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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11076
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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1997n merc 225 optimax. whats the fuel pressure on the high

Customer Question

1997n merc 225 optimax. whats the fuel pressure on the high side. don't think its dfi because there is no fuel rail with injectors by the heads
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: wslow to rev past 1500rpm them slowly catches then goes. does not miss or sputter
JA: Is there anything else important you think the Marine Mechanic should know?
Customer: no
Submitted: 1 month ago.
Category: Boat
Expert:  Tan replied 1 month ago.

Hello, I'm Tan. Welcome to JustAnswer. I'm reviewing your question and will reply momentarily.

Expert:  Tan replied 1 month ago.

Could I get engine serial number please?

Optimax is the DFI motor so if you don't have fuel rails at the cylinder heads this is not an Optimax.

Customer: replied 1 month ago.
Customer: replied 1 month ago.
i'm sorry u r correct its not a optimax
Expert:  Tan replied 1 month ago.

225 EFI fuel pressure is 34-36 psi.

We need to verify spark on all cylinders. This can be done with a spark gap tester or a timing light.

Customer: replied 1 month ago.
if my fuel gauge is correct its around 45psi at idle
Customer: replied 1 month ago.
i have spark on all cylinders
Expert:  Tan replied 1 month ago.

That is way to much fuel pressure this will cause stumble and hesitation. The engine is flooding from to much fuel causing the issue.

To much fuel pressure is a failing fuel pressure regulator.

The regulator is # 20 in this pic.

Customer: replied 1 month ago.
but the engine does not miss or stumble. revs up almost to get on plane then thats it for a minute then will slowly start to rev up then go. once on plane and going it runs fine.
Customer: replied 1 month ago.
evven when reved up to 1500 the engine is not missing or stumbling at that point.
Expert:  Tan replied 1 month ago.

You must verify the fuel pressure. Is that in gear? You cannot test how an engine performs out of gear.

Customer: replied 1 month ago.
thats on the flusher not in gear
Expert:  Tan replied 1 month ago.

I would put the boat in the water and see how it performs. You cannot accurately troubleshoot this engine on a flush attachemnt. These engine require a certain amount of back pressure in the exhaust. If you don't have it the engine can run lean and cause hesitation. So we need to verify fuel pressure and that it is actually acting up. Let me double check that fuel pressure as I was going by memory recall.

Customer: replied 1 month ago.
Expert:  Tan replied 1 month ago.

That spec is correct. 34-36 psi.

Here is what the manual says.

Results: If pressure reading is 34 to 36 psi (234 to
248 kPa), the electric fuel pump is providing
fuel with enough pressure to be used by the
injectors. Pump malfunction is not the cause
of EFI trouble.
If fuel pressure is well below 34 psi (234 kPa), fuel delivery
to electric fuel pump, fuel pump failure or other
related problem exists. Follow low/high fuel pressure
flow charts.
If fuel pressure is above 36 psi (248 kPa) go to fuel
pressure regulator test.

Customer: replied 1 month ago.
thanks let me test on water to verify
Expert:  Tan replied 1 month ago.

Ok. Here is fuel pressure regulator test.