26
Fuel System Testing
The requirements for lower vehicle emissions has in-
creased the need for more precise engine fuel control. Auto
manufacturers began using electronically controlled car-
buretors in 1980 to meet emission requirements. Today’s
modern vehicles use electronic fuel injection to precisely
control fuel and further lower emissions. The digital multi-
meter can be used to test the fuel mixture control s o l e -
noid on General Motors vehicles and to measure
fuel injector resistance.
Testing GM C-3 Mixture Control Solenoid Dwell
This solenoid is located in the carburetor. Its
purpose is to maintain an air/fuel ratio of 14.7
to 1 in order to reduce emissions. This test
checks to see if the solenoid dwell is varying.
Test Description:
This test is rather long and detailed. Refer to
vehicle service manual for the complete test
procedure. Some important test procedure
highlights you need to pay close attention to
are listed below.
1. Make sure engine is at operating tem-
perature and running during test.
2. Refer to vehicle service manual for
multimeter hook-up instructions.
3. Turn multimeter rotary switch to 6 Cyl-
inder Dwell position for all GM vehicles.
4. Run engine at 3000 RPM.
5. Make engine run both RICH and LEAN.
6. Watch multimeter display.
7. Multimeter display should vary from
10
°
to 50
°
as vehicle changes from lean
to rich.
Typical Mixture Control
Solenoid Connection
Mixture Control
Solenoid
