December 2010 Chevy High Performance Q & A

Kevin McClelland Nov 15, 2010 0 Comment(s)
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My other problem is that when the engine is cold at start-up, it sounds like the rods are knocking. After a while it goes away. It's like it's starved for oil. In the winter I start it to warm it up. It still will knock a little now when it's 60 degrees outside. I talked to people with the same motor in trucks, some were different years and they didn't have any add-ons, and they say they have the same noise. One guy blamed the torque converter. I changed the tranny filter and 5 quarts of Dexron 3. It runs better, but the cold start knock is still there. It seems that as soon as I start it and put it in Neutral for a little bit the noise goes away faster.

So what do you think of this? Should I change the torque converter? If so, can I find one that would help my mileage? I like my truck, and it has a lot of miles left in it. I want to keep it, but like many gearheads, I don't want to live with that noise. Something isn't right. I hope you can shed some light on this so I don't throw a bunch or extra money at it. Again, like most gearheads, we have all thrown extra money at things at one time or another. Any insight would be great! Thanks.
Pat Garrison
Via email

A: Glad to hear we can help with insightful stories that you can apply to your truck. Any performance increase is welcomed when you're pulling around a 4x4 fullsize truck with a 5.3L engine. Nice that you picked up a few mpg in the process.

The lean check engine light is because the mass airflow calibration is out of range for the factory program in your computer. The calibrators at the factory take a tremendous amount of time ensuring that the mass airflow sensor will operate in a certain range. If the sensor varies out of this range by over 10 percent it can set a light. Also, if the new mass airflow sensor is reading less airflow at your light throttle and slow engine speeds than the factory sensor did it will lean out the mixture. When this happens, the O2 sensors see that it's lean and begin to trim the fuel richer to accommodate the lean condition. If the O2s run out of trim (they only have so much room), the engine will run lean and set the check engine light. Why your light is going on and off is that in some instances the engine runs in a range that the sensor calibration is close to correct and the fuel trims come back in line.

Looking at your list of mods we'd say the lean condition is from the cold-air intake and the aftermarket mass airflow sensor combination. The mass airflow sensor may stay within factory calibration range with the stock intake system. Contact the manufacturer of your aftermarket mass airflow sensor for recommendations. Changing the mass air calibration isn't something you're going to do with a handheld flash programmer.

Your early-morning wake-up knock isn't uncommon in the LS-based engines. It's not in your torque converter or the transmission. Please save your money. The cold-start engine noise is from the short-skirt piston design in the LS engines. This is a well-documented noise that the factory has known about for years. Over the years of the engine's history, GM has made subtle changes to the piston skirt design, coatings, and clearances to quiet down the knock. To get the performance levels out of the engine, you push the design everywhere you can. The short-skirt piston creates very little friction, which robs horsepower. Try turning up your radio a little during cold start to get past it.

We know the noise can be annoying, but do you remember what a big-block sounded like with forged pistons in the morning? You'd swear that the engine was done until it warmed up.

Technical questions for Kevin McClelland can be sent to him at chevyhi@sorc.com.

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