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Deleting AFM & Upping Power on a Gen IV 5.3L LS Engine

Posted in How To: Engine on October 2, 2018
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Photographers: Trenton McGee

LS swaps all the rage these days. It doesn’t matter whether it’s a street rod, a muscle car, or a 4x4. GM LS engines are finding their way under the hoods of everything. They’re plentiful, reliable as a hammer, nearly bulletproof, relatively inexpensive, and can be built to make stupid amounts of power.

The most desirable non-Corvette/Camaro LS swap candidates are in 1999-2006 GM vehicles, but finding low-mileage examples is getting difficult (though it’s impressive that you can find good-running LS engines with over 300,00 miles on them). In response to increasingly restrictive fuel economy standards, General Motors introduced Displacement on Demand (marketed as Active Fuel Management) in Gen IV small-block LS engines in 2005, and by 2007 most GM cars and trucks were equipped with DOD. These systems greatly increased the complexity of the engine management systems, and they also hurt the legendary durability these engines were famous for. It’s certainly possible to swap DOD engines, and a few aftermarket engine management systems even support the system, but with so many well-known mechanical problems, why not just get rid of the troublesome system?

The good news is that it’s possible to eliminate all traces of DOD/AFM, but the bad news is that it requires internal engine work. Deleting the DOD system the right way involves pulling the heads and installing a new camshaft and lifters, a new engine valley cover, and much more.

We found ourselves with a cheap 5.3L LH6 out of a Trailblazer, and we thought about tackling the delete ourselves until we realized the extent of the internal engine work. Our engine skills are pretty rusty, so we took our 5.3L to the LS gurus at Tilden Motorsports to exorcise the DOD demons. The company offers everything from DOD delete packages and performance modifications all the way up to turnkey, ready-to-run engine packages. The DOD delete procedure is basically the same on all Gen IV LS engines, though there are a few more steps if the engine is equipped with both DOD and Variable Valve Timing. Since we had to replace the cam anyway, we opted to go with something a little hotter than stock with a Tilden Street Rod cam.

While we didn’t dyno the engine when we were done, Tilden consistently sees horsepower and torque numbers in the 400 range with our exact combination. Check out what it takes to eliminate Displacement on Demand and upgrade an LS engine the right way.

The recipient of the upgrades was all-aluminum 5.3L LH6 we plucked from a very wrecked 2008 Trailblazer with 125,000 miles. Power numbers vary by source, but it makes around 310 hp and 330 lb-ft of torque. We were able to run the engine before we pulled it and noticed it had a fairly pronounced tick. This led us to do some research on the Displacement on Demand (DOD) system, which in turn helped make up our minds to delete it. Step one at Tilden Motorsports was pressure-washing the engine. A clean exterior helps ensure no contamination once the engine is opened up.
Truth be told, this was the author’s first foray into an LS engine, which is different in almost every way to a conventional GM small-block. We put the engine in the capable hands of LS guru Kevin Stearns at Tilden Motorsports. The company has developed a reputation as the source for everything from take-out engines to high-performance LS-based engine builds.
On LS engines, accessing the lifters requires removing the heads, so the intake manifold, valve covers, rocker assembly, and valley pan all must be removed. Note the cathedral-shaped intake runners; LH6 engines are equipped with desirable 799 casting aluminum heads that are reputed to be one of the better production heads available. Also note the electrical plug on the rear edge of the engine valley cover. That plug is the best external sign that an engine has AFM/DOD.
Uh-oh! Stearns noted this bent pushrod on No. 7 when preparing to pull the head. Usually indicative of very serious problems, such as a hydraulic event in a cylinder, it turned out that one of the lifters for that cylinder was frozen. Fortunately the pushrod was the only damage. Stearns found that the bleed hole for the lifter must have clogged with oil contaminants, which is very common.
The underside of the valley cover on a DOD-equipped engine has the solenoids that control the oil flow to the DOD cylinders. The raised bosses cast into the block below the valley cover supply oil pressure to those solenoids. All Gen IV blocks have those bosses, but pre-DOD valley covers seal them off. Part of a DOD delete involves installing an earlier, non-DOD valley cover. Note that the inside of the engine was very clean, indicating that the oil was changed regularly and that the owner stayed on top of maintenance.
All LS engines are equipped with roller lifters, but the DOD versions have very different lifters on four of the cylinders. The conventional lifters (left) are on cylinders 2, 3, 5, and 8, while DOD lifters (right) are on 1, 4, 6, and 7. The lifters sit in plastic guides that keep the lifters from rotating, and these will need to be changed along with the lifters themselves when doing a DOD delete.
Stearns took the time to inspect all of the cylinder bores and also measured them to ensure that the short block was in good shape. While the cylinders did have some wear, the engine was deemed healthy enough for the installation to proceed. The carbon buildup on the pistons was about average for an engine with this many miles.
Many things would have tripped us up had we attacked this project ourselves, and this was one of them. Stearns used a small torch to heat up the crankshaft bolt before removing it. He explained that the heat softens the Loctite on the threads, and without it the bolt is very difficult to remove. He said the harmonic balancers are also usually very tight, requiring the need for a heavy-duty puller.
With the timing cover, oil pump, and timing chain removed, the camshaft can finally be pulled. Reusing the stock cam after a delete will result in an engine that runs rough. Factory LS cams use a large single bolt rather than the traditional three-bolt pattern. As a result, regular small-block cam tools don’t work. Note that Stearns made his own removal tool with an old cam bolt and some rod. This was one of several special tools he had on hand to make the process quicker and easier.
The stock camshaft turned out to be in really rough shape. This was one of several chewed-up cam lobes. We would have replaced the camshaft anyway, but seeing this helped make it easier to justify the extra time and expense. Given the evidence that the engine was well cared for, the condition of the camshaft is further proof that DOD components are the weak link in an otherwise very durable engine.
Yet another small but important step is chasing all of the head bolt holes. The factory head bolts are coated with thread locker, and if the threads aren’t chased then the residual thread locker can mess up torque values. No off-the-shelf tap is available to get deep enough, so Stearns makes his own from old head bolts.
Since we had to replace the cam anyway, we went with something a little more aggressive. Stearns recommended the Tilden Motorsports 5.3L Street Rod camshaft. The intake/exhaust lift specs are 0.544/0.538-inch with 206/214 degrees of duration at 0.50 and a 112-degree LSA (lobe separation angle). Nothing radical, but it should wake up our 5.3L while still being very streetable. Stearns coated the cam in Tilden Assembly Butter, a high-quality assembly lube.
The Tilden cam uses a traditional three-bolt timing gear, which requires a new timing set. As a result, the AFM delete kit includes a new double-roller timing chain, which is both stronger and more durable than the stock single roller. If you are eliminating Variable Valve Timing (left, in hand) at the same time, you will also need a new timing cover. Tilden offers VVT delete kits as well.
Engines equipped with DOD have higher-volume oil pumps than those without, and some people recommend replacing them with a non-DOD pump. Stearns says the extra volume doesn’t really hurt anything, and in fact helps on higher-mileage engines. Another important but often overlooked step is centering the oil pump rotor on the crank. Stearns used shims to center the rotor before tightening the oil pump bolts.
After cleaning up the gasket surfaces, Stearns installed new head gaskets and set the heads in place. LS head bolts are torque-to-yield, so they cannot be reused. With this mild build we used standard head bolts sourced from Tilden, but ARP head bolts are available and recommended for high-performance builds.
With the head bolts torqued, Stearns pressurized each cylinder with compressed air to prevent the valves from dropping down into the cylinders and installed new valve springs. The stock springs don’t yield enough seat pressure for high rpm, and increased valve lift requires new valve springs. Stronger springs also required new chromoly pushrods, which are included in Tilden’s DOD delete kit.
After new chromoly pushrods were installed, the final piece of the valvetrain puzzle was reinstalling the rockers. Reusing the factory rockers is just fine for this cam package, but Stearns does recommend replacing the trunnion bearings, as they are a known weak point. Tilden also offers upgraded rockers should your build be more aggressive than ours.
On a Trailblazer the front differential goes through the oil pan, which obviously we weren’t going to need. Plus, LS oil pans are cast aluminum, so they tend to shatter when you hit them against rocks or break a driveshaft. Since we had to have a new pan anyway, we upgraded to a Tilden Motorsports sheetmetal oil pan. The pan is constructed of 18-gauge metal but then has an 1/8-inch skidplate on the sump that ties in to the back of the block. It also has weighted oil control doors to ensure oil pressure in off-camber situations.

What Is Active Fuel Management & How Does It Work?

Active Fuel management (AFM) and Displacement on Demand (DOD) are the same thing: a system developed by General Motors that deactivates cylinders during periods of low engine load. It was used on certain 5.3L, 6.0L, and 6.2L engines starting in 2005 and continued until the release of late-model direct-injected engines.

During periods of low engine demand, the system deactivates cylinders 1, 4, 6, and 7. The valvetrain for these cylinders includes special lifters, and when the system is activated, solenoids positioned on the underside of the engine valley shut off pressurized oil via special passages to these lifters. This causes the lifter to collapse, allowing the body of the lifter to continue moving up and down with the lobes of the camshaft while the plunger that engages the pushrod remains stationary. Since the pushrods are not engaged, the valves for the DOD cylinders remain closed. The ECM turns off the injectors for these cylinders at the same time. When the system shuts off, the solenoids close, the lifters pump back up, and normal operation resumes.

While it’s an interesting system, it brings a variety of problems. The biggest one is that the system is highly susceptible to carbon contamination. There’s also the issue that while the DOD system is activated, the internals of the lifters have only the oil that happened to be there when the system activated for lubrication. As one can imagine, this is hard on the lifters and the engine oil. These systems seem to survive if the engine is well maintained and the oil is changed religiously, but someone with a casual attitude about oil changes or using the wrong oil viscosity can wreak all kinds of havoc. Higher-mileage engines have an even higher likelihood of failure. The most common complaint is valvetrain noise, but misfiring (which is what our engine was doing) and even serious internal engine damage have been traced to this system. While AFM is said to improve mileage by 5-7 percent, in the eyes of most enthusiasts the drawbacks cancel out any advantages.



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