Gas in Oil, rought idle F90TLR

[77tarheel]

Hello, I have a 2011 F90TLR on a 2007 Carolina Skiff 198DLV. I bought the bought used three years ago and have had no issues with engine, which looks brand new.
Late last year, I changed the oil, oil filter and large and small fuel filters with Yamaha products. I have used the boat probably 10 hours since.

I buy non-ethanol gas when I can and put in ethanol treatment when I cannot.

Three weeks ago, a friend and I took it out on the lake. It ran fine for about 90 minutes. As we neared the landing, the engine died and we had to paddle in. It wouldn't restart.

When I got home, I found there was no gas in the fuel filter on the side of the engine. I checked the fuel tank and found the vent screw closed. I loosened it, pumped the primer bulb and the small filter filled up with gas.

The boat started, but idled roughly.

Last weekend, I took it out again and I checked on a couple of deck items while my wife idled us away from the landing. The motor died and wouldn't restart. We had to get a tow back to the landing.

I checked the small fuel filter and it was full.

Then, I checked the oil dipstick and gas-diulted oil came rushing out.

When I got home, I looked around on the interenet and found other folks had the same problem. The suggested fixes always included replacing the thermostat, suspected of running the engine too cold.

I drained the engine oil and replaced with some Castrol 10w30 for testing. I also replaced the spark plugs. The engine started but idled roughly. It seemed to run OK if given throttle and there was no excessive smoke.

I can understand if the thermostat can be bad, but why the rough idle? I ordered a YDS software and cable kit that got here yesterday. In static tests, you can hear the fuel pump and injectors energizing.

I did tests with the engine running and nothing jumps out as wrong from the data there.

Also found the number of hours on the engine: 334.

I found another thread on this site where another owner with the same engine had the same issues and he discovered the diaphram in his fuel pump was in pieces.

So, it seems possible that with the fuel tank vent closed, it could have strained the fuel pump and I'm going to inspect it later today.

Am I thinking straight on this?

I'll also order a thermostat for preventive maintenance and will do a leak-down test. Waiting on a long-reach adpater for the spark plug hole.

Thanks for any insights on this.

[TownsendsFJR1300]

Yours is fuel injected so you don't have a fuel pump diaphram that can leak into the crankcase:

https://www.boats.net/catalog/yamaha...jection-pump-1

An engine "running too cold / thermostat issues refers to WATER in the oil, definitly not gasoline..


I'd drain the VST, part/screw #13 and catch what comes out. Looking for any crap in there.

Potential issues could be a leaky fuel injector as well.



Capture.JPG

[fairdeal]

Yours is fuel injected so you don't have a fuel pump diaphram that can leak into the crankcase:

You've having a momentary lapse there, Scott.

Just as your F150 does,

(actually you have two)

his F90 has a camshaft driven diaphragm "low pressure" lift pump

[99yam40]

and a cold motor does not vaporize and burn all of the fuel and it will contaminate the oil according to what I have read
too much low RPMs can also cause it.
that is why the recommendation to run at high RPMs for the some of the trip to heat up the motor

a malfunctioning/ leaking /stuck open injector can cause fuel in oil also

not breaking in a motor properly to get the rings seated well can also cause problems

[brian3006]

Had this issue with 2 F90's and one F115. All 3 were caused by the thermostat stuck open. Replace the thermostat and also remove and check the poppet valve on the side of the motor. (near the oil filter). one of mine was also slightly stuck open with debris.

Basically the motor runs too cold and the parts don't heat up and seal properly. The fuel goes past the rings and into the crankcase oil. in the process, the cylinders get "glazed" and the rings do not seat well. after replacing the thermostat, let it warm up then take it out and run it hard. High RPM and varying if possible. this will help heat it up and help the rings seal a lil better. it might take a couple of trips to get it back to normal but probably not.

I have had people tell me that Yamaha ringfree will help get it back right quicker. I have used it on one boat and not the other. both are running fine now.

[77tarheel]

Update: A compression check shows the same for all four cylinders -- about 200 psi. Did not get a leakdown gauge yet.

But, a pressure and vacuum check on the mechanical fuel pump showed it would not hold pressure or vacuum. I removed from engine and tested again and could hear air sounds on the end where the camshaft drives the pump shaft. Removing the cover and shaft revealed a diaphragm that looked dry rotted and a floating fragment of rubber that had come loose.
So, for now I'm figuring the compromised pump was putting gas in the head and diluting the oil.
I have ordered a replacement pump, O-ring, thermostat and seal, new oil, oil filter and both new fuel filters.
Thanks,

[77tarheel]

It was a bad diaphram in the fuel pump. The mechanical lift pump runs off the camshaft. If the diaphragm goes bad, it can dump gas right into the crankcase. The thermostat also looked pretty ragged, so I replaced it, too. But that wouldn't explain the poor idling or the huge amount of gas showing up in the oil. The bad diaphragm does. It didn't have large holes in it, but looked dry rotted and a small chunk had peeled off. So it would send some gas to the injectors and dump a lot in the crankcase it appears. You can see where the pump goes through the valve cover and is driven by the camshaft. A perfect entry into the crankcase.
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I also replaced large fuel filter (checked gas from it, there was no water), small fuel filter and housing O-ring, oil filter and oil. Good prices at SIM Yamaha.fuelpump.png

[pstephens46]

Whoa!!!! You realize this forum is owned/operated by Boats.net? Competitor to the company you mentioned.

[77tarheel]

Sorry, no disrepect intended. I have purchased parts from boats.net in the past as well. I appreciate boats.net having this forum where I found help for my issue and will purchase parts from them again.