2005 Mercury 90 FOURSTROKE Fuel Pressure Measurement

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
howlingdogsteve
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Joined: Sat Sep 03, 2016 7:45 am

2005 Mercury 90 FOURSTROKE Fuel Pressure Measurement

Postby howlingdogsteve » Mon Oct 14, 2019 12:28 pm

Hello All. I have been trying to diagnose rough running and misfire on my 2005 Mercury 90 FOURSTROKE, actually a Mercury-Yamaha engine.

I am unable to get a fuel pressure reading on the Schraeder valve on the VST [vapor separator tank]. I have tried several fuel pressure gauges from both AutoZone and Harbor Freight with no success; all readings are zero.

Is there a special adapter available from Mercury or Yamaha?

Any advice would be greatly appreciated. Thanks so much--Steve

jimh
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Re: 2005 Mercury 90 FOURSTROKE Fuel Pressure Measurement

Postby jimh » Mon Oct 14, 2019 4:03 pm

Are you certain the fuel is under pressure in the vapor-separator tank?

macfam
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Re: 2005 Mercury 90 FOURSTROKE Fuel Pressure Measurement

Postby macfam » Mon Oct 14, 2019 5:17 pm

I share a dock with a friend who has a 2003 Mercury-Yamaha with carburetors.

My friend's engine had running problems exactly like yours; the engine would start, but ran very roughly, and could accelerate to only about 4,200-RPM.

In the summer of 2019 my friend lost use of the engine for three weeks while the engine was being repaired at Nauset Marine--with all new carburetors. They were a corroded mess. This was a large expense for my friend of maybe between $3,500 to $4,000. [The 2003 Mercury-Yamaya engine of unspecified horsepower that is owned by my friend] runs perfectly now [that the carburetors were replaced].

jimh
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Re: 2005 Mercury 90 FOURSTROKE Fuel Pressure Measurement

Postby jimh » Mon Oct 14, 2019 5:44 pm

MACFARM--do you know if the engine owned by your friend indicated fuel pressure from the Schrader valve on the vapor-separator tank?

Engines with carburetors usually do not have a fuel-vapor separator tank.

The Mercury-branded engines made by Yamaha in the early 2000's generally were carburetor engines. Those engines were somewhat infamous for problems with their carburetors. There are many prior discussions in the old forum about problems with the carburetors of Mercury 90 FOURSTOKE engines that were actually re-painted Yamaha engines. Here is one such older discussion about problems with the Mercury 90 FOURSTROKE carburetors:

Mercury 90-HP FOURSTROKE: Carburetor Problems
http://continuouswave.com/ubb/Forum3/HTML/009936.html

Regarding the possibility that a Yamaha-made powerhead engine re-branded as a Mercury FOURSTROKE in c.2005 might have used some sort of simple fuel-injection (not direct-injection), there is a discussion of that possibility also in the old forum. Please read for background:

Mercury 90-HP Four-stroke Change to EFI
http://continuouswave.com/ubb/Forum1/HTML/015235.html

biggiefl
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Re: 2005 Mercury 90 FOURSTROKE Fuel Pressure Measurement

Postby biggiefl » Tue Oct 15, 2019 12:08 pm

HOWLINGDOGSTEVE:

Q: Have you tried new spark plugs?

Every time [my engine] starts to misfire, [the cause] is the spark plugs and [to replace them] costs $10.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

rtk
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Re: 2005 Mercury 90 FOURSTROKE Fuel Pressure Measurement

Postby rtk » Wed Oct 16, 2019 7:17 am

If I remember correctly it was usually vacuum readings that were used to diagnose fuel system problem on the carbureted outboards. A vacuum gauge was placed inline with the fuel line between the fuel tank and the engine. I have an old one that has the Johnson/Evinrude fuel line quick disconnects so it may be placed inline easily. In applications where quick connects were not used barbed nipple attachments were used.

For fuel injected engines a vacuum gauge may serve the same diagnostic purpose but I do not have actual experience using a vacuum gauge on a fuel injected engine.

Rich