The Tifler Blog

Interference Engines

Interference Engines: ……..Free Useful information

When buying a used car always insist on determining if the
vehicle has a rubber timing-belt. Be aware that about five years
ago an International Oil Company did a follow up on 5,000 cars
it had turned back after 3 year leases and traced them to their
eventual private owners. All the cars had by then passed through
wholesale auction markets and likely one or more retail dealers
before being sold to a private owner. The survey disclosed that
50% of the cars had their odometers illegally turned back.

When buying a used car, supposedly with 40,000 miles for
example, and determining it has a rubber timing belt, insist on
a written guarantee from the seller to guarantee in writing to
replace the timing belt at no charge if it fails within another
20,000 miles, a typical recommended total amount (Call any
Dealer to get the recommended amount for the particular make of
vehicle). After all, the vehicle may in fact already have 55,000
miles on it. If the seller will not make that guarantee, then he
is admitting that the mileage is probably not accurate and by
implication may well have been turned back. If the seller will
not make that guarantee, consider a compromise, such as $100
maximum cost. If not acceptable, walk away and look elsewhere.”

Before buying any car, especially 4-cylinder foreign cars, or
even 6-cyl. BMW, be aware of the unavoidable cost of $400-$800
to replace the timing belt at anywhere from 50,000 to 70,000
miles if the car has an ‘interference’ type of engine. The sales
person will invariably not mention that an ‘interference’ type
engine powers the vehicle and may not even know what one is. If
a timing belt on an interference engine is not replaced at
recommended intervals, the repair cost when the belt breaks (not
gradually, but always catastrophically) could increase to $3,000
to $5,000 due to engine failure because parts have smashed into
each other

An ‘interference’ engine is an engine design that has been
avoided by some manufacturers for well over 80 years. General
Motors, Chrysler, etc., typically use a metal chain-type timing
belt on push-rod engines (often called a timing chain) to
transmit torque from the engine crankshaft o the engine camshaft
that opens the valves that admit air and fuel. (Note: on some
new cars the fuel is admitted not through the valves but through
injectors in the top of the cylinder. Rather than use a steel
timing chain, interference engines may use a rubber timing belt
with its limited life, whereas steel timing belts typically last
150,000 to 200,000 miles or more.

Valves open further in an interference engine and project
further into the combustion chamber than in a ‘free-running’
engine. This allows outside air at atmospheric pressure flow
faster into the combustion chamber through the larger valve
opening. The engine can therefore inhale more air, be a little
smaller, and still create as much power while reducing its.
manufactured cost and also guaranteeing future repair business
for its dealer. If a rubber timing belt breaks by not being
replaced soon enough, some of the valves stuck in their open
position will collide with the top of the pistons, thereby
breaking or irreversibly damaging one or the other or both. To
make matters worse, it is not possible to measure the wear on
such a rubber belt so that it could be replaced when there is
some indication of imminent failure. Failure in these belts is
catastrophic, without warning. This will require a whole new
engine be installed. Woe to the owner. Finally, the rubber belt
may have to be replaced long before 60,000 miles solely due to
its age. This is really playing a bad poker hand. Interference
engines are like a time bomb waiting to explode unless replacing
the timing belt at the recommended interval. Be aware of that
guaranteed future expense before buying a new car, or especially
a used car, ” with such an engine.

For details on this subject or for recommended mileage to
replace rubber timing belts on interference engines, connect on
the Internet to Gates.com. When its web site appears, click on
Replacement parts/Automotive. Look for ‘Timing belt replacement
Guide’.