Q: The culprit is a 1989 GMC 1500 4×4 with a 350, automatic with overdrive, a 3.42 axle, and over 150,000 miles. It has recently developed a powerful thirst for Texas tea, consuming 1 quart every 100 miles.

A new PCV valve didn’t fix it, and a valve job only eliminated the smoke at startup. A compression check before the valve job showed one cylinder at 150 pounds and the rest at 170 lbs. with clean spark plugs.

The truck is used mostly for short runs with occasional longer trips.

Any suggestions?

Assuming a rebuild is in order, I’d love to improve mileage while I’m at it, but my dollar tree is suffering a bit of a drought! I’d probably do a .030” overbore, but how many mods can I make before I have to change the chip?

How about a roller cam and rockers?

putting intake manifold gaskets onto a cylinder head prior to intake manifold installation for a small block chevy v8 engine
A proper intake manifold gasket seal is critical. Jeff Smith will show you how to do it here. (Image/Jeff Smith)

A: With the symptoms you’ve described, we believe the problem is either the intake gasket or the piston rings. Try replacing the intake gasket first, and if that doesn’t fix the problem, then that rebuild—and a new set of piston rings—is definitely in order. (Given the age of the truck, you’re due for new rings anyway.)

To improve mileage, we’d recommend a K&N air cleaner and an MSD ignition box. With your usual rpm range, a roller cam and rockers would be of minimal benefit.

…

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.