecting rod with an oil passage off of the bearing that shot oil on the lower walls... cured the problem.


Beaches are usually near water She lived in Orange Beach, and Gulf Shores as well. Loved it and wants to move back.