Breathing Problems For Engine

An engine that cannot breathe properly is suffocating and will not be able to develop sufficient compression. Engine vacuum will drop off, further lowering compression. Breathing problems can be traced to such things as worn camshaft lobes that do not open the valve far enough (Figure 3.16),or late valve timing . Valve timing can
become retarded (late) when a timing chain becomes so worn that it skips a tooth.

FIGURE 3.16 A severely worn cam lobe will not properly open a

valve.

If the timing chain has skipped and valve timing is retarded, suction will be felt at the exhaust
pipe. This happens because the exhaust valve is still open during the piston’s intake stroke.

Breathing problems can also be traced to carbon buildup around the neck of the valve (Figure 3.17)
or to restrictions such as a dirty air cleaner or a blocked exhaust.

FIGURE 3.17 A lack of carbon formation on the top of the piston,
beneath the intake valve, indicates oil leakage through the valve guide.
The valve in the photo was found when the head was disassembled

Catalytic converters in the exhaust system can become plugged after running for a prolonged period with an ignition system defect. A rich air-fuel mixture can also cause a converter to plug when it overheats and melts internally. OBD II vehicles (later than 1996) will go into limp-in mode when the computer senses a catalyst damaging misfire.


An exhaust restriction can also cause an automatic transmission to shift harshly or late due to the resulting faulty vacuum signal or increased throttle pressure in the transmission. Exhaust backpressure can be tested using a fuel pump vacuum/pressure tester connected to thesmog pump lines into the exhaust manifold, or an
adapter can be substituted for the EGR valve. According to TRW, removing the oxygen sensor to
perform the test can give an inaccurate reading due to a venturi effect in the exhaust system. Specifications
vary among manufacturers. As a general rule, pressure should not exceed 1.75 psi at wide-open throttle (WOT) under full load.

Breathing problems can also be found using a vacuum gauge


SHOP TIP
A blocked exhaust will be evident when the engine rpm is raised quickly. A roar will be heard on the engine side of the air cleaner.





SAFETY NOTE
A catalytic converter on a pre-OBDII vehicle can become so hot that it can start a fire if the vehicle is parked above dry grass.