7.5L Diagnostic Tips

If the diagnosis rules out a plugged catalyst, refer to the following tips to narrow down the cause of a loss of power in the vehicle. If the catalyst is plugged, review the procedures listed under "Supplemental Diagnostic Instructions" found near the end of this article.

In addition, the following tips may be useful:

^ Inadvertent operation of the vehicle with the "SPOUT" bar disconnected will lead to "Lack Of Power" and excessive heat in the exhaust system due to the spark retard condition in this mode.

^ Running out of fuel at highway speeds, either by the customer or because of poor fuel delivery, will cause engine misfire and subsequent high catalyst temperatures, resulting in damage to the catalyst.

^ Vehicles with manual or C6 automatic transmissions built before 10/30/89 and vehicles with E40D transmissions built before 11/30/89 may exhibit unsatisfactory engine performance, hard starting after a short hot soak in high ambient temperatures, or while the truck is under heavy load. This may be caused by the strategy of the EEC processor. Refer to TSB 89-24-9 for service procedures.

NOTE:
IF TSB 89-24-9 WAS PREVIOUSLY PERFORMED ON THE VEHICLE, DO NOT REPEAT.

^ Vehicles built after the above mentioned dates and which had TSB 89-24-9 performed may have an EEC IV processor containing a strategy that shuts off half of the injectors at 4600 rpm. This further protects the catalytic converter and may give an indication of low power when it is really a measure of cutting back the fuel. Super Duty vehicles with higher axle ratios may be susceptible to this system.

^ In hot weather, a failed low pressure in-tank fuel pump will result in a sense of low power or stalls. Refer to TSB 91-13-8 for pre-1992 model year E-Series and pre-1991 model year F-Series.

^ An electrical short circuit in the E40D transmission electronic power supply solenoid (self test code 99) turns off half of the fuel injectors for protection of the E40D transmission clutch. This will yield a "lack of power" condition.

^ Altering spark timing (reductions) due to spark knock caused by the use of low octane fuel can cause a "lacks power" condition.

^ Functional or performance issues, caused by a failure or malfunction of other engine or vehicle components, can result in a "lacks power" condition.

These include...

a. Injector deposits

b. Fouled spark plugs (root cause can be stuck open fuel injectors)

c. Failed fuel pressure regulator

d. Blocked or kinked fuel tank return line(s) (Refer to TSB 91-23-10.)

e. Broken or otherwise disconnected MAP sensor vacuum line

The following items in themselves can be responsible for vehicle functional issues.

^ Dirty air filter

^ SEVERELY restricted fuel filter (Refer to TSB 91-23-11.)

^ Vehicle brake drag

^ Kinked or squashed exhaust system components

^ Various EEC IV sensor malfunctions, including broken or chafed wiring. Self-test code 42 (HO2S always rich) can be caused by a broken wire to the H02S (HEGO) sensor. This will drive the fuel system in the lean direction which will cause a "lacks power" condition.