FUEL MIXTURE PROBLEMS FOR ENGINE

Emission control and fuel system malfunctions sometimes mimic engine problems. Occasionally, important items are neglected during an engine job. Larger engine shops often employ specialists capable of diagnosing these complicated problems.

An air-fuel mixture that is too lean (too much air/too little fuel) can cause burned internal engine parts.


Oil/Fuel Wash
An overly rich mixture (too much fuel/too little air) can cause fuel wash or oil wash (when oil is washed from cylinder walls, resulting in cylinder wall wear). Leaking fuel injectors can result in cylinder wall oil wash, too. They can also cause intake valve deposits that will affect engine idle and emissions. A bad vapor canister purge valve can also cause oil wash. When only one side of a piston is worn, this is often due to oil wash. The worn side will be the major thrust side. An explanation of piston thrust surfaces is provided in Chapter 13.

(a)

(b)

(a) A burned exhaust valve like this one causes lower or no cylinder compression pressure.

(b) Severely burned valves

 Many areas use reformulated gasoline (RFG) to lower air pollution. RFG does not cause problems with
engine wear under normal conditions. But a richer than normal air-fuel mixture can result in accelerated
engine wear because the alcohol or ether in RFG dilutes the oil on the bearings, cylinder walls, and piston
rings. When mixed with the oil, it does not evaporate as easily and the diluted oil loses much of its lubricity. When a low mileage engine fails and there is no evidence of excessive dirt, abrasives, or machined material, diluted engine oil is a prime suspect.


Oxygen Sensor Problems
Modern engines use oxygen sensors in the exhaust to compare the oxygen content of the exhaust

with that of the outside air. After a short warm-up period, it gives the computer information to control
the air-fuel mixture. If the engine runs rough when cold, but the problem goes away after a short
warm-up, the oxygen sensor could be masking an air leak.
Check to see that the sensor is not dirty. Dirt or undercoating can plug the sensor’s outside air intake port, affecting the signal from the sensor. This can result in a richer than normal air-fuel mixture.


COMPRESSION LOSS
Compression loss, another reason for an engine overhaul, can be traced to two causes: compression leaks and engine breathing problems.

Compression Leaks Compression can leak due to several causes:
a blown head gasket , burned valves , worn or broken piston rings, a damaged piston , or a broken valve spring

When valve clearances are too tight the valves cannot seal against their valve seats, resulting in a compression leak. Tight valves can result from wear to the valve faces or valve seats, either of which allows the valve stem tips to move deeper into the cylinder heads. Closer valve clearance can also result
when valves have been adjusted incorrectly.
Exhaust leaks make noise at ½ crankshaft rpm

FIGURE 3.12 A damaged piston resulting from detonation

FIGURE 3.13 A broken valve spring.

FIGURE 3.14 Valve face wear will cause the valve stems to move
into the head, eliminating valve adjustment clearance

FIGURE 3.15 This broken exhaust manifold bolt resulted in a
burned exhaust valve due to thermal shock