Basic Engine Diagnosis

It’s a good thing we opted to change the oil and filter first, as the old stuff was asphalt-thick and reeked to high heaven. If we hadn’t changed this sludge, the ’Bu’s exhaust emissions would likely have been higher. The plugged air filter was also swapped for a new free-flowing replacement.

It’s hard to believe we didn’t notice the duct-taped choke assembly when we bought this thing. It sure explains the 305’s cold-start roughness. We later found a good choke assembly at the local Pick-Your-Part, but not in time for the smog check.

Another ingenious idea: Hey, when the kick-down cable breaks, why not just tie it back together (arrow)? We bought a replacement cable for the Turbo 350 at a Chevy dealership, and no, we haven’t installed it yet. We’re just having too much fun manually downshifting the tranny—you know, for that extra surge of power.

CC Editor Matt King hypothesized that the No. 1 had poor valve sealing, so we shot some 80-weight gear oil in the No. 1 hole and retested it—185 psi. Sorry Matt, survey says: worn rings. If the compression hadn’t changed, we would have suspected a problem with the valves.

Next, we fired up the air compressor for the leakdown test. Powerhouse Products’ leakdown tester requires an external air supply to pressurize the cylinder. The tester has two gauges; one measures the supply pressure, and the other measures the resulting cylinder pressure. We set the supply pressure to 100 psi, rotated cylinder No. 1 to TDC, threaded in our tester, and attached the air hose from the compressor. Proceed in the correct firing order and test each cylinder, and rotate the motor 90-degrees clockwise after each test to ensure each cylinder is at TDC with both valves closed. We plugged the observed numbers into the formula %leakdown = [(supply pressure - cylinder pressure) / (cylinder pressure)] x 100 with the following results...

The leakdown test again proved that the No. 1 was especially weak, but ideally we would like to have seen no more than a 10 percent leakdown on any of the cylinders. You should be able to isolate the cause of a problem cylinder by listening for escaping air. We heard air escaping when we removed the 305’s oil fill cap, a sure sign of ring blow-by. If it’s audible in the exhaust pipe, suspect an exhaust valve. Think the intake valve isn’t sealing properly? Listen for air blowing out through the carb/throttle body. If you detect bubbling coolant, bet on a poorly sealing head gasket.

At one time or another (possibly many times), most of us have bought at least one project car without giving it a thorough once-over. You remember—it’s that car that you ecstatically “stole” from its previous owner, only to find plywood where the floors once were, a hose clamp “securing” the transmission, and metallic-tinted 80-weight oil in the crankcase. There’s no known cure for the adrenaline rush that comes with buying a new project, but there’s nothing wrong with calming yourself long enough to perform a few basic tests on its engine—otherwise you’ll end up like us.

We forked over the cash for this ’78 Malibu after reveling in the original-paint cleanliness of its bodywork. We didn’t really think about the mechanicals until the next morning when the car refused to start. We convinced it to run, and while cruising around, noticed that the transmission only occasionally shifted into Third gear. Disappointed, but not discouraged, we crossed our fingers and dove in to assess the condition of the drivetrain. The cold-start and shifting problems were easily remedied with a new choke and transmission kick-down cable, but we figured this was a good time to perform a compression and leakdown test before tuning the car up for a required smog check. The test results hinted that the 305 probably hadn’t been rebuilt (as the previous owner claimed). It did, however, run well enough to pass smog and cruise us around Southern California. We think it’s a pretty happy ending to a $1,200 investment.

Power-hungry readers need not stress, because this is only the beginning of our late-model Malibu saga. Don’t count on the wee-oh-five surviving the transformation.