Checking the Basics

When attempting to diagnose or repair a customer's complaint, always follow a logical sequence as outlined in FLOW OF DIAGNOSIS. Then look for any obvious problems. Some common problems associated with fuel systems are:

1. NO FUEL IN TANK
Do not rely on the gauge or customer. Remove the gas cap and bounce or rock the vehicle and listen for fuel sloshing. Or if in doubt, add a few gallons.

2. A CLOGGED OR PARTIALLY RESTRICTED FUEL FILTER OR FUEL LINES
Most auto makers now use more than one filter. There is usually a screen on the fuel pick-up tube, in the fuel tank, that may also clog.

3. AN OPEN FUSE OR FUSE LINK
All electrical circuits are protected with a fuse or fuse link. Some fuses protect more circuits than may be listed on the fuse panel cover. Always check all fuses and fuse links.

4. BATTERY VOLTAGE
Electrical components used in the fuel system have a minimum voltage requirement to operate properly. If the cranking or operating voltage of the battery drops below 9.5 volts, some components may not operate properly.

5. DIRTY, RESTRICTED OR MISSING AIR FILTER ELEMENT
Many of todays engines are designed to compensate for the amount of restriction that a clean air filter element normally creates. A minor deviation from the normal air flow (too much from a missing element or too little from a dirty element) can cause the engine to operate abnormally.

6. VACUUM LEAKS
Minor vacuum leaks at any point in the intake system can cause a lean mixture. Always confirm that all vacuum hoses are installed properly, check for leaks at intake manifold, throttle chamber, carburetor base, and ducting. Check any point between the air intake and the cylinder head for leakage.

7. RESTRICTED EXHAUST
A partially restricted exhaust system can cause the engine to run poorly or in severe cases, not at all. An exhaust leak may be an indication of a restriction down stream of the leak.

8. SATURATED CHARCOAL CANISTER
A charcoal canister that is saturated with fuel can cause an overly rich air fuel mixture. This may cause the computer system to malfunction.

9. FUEL DILUTED ENGINE OIL
If an engine has been operating in an overly rich air fuel mixture, the chances that the engine oil has been diluted are great. If the oil smells like fuel or the level is above normal, suspect this problem.

10. IMPROPER FLOAT LEVEL
A high float level may cause fuel to enter the throttle chamber during braking, cornering or in extreme cases, normal operating conditions. This causes a rich air fuel mixture. A low float level may cause insufficient fuel delivery, causing a lean air fuel mixture.

11. IMPROPER FUEL PRESSURE
High fuel pressure can cause excess fuel delivery, causing a rich air fuel mixture. Low fuel pressure can cause insufficient fuel delivery, causing a lean air fuel mixture.

12. CONTAMINATED OR WRONG FUEL
Dirty fuel. Water in fuel. Excessive alcohol content. Diesel fuel being used in a gasoline engine or visa versa. All of these can cause a variety of driveability problems.

NOTE: Always verify a customer's complaint, then perform a visual inspection of the system before performing any diagnostic procedures. Don't forget that your most important tools are the tools you were born with. Good luck.