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Optimax (Mercury) stalls but running great then restarts runs great stalls ....

Snakeriversailor77

Contributing Member
Working on a Mercury jet boat with Yamaha Optimax engine. The engine starts and runs great but then will just stall. Restarts and runs great again only to stall out. Previously had a fuel pump issue and replaced the internal high pressure pump in the VST and the tiny lift pump. The other low pressure pump was not replaced but tested ok and was pumping fuel. Hate to replace that pump too as they are pretty pricey. Is there another test I can do to the pump to test it? The engine has had all its filters replaced and the fuel is good. Im thinking perhaps the float in the VST is sticking closed and is not refilling with fresh fuel. But why is it refilling after the engine shuts down?
 
Hook up a two fuel pressure gauges, one to the fuel rail and one to the air rail. See if your fuel pressure is dropping off, If the Air pressure drops the engine will stall also. 80PSI on the Air and 90PSI on the fuel unless it's a Pro Max.


Make sure the belt is in good shape and the spring tensioner is working correctly.
 
I'll check the fuel and air pressures as well as the belt and tensioners. Does this engine have a data interface connector? I have a Mercury computer system for Mercruisers and outboards. Since this was a Yamaha product, I hadn't tried a computer diagnostic yet.
 
Check the air and fuel pressure as suggested. The air was @78-82 psi. The fuel was right at 90 psi. Ran a shop tank but had the same problem. Checked the tps unloaded and saw that resistence increased and decreased smoothly. The engine started and idled great, idled out and increased rpm above 2000 no problem. Then continued to add throttle, got just up to 4500 rpm when it cut out. Restarted at an idle and headed back to the dock, eventually made it after the engine stalled several times. Restarted after letting it sit for a while. Anyone have further thoughts?
 
Actually it is more like the engine is turned off. It doesn't sputter or buck but rather just stops. Normally it starts right back up but will stall again if throttle is advanced at all even just putting it in gear will stop it.
 
Like I said, hook both gauges up and take it for a ride and see if you are losing fuel or air. This is the first step in diagnosing this issue after the obvious has been checked.
 
After replacing the fuel and air regulators on the port fuel along with the regulator springs and injectors seals the engine started right up once pressure was reached. Ran better than ever for hours and then suddenly shut down like it was turn off. Was not able to be restarted. This sounds more electrical than fuel now though. Any direction to go now?
 
Check your fuel and air pressure again. If there is no fuel pressure it will not even try to start. If the air compressor failed it will not even try to start.

FYI, this engine is made by Mercury not by Yamaha.
 
After the replacement of fuel and air regulators on the port side the engine started and ran great without stalling. At least for a few hours anyway. Then it just stopped and would not restart. Upon testing, there is no spark what so ever. Engine turns over just fine and fuel and air pressure pump up as they should. Gotta love boats.
 
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