1999 Sierra engine replacement?

DavidP2001 · July 7, 2020

I have a nice 99 stepside that I inherited from grandad and it has great sentimental value to me. Although it has 185,000 miles on it, and the 4.8L engine doesn't burn a drop of oil, it leaks oil rather significantly. Also, as most will know that have a truck with this engine, it is greatly underpowered. Would it make sense to replace the engine with a 5.7L LS-6 available from a friend who owns a salvage yard? Is this a difficult swap? What about the computer/ECM currently in the truck, do I replace the computer/ECM with one for an LS-6? One other data point is that the frame has fairly significant rust. Many thanks for you comments!

Mike GMC · July 7, 2020

It should drop right in. Use your existing intake manifold, fuel rails and injectors to make sure everything hooks up the same. Use your existing flex plate and exhaust manifolds. You didn't mention the year of the LS6, but if it's an 02, all the electronics will be the same. Reseal the new engine and put all new sensors in it. Taking it apart to reseal also gives you the opportunity to inspect everything and make sure it's in good shape. You may also want to put a new timing chain and oil pump in it. It will run and drive with your existing computer, but you will want to get it Dyno tuned once your done.

DavidP2001 · July 11, 2020

Thank you Mike, I appreciate your feedback. So, since you specify that an 02 LS6 will work with existing components, I assume that a 99-02 LS6 will work?