Tom's Corner Garage: Engine Knock, a Sign of Engine Damage
This weekly feature for Edmunds Daily about car repair and maintenance is written by Tom Torbjornsen, the popular host of America's Car Show on satellite radio.
Dear Tom,
I own a 1997 Ford Escort. There is a knocking noise coming from the engine when I press down on the gas pedal. Where should I look for the problem?
Sandy from Wilmington, DE
Sandy is hearing the dreaded "Engine Knock," a sign that her power plant is about to give up the ghost. Not good news.
What is "Engine Knock"?
An engine knock is usually the result of a worn bearing inside the engine. Have oil pressure readings done and drop the oil pan to check rod and main bearing clearances. You'll probably find that major engine work is necessary.
What causes it?
Engine knock is a condition that results from internal wear on the crankshaft and/or related components. When the gas pedal is depressed, a loud metallic clanking or "knock" can be heard that intensifies as the engine accelerates. Typically, this internal wear is caused by a lack of lubrication, or by parts wearing out over time. Causes run the gamut from: lack of oil/filter changes, excessive sludge buildup, low oil levels, lack of oil pressure caused by a bad oil pump, loose piston wrist-pins, and other factors too numerous to list.
How do you fix this condition?
First, a thorough inspection must be performed to determine the extent of the damage. The inspection includes: performing an oil pressure test, dropping the oil pan for visual inspection, measuring oil tolerances of the main and rod bearings (compared to factory specs), inspecting the oil pump, and measuring the crankshaft journals for wear.
Sometimes the engine can be partially rebuilt; bearings can be replaced if the collateral damage is not too significant. Most times the engine needs either a complete rebuild or replacement. Today, complete rebuilt engines are available at less cost than rebuilding the existing engine. In addition, the warranties are so good that it may not make sense to rebuild.
When faced with the possibility of engine repair or replacement, ask the shop for your options. It usually comes down to three options: replace the engine, repair the old engine if it's cost effective, or (if replacement is imminent but funds are low) find a good used engine. If you opt to go with a used powerplant, make sure the tech sees and hears it running before installation; make sure it has a warranty that covers parts and labor; and make sure it is compatible with your old engine. Power plant swaps can be a nightmare if the engines are just a few years off, especially when it comes to emissions and performance systems. These systems must be in proper operating condition to pass inspections in states that have emissions laws.
How long can I drive the car before repairing the engine?
When an engine knock is evident, complete failure is not far behind. The constant hammering of the rod against the crankshaft journal usually results in complete engine failure in a matter of hours. At 60 mph, the piston cycles hundreds of times a second. Couple this high rate of speed to the downward hammering force of the combustion cycle and you have a cocktail of destruction for an engine with a knock. Get it fixed or junk the car.
'Til next time...Keep Rollin'
- Posted by
- Ron Montoya September 30, 2009, 11:00 AM
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- Car Maintenance
Good advice and all, but it is confusing to refer to this phenomenon as "Engine Knock" instead of a knocking noise coming from the engine. "Engine Knock" is almost always used to describe the uncontrolled detonation of the fuel/air mix in the engine (also called "pinging").
Considering the imprecise description of the original symptom, how do we know this isn't just pinging (a/k/a "spark knock") caused by carbon buildup leading to an increase in the effective compression ratio? Or the relatively harmless but relentlessly annoying phenomenon known as "piston slap"?
I too was expecting a description of pinging. But what he decribes sounds pretty bad; it made me cringe reading about it.
Hundreds of times a second, hey? It would have to be turning 6000 rpm just to go one hundred times a second. Maybe that's why the engine is so worn, or maybe this 'Scort races in F1.
Sorry, Tom, but I believe Sandy is referring to pinging. "There is a knocking noise coming from the engine when I press down on the gas pedal." If the bearings are shot she should hear knocking all the time, worse as you point out with increased RPM, and perhaps a little more with more throttle. Worse pinging occurs with depressing the gas pedal due to increased pressure in the cylinder with an open throttle.