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Yamaha YZ400F (1999)


Owner: Thumpacular
Added on January 18, 2017
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10' Yz450F Fuel in the oil?



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64 replies to this topic
  • Chazmagnum

    TT Bronze Member

110 posts
Location: Washington
Garage View Garage

Posted January 19, 2011 - 07:03 PM


Has anyone had this problem? I look for a sticky but could not find one. Is it just the cold weather? What is the cause and or cure? Thank you.:smirk:

  • brentn

    TT Addict

4,015 posts
Location: Alberta

Posted January 19, 2011 - 07:17 PM


Cause and cure are still sorta unknown, everyone has their theories.

If you search you can find alot of information.

To me, it varies, and sometimes it smells like gas, other times it smells like burned fuel... I've had my own thoughts to improper break in (ring seal is not as good as it could be), to a leaky injector... Haven't found any part #'s to compare 2010/2011 injectors to see if yamaha made an update.
I'm on the verge of pulling it and priming the pump with a 12V source and checking for leaks while using a pressure gauge on the fuel line...

  • grayracer513
43,170 posts
Location: California
Garage View Garage

Posted January 19, 2011 - 08:54 PM


From everything I've seen, and from the work TT'r tech24 has done on his own bike, the trouble appears to be that the lack of a thermostat in the current weatherkeeps the coolant temp very low (under 100 ℉, by one account), and since the EFI corrects for cold temps, the mixture is kept rich, contaminating the oil. Try taping over one of your radiators, and see what sort of difference it makes.

Read:

http://www.thumperta...ad.php?t=946026

  • MXR176

    TT Silver Member

522 posts
Location: Iowa

Posted January 20, 2011 - 04:16 AM


My buddy had an injector go bad on his Honda 250f and it was dumping gas in the oil. Put a new injector in it and it solved the problem.

  • dgcars

    TT Titanium Member

2,459 posts
Location: Northern Ireland

Posted January 20, 2011 - 04:39 AM


Try taping over one of your radiators, and see what sort of difference it makes.....


A guy from Italy posted the same problem with his bike not so long ago. After a lot of testing injectors and diagnostic analysis....he ended up doing whay gray suggests. Worked for him.

  • Chazmagnum

    TT Bronze Member

110 posts
Location: Washington
Garage View Garage

Posted January 20, 2011 - 06:37 PM


So we dont know if the injector is continueing to squirt fuel after shut off? It seems to be a problem with everyone I know who has this bike. So by taping off one of the radiators, you suggest that there is an excess of fuel during normal operation. Is it common for a 4 stroke to have unburnt fuel in the cylinder after combustion. The 12v trouble shoot sounds interesting also. Im not sure I know how to do that. But if you try it I would be interested to know the results. Thank you.

  • brentn

    TT Addict

4,015 posts
Location: Alberta

Posted January 20, 2011 - 07:32 PM


The problem I have with the theory of the bike not reaching operating temp is that quite a few owners are reaching operating temps and still have their oil smelling of fuel/exhaust.
In the summer it was scorching here when I was riding mine, and oil was going black and stinking of gas/exhaust in the first hour of run time every change. Bike was def in the operating temp, must have been at those outside temps.
However, I never had a temp gauge on it so maybe the bike is overcooling, maybe the rads are just too good for the new style of engine?
Cool thing with my new trail tech vapor is that it will log top speed, top rpm, and top temp. So in the summer I can reset it, go on a good trail ride without stopping and check the top temp log and just see if we're in the magical 80-90 degree temp range that the bike was designed to run at.
If we are, and the fuel still stinks after a good hour ride, then that idea is debunked.

  • thestuz

    TT Gold Member

1,427 posts

Posted January 21, 2011 - 02:14 PM


My mates oil goes black quickly and always stinks of fuel.

  • tech24

    TT Platinum Member

1,631 posts
Location: Maryland

Posted January 22, 2011 - 05:04 AM


I serioulsy doubt that the injector is spraying or leaking any fuel after shut off. Injectors can hold pressure indefinently and the only way fuel with come out is it it is pulsed by the ecu which will not happen unless it recieves a crank signal indicating the bike is running or trying to be started.

I also think some are being over sensitve to the issue since they have heard somewhere that this may be a problem. A little fuel smell in the oil is not going to be an abnormal occurance. There is always some fuel vapor that is going to enter the crankcase no matter what and its the engine oils job to hold it in suspension. If the engine isn't able to maintain normal operating temp fuel vapors and condensation may not be released and strenghen the "fuel" smell. With cold coolant temps the efi is dumping so much extra fuel which in turn can only make this condition worse.

Test: Smell a carbed bikes oil after a good ride then smell an efi bike after a good ride....I bet they smell the same :smirk:

  • thestuz

    TT Gold Member

1,427 posts

Posted January 22, 2011 - 11:14 PM


I serioulsy doubt that the injector is spraying or leaking any fuel after shut off. Injectors can hold pressure indefinently and the only way fuel with come out is it it is pulsed by the ecu which will not happen unless it recieves a crank signal indicating the bike is running or trying to be started.

I also think some are being over sensitve to the issue since they have heard somewhere that this may be a problem. A little fuel smell in the oil is not going to be an abnormal occurance. There is always some fuel vapor that is going to enter the crankcase no matter what and its the engine oils job to hold it in suspension. If the engine isn't able to maintain normal operating temp fuel vapors and condensation may not be released and strenghen the "fuel" smell. With cold coolant temps the efi is dumping so much extra fuel which in turn can only make this condition worse.

Test: Smell a carbed bikes oil after a good ride then smell an efi bike after a good ride....I bet they smell the same :smirk:



i beg to differ. carbed bikes oil smells like oil.
10 efi oil smells like im sniffing the gas tank.
you obviously have not got a friend with a yzf10 efi fuel problem?

  • gscx

    TT Platinum Member

1,876 posts
Location: Delaware

Posted January 23, 2011 - 12:59 AM


LOL. He has a 2010 with a fuel problem. But not anymore

  • tech24

    TT Platinum Member

1,631 posts
Location: Maryland

Posted January 23, 2011 - 10:04 AM


Can't say I do. I only ever got a hint of gas smell in mine. With thermostat installed no smell at all. My '04 carbed bike smells about the same, a hint of gas smell from time to time. I just wonder when everyone started smelling their oil or if this has been common pratice that I've never been aware of?

  • tech24

    TT Platinum Member

1,631 posts
Location: Maryland

Posted January 23, 2011 - 10:05 AM


LOL. He has a 2010 with a fuel problem. But not anymore


Oops just seen this post, yeah no problem here and I'm not sure mine was ever severe as others, mine never smelled like a fuel tank so I don't know??????

  • brentn

    TT Addict

4,015 posts
Location: Alberta

Posted January 23, 2011 - 03:52 PM


Injectors can and do leak, this is why we have injector tests. Quality control can fail on an injector, it happens.

I've seen videos of brand new bosch inkjectors leaking for cars... so...Just saying I think these bikes deserve to be at least checked out.
If I get around to it, or find out exactly how to prime the fuel pump I will try it on my own bike and report back the results.

  • gscx

    TT Platinum Member

1,876 posts
Location: Delaware

Posted January 23, 2011 - 09:24 PM


So is that the definite cause though? No thermostat? I would think that someone would have figured it out before this?

  • thestuz

    TT Gold Member

1,427 posts

Posted January 24, 2011 - 12:15 AM


the only thing im sure of is that

1, only hondas and yam 450s have the problem

2, nobody knows the solution to fix it. from leaky injectors, thermostats, to mapping, ive heard it all. but not all solutions work for all people.

3, sniffing your oil for fuel started in 09 when honda released its crf450.

  • Darvo

    TT Platinum Member

1,805 posts
Location: Idaho
Garage View Garage

Posted January 24, 2011 - 07:18 AM


the only thing im sure of is that

1, only hondas and yam 450s have the problem

2, nobody knows the solution to fix it. from leaky injectors, thermostats, to mapping, ive heard it all. but not all solutions work for all people.

3, sniffing your oil for fuel started in 09 when honda released its crf450.


I'm having the same thing happen on my RMZ. Pretty sure its being caused by cold operating temps, out of the 7 snowbikes that I ride with, only the 2 injected bikes are having this problem.

  • dgcars

    TT Titanium Member

2,459 posts
Location: Northern Ireland

Posted January 24, 2011 - 07:56 AM


the only thing im sure of is that

1, only hondas and yam 450s have the problem

2, nobody knows the solution to fix it. from leaky injectors, thermostats, to mapping, ive heard it all. but not all solutions work for all people.

3, sniffing your oil for fuel started in 09 when honda released its crf450.


The big hype from all the manufacturers was efi would the be all and end all. Kinda puts you off....:thumbsup:

  • brentn

    TT Addict

4,015 posts
Location: Alberta

Posted January 24, 2011 - 11:53 AM


I want to know how bad this really is.
An oil analysis after a one hour ride and the oil stinking of gas would be excellent information for everyone on this forum. IMO, the gas is breaking down the oil bigtime, and 1-2 hour changes are actually needed cause the lubricating properties are gone!

  • grayracer513
43,170 posts
Location: California
Garage View Garage

Posted January 24, 2011 - 12:20 PM


Do one, then:

http://www.blackston...e-test-kits.php





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