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Jeremy R.
Jeremy R., MarineTechnician
Category: Boat
Satisfied Customers: 9020
Experience:  Multiple factory certifications 20+ years of experience.
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i have a 1998 mercruiser 7.4l mpi. i am losing fuel pressure,

Customer Question

i have a 1998 mercruiser 7.4l mpi. i am losing fuel pressure, i replaced fuel pump and regulator. started and ran ok at dock. went up river and lost fuel pressure ( slow speeds wake area) happen week earlier. ran ok to where we docked and when we prepared to leave lost fuel pressure. sat for approx. 1.5 hours and cycled key we got fuel pressure back and ran home ok. i then replaced pump. both times outside temperture hot and humid. this last trip engine seemed to run hotter than normal. could this cause vapor lock in pump. i was towed in this last time and after 2 hours still no pressure. tank level is between 1/4 and 1/2.
Submitted: 7 years ago.
Category: Boat
Expert:  Jeremy R. replied 7 years ago.
Welcome, how are you checking fuel pressure and where are you checking it, what are the serial numbers of the motor
Customer: replied 7 years ago.
I am using a fuel pressure gauge at service port valve. when running pressure is 30 - 35 psi with vacuum connected to regulator. vacuum disconnected pressure is approx. 45. I had same readings when pump I replaced was running OK. when stopped pressure would only rise to 10psi. serial #s 57-861049 Ol0149962
Expert:  Jeremy R. replied 7 years ago.

OK that serial number is XXXXX but here is the factory manual


ok we will need to check for codes, they also had vapor locking problems with the gen 2 fuel cooler


also they have a bulletin about vapor locking and problems with the fuel pumps sticking, get me the right serial number and i will tell you the bulletin



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Customer: replied 7 years ago.
I have been traveling, will try to get down to boat today and fine serial# XXXXX would like that fuel overheat bulletin, i suspect that may be whats happening. i still have gauge hooked up. i will see if pressure returns. the engine seemed to be running hotter than normal before losing pressure about 190, normal is 170. hose at fuel cooler was hot to touch.I have no scanner to pull codes.
Customer: replied 7 years ago.
try this number I put in incorrectly 1st time
OL014962 (this is number I use for parts) also if this will not work, it may be 2-3 days before i get back to boat, weather bad here. if i accept for you to be payed will still send me service bulletin?
Expert:  Jeremy R. replied 7 years ago.
Customer: replied 7 years ago.
one last question, i am starting to think i may have a raw water pump propeller problem. i am going to disassemble and check. I always install new thermostat at start of season, specs indicate using a 170 deg. would you recommend dropping to a 160?
Expert:  Jeremy R. replied 7 years ago.
use a OEM mercruiser t-stat and not aftermarket!
Expert:  Jeremy R. replied 7 years ago.
Mercruiser calls for a 160 degree thermostat for that motor,not 170.Part number 807252T2