In this stage of the refresh of our ’04 Chevy Avalanche 1500 Z71, we’re tackling the engine. Like millions of GMT800 pickups and SUVs, ours is equipped with the venerable LM7 iron-block 5.3L Vortec V-8 engine. New, the engine produced a respectable 295 hp and 330 lb-ft of torque. Under light throttle, the 164,000-mile engine seemed to drive fine. However, once we pushed the old mill a little harder, some problems began to pop up. The engine initially detonated badly under full throttle, indicating a buildup of carbon in the combustion chamber. A few weeks later, the check engine light came on.
A diagnostic scan revealed a faulty knock sensor. With the engine control module (ECM) pulling spark way back, the LM7 clearly wasn’t living up to its full potential. Since there were close to 200,000 miles on the engine, we were under no illusions we’d get “like new” performance no matter what we did, but we did want to improve driveability and performance as much as possible without opening up the engine.
| One of the first things to do on a gasoline-powered used vehicle with an unknown service history is to check and replace the spark plugs. Good thing we did. Based on the 0.06-inch electrode gap and crusty electrode, it appears the plugs in our 165,000-mile Avalanche were original.
While maintenance records provided by the original owner of the vehicle indicated the oil had been changed regularly, there was no record of the spark plugs ever being changed. Even long-life plugs have a life of 100,000 miles. Based on the brand and condition of the plugs once we pulled them out, it was likely they had never been changed. With crusty electrodes and a wide .060 gap, it was time for a new set, so we ordered Denso Iridium plugs from RockAuto, along with new plug wires.
As bad as the plugs were, the knock sensors were even worse. Gen-III GM small-blocks are notorious for going through knock sensors, based on their location in the middle of the “V” under the intake manifold. The rear knock sensor is especially prone to corrosion due to the engine being tilted slightly backward. Moisture tends to accumulate in the sensor cavity. While our front knock sensor looked to be in decent shape, the rear one was so severely corroded it was barely recognizable. While we were replacing the knock sensors, we also replaced the sensors’ wiring harness, eliminating another potential culprit. Based on the amount of dirt and rodent feces we found under the intake manifold, replacing the wiring probably wasn’t a bad idea.
| The new Denso Iridium plugs came out of the box with the specified 0.04-inch gap. Nonetheless, it’s never a bad idea to check the gap on new plugs anyway.
Finally, we performed an oil change with Amsoil 5w-30 full synthetic and the company’s oil filter, which it claims outperforms OE specifications. With new plugs, sensors, and oil, the engine was smoother and more responsive. Though it didn’t demonstrate “like new” performance, it was a big improvement.
| We also replaced the spark plug wires. Over time, old wires can arc and cause misfires. Be sure to retain the metal sleeve off the old plug wire.
| We applied some antiseize compound to the thread of the plugs to facilitate future service. Be careful not to get the compound on the electrodes. Hand-thread the plugs into the head to get them started. Check the recommended plug torque specifications of your vehicle’s engine.
| Now comes the fun part. We were getting scan codes indicating a faulty knock sensor. Removing the knock sensors on Gen-III GM small-blocks requires removing the intake manifold to access them. The first step is to depressurize the fuel line and disconnect it from the fuel rail and manifold, which on our engine can be removed as a single assembly.
| As we expected, it was a literal rodent’s nest underneath the manifold, with ample telltale amounts of guano to indicate it was inhabited at some point.
| We carefully vacuumed up the guano and debris from underneath the manifold, making sure none dropped into the intake ports. If you’re extra-cautious, you can cover the ports with masking tape.
| Once we got the large dirt and debris out of the way, we wiped down the valley cover and area around the intake ports with brake cleaner to clean off any residual grime and grease. Depending on the age and condition of the engine, this may take a while.
| When we removed the old knock sensor harness and sensors, it was easy to see why we were getting the fault codes. The rear sensor was so badly deteriorated, it was barely recognizable.
| After thoroughly cleaning the valley cover and around the intake ports, we installed a new harness and the new knock sensors. Before reinstalling the manifold, we added a U-shaped bead of RTV silicone around each of the sensor covers to prevent unwanted moisture intrusion. While we had the manifold off, we also replaced the oil pressure sending unit, since we had easy access to it.
| It’s recommended that the intake manifold gaskets be replaced if you take the manifold assembly off. The ones we got were one-piece stamped metal units that conveniently aligned on the intake ports, making reassembly easy.
| We also replaced the foam blocks at the front and rear of the manifold that act as debris and moisture barriers to the valley cover. Some technicians recommend leaving the rear block out to let moisture roll off the back of the valley cover and not accumulate in the rear knock sensor cavity.
| It’s important to reconnect all harnesses, connectors, and fuel lines when you reinstall the manifold. Late-model engines have myriad sensitive electronics that require the proper signals and feedback to run correctly.
| Finally, we performed an oil change on the engine with Amsoil 5w-30 synthetic motor oil and filter. Most of the LS-type truck engines take about 6 quarts. If you have an auxiliary oil cooler on your truck, you may need to add a little more.
| Resetting the oil life monitor is a relatively easy process. We were able to do it through the Driver Information Center (DIC) menu. Check your owner’s manual for the precise sequence.
After the plug and oil change and new knock sensors, the old LM7 seemed to run smoother and peppier, and the dreaded check engine light stayed off. We’re now ready to move on to more fun things down the road with our Avalanche!