HANDS ON
OUT OF TUNE OR OUT OF TIME?
HOW TO KNOW WHEN YOUR ENGINE NEEDS A COMPLETE REBUILD
Don Sherman
When your pride and joy’s engine room cries out distress signals, the
correct response could range from a routine tune-up to a complete over- haul. If the issues are rough running, high fuel consumption and diminished power, the remedy might be fresh plugs, new ignition wires, and reset- ting the dwell and timing. But if there’s a low oil-pressure reading at idle, blue smoke out back and abnormal noises under the hood, you’re in for more.
Before you hook your lift chain to the block, a comprehensive diagnosis is essential. Start by measuring each cyl- inder’s compression pressure during cranking with the engine warm and the ignition disabled. If one or more holes reads low, that’s a serious fault that needs to be remedied. Another means of identifying worn valves and piston rings is to conduct a leak-down test. A leak-down tester applies compressed air through each cylinder’s spark plug hole with the piston at top dead center and both valves closed. Two gauges report the supply pressure and one cylinder’s sealing pressure. If the low reading divided by the high reading is less than 80 percent, you have inade- quate sealing. When you hear air leak- ing into the intake manifold or exhaust pipe, the valves are at fault. Bubbles at the radiator filler expose leakage into
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the cooling system. If you trace the es- caping air sound to the crankcase, the pistons and/or rings are the culprit.
Using a mechanic’s stethoscope is the best way to identify where air is exit- ing the cylinders. That same tool can also be used to pinpoint the location and cause of rattles, knocks, ticks and slapping sounds while the engine runs. Heavy knocks indicate bad bearings or cracked pistons. Lighter clicks reveal valvetrain issues such as a collapsed lifter or excessive lash.
If burned or poorly sealing valves are positively identified as the culprit, you should be able to get by with a top- end overhaul. That means removing and refurbishing the head(s) by grind- ing the valves and their seats. It’s wise to have your machine shop renew the head-to-block surface at this time.
Removing the heads or the complete engine are within the means of any home mechanic with the proper tools. But from that point on, you’ll most likely need the services of a reputable shop. Before you commit, check with fellow club members, local auto parts stores, and customers of the prospec- tive shop. Online sites like
yelp.com and
angieslist.com can help you here. Doing the job right is your best as- surance that you’ll enjoy many happy miles before major engine work is again required.
RESOURCES
While many auto parts stores rent, sell or loan en- gine diagnostic tools, they can also be purchased here:
eastwood.com harborfreight.com sears.com snapon.com
PHOTO: GETTY IMAGES
PHOTO: CORBIS
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