2002 GMC Sierra Repair Question
Replacing stock camshaft for 4.8 V8 with a mild performance camshaft
REAL bad idea. Since the OBD2 emissions systems that started in '96 models, there is just too much stuff the Engine Computer monitors. Changing any one of those variable can confuse it. On carbureted engines, fuel flow was determined by airflow which was a byproduct of cam lobe lift and duration. On your engine, fuel flow is calculated from air flow through the mass air flow sensor, AND by the profile of the cam, fuel pressure, engine load, engine speed, and the size of the injectors, five things the computer has programmed in.
The computer is going to try to reconcile why more air is going in and why more fuel is needed relative to throttle position and engine speed. It starts out running on a set of numbers programmed in at the factory called "fuel trim" numbers. As soon as the truck is driven, the computer watches readings from the two front oxygen sensors and makes fine tuning adjustments in fuel metering. Those are called "short-term" fuel trims and are constantly being updated. There are numbers to add or subtract fuel for any given combination of engine speed, load, throttle position, rate of increase, direction of travel, coolant temperature, air temperature, and barometric pressure.
Those short-term fuel trims, (STFT) update constantly as you drive. When the computer sees the same numbers continuously, it uses them to update the long-term fuel trims, (LTFT). Those are the numbers programmed in at the factory that it starts running on. That data is lost when the battery is disconnected or run dead, but it gets rebuilt without you noticing as soon as you start driving again. What most people don't realize is based on the data from the oxygen sensors, and those stored fuel trim numbers, the computer can only modify fuel delivery by about plus or minus ten percent beyond the factory-programmed values. That is enough to cause a lean misfire or stumble, or black smoke from the tail pipe. Either condition increases tail pipe emissions beyond federal limits. A different cam lobe profile can send those fuel trim numbers beyond what the computer can accept. That can set a diagnostic fault code and since it affects emissions, it will turn on the Check Engine light. When a code is set in memory, it blocks some self-tests from being run and it can prevent the computer from watching for certain other problems because it knows it can't rely on the data it compares to other sensors and conditions.
What you're asking your new camshaft to do is allow more air and fuel into the engine. That's what the throttle blade and mass air flow sensor are for. What you might try instead is changing the cam timing. We used to do this on Chrysler engines by installing an offset key between the camshaft and sprocket. It was done on GM engines by using an offset dowel pin or by purchasing a cam sprocket with a series of offset dowel pin holes.
If you think of "L" for loose, late, and low, a loose timing chain, (or a purposely retarded camshaft), causes late valve timing which greatly increases low-end torque. This is what is done to change the personality of a motor home engine to give it more low-end torque to leave a stop sign. It is also what used to be done to create the "gas mileage camshafts. I put one in an old heavy Chrysler station wagon and I could squeal the tires anywhere and still get over 20 mpg on the highway.
Think of "T" for tight, top, and, uhm, advanced. Tightening up the tugging side of the timing chain would advance valve timing and increase top end power. That was commonly done on police cars used in highway pursuit. Changing cam timing two degrees makes a huge difference in engine response. The offset keys and dowels were available in two, four, and six degree offsets.
It sounds like your mechanic thinks your engine has mushroom lifters. Those have to be installed before the camshaft goes in but they make tools for holding them up while the camshaft is removed. Your engine uses roller lifters and must be used with a special camshaft. Regular lobes are tapered slightly to cause the lifters to rotate which evens out the wear. Roller lifters run on a perfectly flat lobe and must not rotate. The only replacement camshaft I would consider is one from the dealer made specifically for your engine and model year. They might also specify replacing the Engine Computer to a different part number.
I've heard and read too many stories about people trying to do the fun things we used to be able to do, and having nothing but trouble. If you pursue this modification, be sure to hold on to all of your old parts so you can put them back in if you're unhappy with the results.
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