1999 Sierra light-duty trucks

GMC's first complete redesign since 1988 features new engines and a fresh face for its 1500and 2500-series pickups

GMC has added its name to the new-truck-introduction marquis with it's first complete redesign since 1988: The 1999 Sierra light-duty trucks (and their near-identical-twin counterparts, the Chevrolet Silverados) are powered by an all-new engine family, have an improved frame and a restyled body that follows the trend set by Dodge in 1994 and Ford with its 1999 models.

GMC will only build 1500- and 2500series models in 1999, marking the first phase of new-truck introductions. The 3500-series and crew cab models will be introduced as 2001 models, so for the next two years, the heavier-truck customers will still be buying the current body-style trucks.

POWERTRAIN, CHASSIS

GM has tossed its entire line of smallblock V-8 gasoline-fueled light-truck engines in favor of three all-new Vortec V-8 engines. Unlike the companies that have embraced the technology of overhead camshafts, the new GM engines are refinements on the tried-and-true pushrod technology that's worked so well for all these years.

The old standby 4.3-liter V-6 is rated at 200 hp at 4,600 rpm and 260 lb-ft of torque at 2,800 rpm. A Vortec 4800 V-8 replaces the Vortec 5000 5-liter engine; the new version produces 255 hp at 5,200 rpm and 285 lb-ft at 4,000 rpm. The 4800 is 25 hp stronger than the 5000, and rated at the same torque.

Moving up a bit, a Vortec 5300 replaces the venerable Vortec 5700 V-8. The 5300 produces 270 hp at 5,000 rpm, which is 15 hp better than the old 5700 engine in trucks with an 8,60010,000-pound gross vehicle weight rating (gvwr) and 20 hp more than the 5700 in trucks with a 10,000-pound or greater gvwr. Torque figures have dropped from 330 lb-ft at 2,800 rpm for the old engine to 315 lb-ft at 4,000 rpm for the 5300.

At the top is the new engine lineup is the Vortec 6000 V-8 that's rated at 300 hp at 4,800 rpm and 355 lb-ft at 4,000 rpm. This engine is not a direct replacement for any one power plant, but rather, is an addition for those who need considerably more guts than offered by the Vortec 5300 model, but don't quite need the 7.4-liter big block. The replacement for the big-block Vortec 7.4-liter engine will be introduced with the redesigned 3500-series trucks.

All of the new engines have higher operating rpm ranges and flat torque curves, so they can run easily at higher rpm. This comes in handy when towing up a hill and it's necessary to shift down to maintain road speed.

A variety of technical upgrades have improved the gasoline engines' reliability and performance. A new deep-skirt castiron block, hydraulic roller lifters, a new valve angle, new Teflon main seals, a coilnear-plug ignition system and other details have been added across the board.

Also upgraded for 1999 is the 6.5-liter turbodiesel engine that's now rated at 215 hp at 3,400 rpm and 440 lb-ft at 1,800 rpm, compared to last year's engine at 180 or 195 hp (depending on application) at 3,400 rpm and 360, 420 or 430 lb-ft (depending on application) at 1,800 rpm. Increased coolant system flow, an upgraded fuel pump and doublefiltration fuel filter with a water separator are among the engine's refinements.

The new engines are cradled in a new three-piece frame that incorporates hydro-formed steel sections for strength and rigidity.

Very little has changed in the transmissions, except for an all-new "Tow/Haul" mode that's available in trucks with the towing package. When the driver engages the Tow/Haul mode via a shiftlever-mounted button, the transmission shift points are moved farther apart and the shifts are firmer If the transmission made a first to second shift at 10 mph in standard driving mode, for example, use of the Tow/Haul mode ups that shift point to 22 mph.

Four-wheel disc brakes with a fourwheel anti-lock brake system (ABS) are standard on the 1999 trucks. Rear leaf springs and front coils (2WD) or torsion bars (4WD) are standard equipment.

Most tow ratings have been changed somewhat due to the new engine lineup. At maximum, the 2500-series GMC can be rated to tow 11,000 pounds with the correct equipment, which includes the 6.5-liter diesel engine, manual transmission and 4.10 axle. The same truck with the 6.0-liter gasoline engine can be rated to tow as much as 10,500 pounds. These figures are improvements over this year's trucks, which for comparable models are rated at 8,500 pounds for the 6.5-liter diesel truck and 8,000 pounds for the 5.7-liter-powered truck.

The 11,000-pound tow-rated truck carries a 17,000-pound gross combined weight rating (gcwr).

BODY, INTERIOR

The new Sierra's body is different enough to stand out, yet the changes are subtle and evolutionary in a low-key way. Most noticeable are the front clip and hood, which, like Ford's new Super Duty truck models, display just a hint of Dodge Ram influence with its slightly raised center section and gently dropped fenders. Recontoured body lines with small wheel-well flares and integratedstyle bumpers create a truly unique image. To better differentiate the Sierra from its Chevrolet sibling, the hood, fenders, bumper, fascia and headlamps are unique to the Sierra and cosmetically identify it as a GMC product.

New composite headlights are designed to provide superior forward illumination, with 120 percent more lefthand-side driving light for better visibility, and structure improvements make the body stronger for enhanced occupant safety, according to company engineers. Safety is further improved by way of the passenger-side air bag shutoff switch, and the new hood is said to allow greater visibility up front.

Occupant comfort was also high on the company's priority list when designing the new truck. The new body has a lower floor step-in area, and the cab is somewhat larger. For example, the 1999 extended-cab model is 3.7 inches longer than the current truck. The rear seat in extended-cab models is angled somewhat more for greater passenger comfort. All interior details, such as the seats, headliner, upholstery and such, are new for 1999.

Each truck manufacturer building extended-cab models offers a third door configuration with several options for handling the shoulder-belt attachment issue. The Sierra follows the Dodge example with a shoulder belt that's integrated with the front seat. This avoids having a roof-mounted shoulder harness to entangle aft-seat occupants.

Located in the upper left corner of the instrument cluster is the new Driver Message Center (DMC) signal that works with the truck's on-board computer to convey important information (18 functions). This liquid crystal display readout delivers progressive warnings as the systems require more imperative attention.

On THE ROAD

We spent a short time towing with an extended-cab 2500-series Sierra 4WD with the 5.3-liter engine and 3.73 axle. As a payload, we towed a Fleetwood Terry trailer that weighed approximately 3,500 pounds (plus 500 pounds of packed, frozen "Thanks a lot El Nino" snow on the roof), a figure that certainly didn't overstress the truck's 7,500pound tow rating. The drive took place on Sierra Nevada foothill- and mountain-type roads in elevations between 3,000 and 5,000 feet.

The new engine is smooth and quiet. The truck's sound insulation is excellent, and the low vibration makes the driving experience even better.

The current GMC trucks do a good job of towing trailers, and the 1999 models follow admirably in those footsteps. While the trailer was a relatively small load, it still affected the truck on the curving roads and dips and hummocks of the Sierra Nevada foothills. At all times, the trailer was fully under the truck's control. Steering and handling stability were excellent, as was braking. Acceleration was brisk, considering the load.

A casual 6 percent grade found us motoring over the top of the hill at 56 mph at 3,800 rpm in second gear A somewhat steeper 7 to 8 percent grade slowed us a bit to 52 mph at 3,400 rpm in second gear, but that was still fast enough to flow with the traffic.

Engine compression down the same hills netted 47 mph at 3,500 rpm in second gear on the 8 percent grade, and 54 mph at 2,500 rpm in third (direct) gear on a 5 percent grade.

Use of the Tow/Haul shift mode does indeed improve the acceleration feel with a trailer in tow. Because it delays the transmission upshifts, the engine can wind out farther before the upshift occurs. This helps avoid having the engine drop below its optimum rpm range after the upshift, which keeps it closer to its desired operating torque range.

The 1999 GMC trucks represent a sizable leap forward for General Motors in light-truck technology and design. Buyers have an even more sophisticated light truck for towing and general daily use, and user comfort has increased to make the driving experience more enjoyable. Look for a complete test of the new GM truck in a future issue. TL

Copyright T L Enterprises, Inc. Jul 1998
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