Mercury V6 175 Diagnostic Manual page 70

Troubleshooting with the Computer Diagnostic System (CDS G3)
3. Check the fuses.
4. Check the battery cable connections. Ensure that they are clean and tight. If wing nuts are being used, discard them and
replace them with corrosion‑resistant hex nuts. A corrosion‑resistant terminal cable washer nut should be installed between
the battery terminal and the cable end (stack up order must be: battery terminal, washer, cable, nut).
5. Ensure that the cable connections are tight at the hot stud on the engine. Also ensure that the ground stud is not loose in
the engine block and that the nut is tight.
6. Check that all grounds are clean and tight.
7. If there is any doubt about the mechanical condition of the engine, perform a compression test and a cylinder leak‑down
test.
Performing tests with the engine running may cause the propeller to rotate and result in serious injury or death. Use caution
when performing a test that requires the engine running, and remove the propeller to avoid injury.
8. Unplug and inspect the main harness connector between the engine and boat harnesses. If there is any doubt about the
boat harness, substitute a shop harness and key switch assembly and run the boat. If the problem disappears, the problem
is in the boat harness, not the engine harness. A transom mount key switch harness can be purchased for this purpose.
9. Check for adequate fuel pressure at the fuel rail.
a. If there is no fuel pressure, check that the fuel pumps are operating. The pumps must run for at least three seconds
each time the key is turned to the on position. If the fuel pumps and the warning horn are not operating as the key is
turned on, ensure that the PCM is powering up (check the fuses and the wake‑up power), and that clean power is
available.
b. Fuel pressure varies with engine vacuum. Fuel pressure will be high during cranking, low at idle, and increase
proportionally as the throttle is opened to the wide‑open position. Disconnect the reference hose to find the
regulator's rated pressure. Then reconnect the reference hose to ensure that the pressure drops at idle.
Pressure usually drops about 41–45 kPa (4–6 psi) at idle (from the regulator's rated specification).
c. If the fuel pressure drops at higher speed and higher engine loads, check the boat's fuel system (the supply system)
for restrictions with an accurate vacuum gauge and clear hose at the water‑separating fuel filter inlet. As the engine is
run from idle to wide‑open throttle and back to idle, the clear hose must not show the presence of any air bubbles and
the vacuum gauge must not read higher than 6.7 kPa (1 psi) (2.0 in. Hg).
d. If the supply system is working correctly, but the fuel pressure is low at high speeds and loads, replace the
water‑separating fuel filter and retest. If pressure is still low, the fuel pump is most likely defective.
10. Check the two vacuum lines for splits, kinks, and proper connections.
11. Check for any other air leaks in the induction system, such as at the throttle body and intake manifold gaskets.
12. Unplug and inspect the PCM connectors. Ensure that there are no bent PCM pins and that all of the correct pins are
present. Refer to Section 2B for the pins used and not used on PCM 112 engines. Look for signs of tampering, corrosion,
damage to the pin locking mechanisms, melted insulation, any other evidence of shorts, or other damage.
13. Unplug and inspect as many of the sensors and actuators as you can reasonably access. Look for signs of tampering,
corrosion, damage to the pin locking mechanisms, melted insulation, any other evidence of shorts, or other damage. Based
on the results of this inspection, further inspection of the harness may be necessary. Remember that there are many
internal splices in the harness that may be damaged or defective. If there is damage at the external connections, you will
have to inspect several of the internal splices to verify that the damage is not also present at these locations.
Page 3B-16
WARNING
!
Rated Fuel Pressure at Idle
390–340 kPa (42.1–43.9 psi)
© 2018 Mercury Marine 90-8M0146617 eng JULY 2018