TBI Fuel System - Power Curve Information

Number: 93-74A-6

Section: 6

Date: FEB. 1993

Corporate Bulletin No.: 276501

ASE No.: A2

Subject:
PERCEIVED LOW POWER INFORMATION

Model and Year:
1987-93 C/K, G, P, AND R/V TRUCKS WITH 7.4L ENGINES AND THROTTLE BODY INJECTION (TBI) FUEL SYSTEM (VIN N)

THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 93-74-7A, DATED JAN. 1993. THE SECTION NUMBER HAS BEEN CHANGED. ALL COPIES OF 93-74-7A SHOULD BE DISCARDED.

ENGINE APPLICATIONS:
7.4L (RPO L19)

ENGINE VIN (Models):
N

SUBJECT:
Perceived Low Power, 7.4L Engines with TBI Fuel Systems

VEHICLE APPLICATIONS:
1987-93 Model Year C, K, G, P, R, and V Trucks





Some owners with Chevrolet trucks equipped with 7.4 liter throttle body injection (TBI) V8 engines may complain of low power under some road/load conditions.

Engines equipped with TBI fuel systems respond differently to accelerator pedal positions than those engines equipped with a 4-barrel carburetor. While a TBI system and a carburetor accomplish the same task, the TBI system maintains a constant air to fuel (A/F) ratio over different road/load conditions and air flow rates.

The air flow curve of the TBI unit has a more uniform, smooth response as the throttle angle increases. The flow curve of the carburetor shows the characteristic "sudden jump" in flow rate as the secondary throttle plates open. Since a TBI unit does not have secondary throttle plates, there is no sudden increase in flow when the accelerator pedal is suddenly depressed from part throttle to W.O.T.

By using a TBI fuel system, improvements in cold startability, smoother engine operation, increased fuel economy, and improved exhaust emissions are obtained.

Customers may perceive little or no noticeable increase in engine power between throttle positions of 3/4 open (60 degree throttle angle) and wide open (80 degree throttle angle) under some road/load conditions. Extensive tests have shown the engine is not losing power in these situations.

Assuming all components and systems are operating correctly, no further action is required.