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Ask Dominick Your Own Question
Dominick, Marine Mechanic
Category: Boat
Satisfied Customers: 1009
Experience:  25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified
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I have a nagging problem that has been in place since I

Customer Question

I have a nagging problem that has been in place since I bought this boat in 2010. I am the third owner. The problem is, the Mercruiser I/O V8 idles roughly, stumbles, then dies when in gear after it is thoroughly warmed up. In all other respects it runs quite well, making full power all the way up to WOT rpm, pulling two skiers up with ease. It never over heats, its temp gauge indicates just over 170 deg F when fully warmed up.
The lack of idle problem manifests itself as idling okay for a few minutes, then becomes rough, then stumbles enough to die. This seems to be most aggravating when I really need it - such as when maneuvering for loading on the trailer. When preparing for an outing, it idles fine when it is on the trailer, operating with the outdrive hose muffs. But when the propeller is submerged, the additional load when it is put in gear (either forward or reverse) is sufficient to cause the idle to become rough, then it stumbles and finally dies. If the in gear load is lessened, such as when the boat is idling along in, for example, a trolling for fish scenario, it will keep running but stumbles some; the exhaust fumes that bubble up from behind the boat smell like it is running quite rich.
I have the outdrive serviced annually, and this past year, my mechanic listened to this detailed description of its idle problem closely, then examined the entire system for a cause for this idle problem. He stated that the fuel pressure is correct at 30 psi, the computer does not report any trouble codes and for everything he can see it runs quite well, but it is in his shop on the trailer, using a hose and outdrive muffs. (He does not have a facility to lower the propeller into water to simulate the lake water load.) He suggested that a local auto repair house could rebuild the fuel injectors. The engine seems to work better after they were rebuilt (maybe hopeful thinking?), it does seem to idle slightly better, but the stumble and die problem persists. Careful inspection of the throttle body does not reveal any liquid fuel except the injector spray shots.
Two breather hoses lead from each rocker arm cover to ports on the flame arrestor. This Mercruiser 5.7 has no PVC valve, so there is no hose leading from each rocker cover to a vacuum port under the throttle body. In fact, both vacuum ports below the butterflies are capped with rubber plugs.
My question to you is: my Seloc manual has a diagram for this engine’s fuel system that shows the fuel pressure regulator has a vacuum hose that leads up from where it is mounted on the fuel cooler (below the port side exhaust manifold) to a vacuum port. However, on my engine, that hose leads up to a port that is part of the flame arrestor on top of the throttle body. Is this normal? If not, could it be the cause of my idle problem that seems to be coincidental to a rich idle fuel mixture? Should this engine have a PVC valve?Reference info:’96 Mercruiser Engine, Model No. MCM 5.7L EFIEngine Ser. No. 0K007156
Transom Assy Ser. No. 0K109134
Alpha Gen II Drive Unit 1.62R 0K0518381996 Searay Bowrider 210
Mercruiser 5.7 L, TBI EFI
Submitted: 2 months ago.
Category: Boat
Expert:  Jason replied 2 months ago.

Hi Tate, my name is***** read what wrote. That engine has an old MEFI 1 ECM. Those don't throw codes, they simply show the engine data in real time. Which scan tool are you using? Will that tool do a copy of the live data or can you transcribe it? Has a compression test been completed yet?

Customer: replied 2 months ago.
you didn't address my questions:
1) Should my engine have a vacuum hose from its fuel pressure regulator to a vacuum port on the intake manifold or throttle body?
2) Would Mercruiser issue these engines without a PCV valve?
Expert:  Jason replied 2 months ago.

Hi Tate. Good morning and sorry for the delay. When you replied back last night I had already logged off for the evening. Did you still need help with this or did you get it figured out?