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1998 yamaha 250 ox66 fuel pressure issue

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1998 yamaha 250 ox66 fuel pressure issue

Old 05-12-2019, 06:47 PM
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Default 1998 yamaha 250 ox66 fuel pressure issue

I tested the fuel pressure on my engine and its a little high, 39psi. I checked and cleaned the vst filter, and the fuel pressure regulator and retested, still 39psi. Next i tested per manual with a vacuum pump attached to the fuel pressure regulator. I tried to apply 7 hg vacuum, but could barely hit 5 while pumping continuously, it would not hold vacuum at all, went back to 0 as soon as i stopped pumping. The fuel pressure did drop a little momentarily, but went back to 39 quickly.
The high pressure fuel pump was replaced about 1 1/2 yrs ago with a non oem version. Does this sound like i have a bad pressure regulator?
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Old 05-13-2019, 05:28 AM
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Yes, The regulator should hold a vacuum.
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Old 05-13-2019, 05:46 AM
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Were you having a particular problem or just general maintenance?
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Old 05-13-2019, 09:41 AM
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A little of both, during regular maintenance I noticed a couple of issues, the fuel pressure for one and the O2 sensor sniffer tube was clogged pretty bad, almost completely, and there was black oily fluid in the chamber. It was replaced about a year and a half ago. At that time the whole engine was gone thru, sync, spark, TPS, etc. All fuel filters replaced, low pressure fuel pumps replaced. The odd thing is that the #1 spark plug looks fine, which I would expect to be fouled since it is that same cylinder that the O2 sensor is connected to. I checked compression and it is good on all cylinders.
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Old 05-19-2019, 04:59 AM
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I tested the fuel pressure on the water yesterday and it dropped from about 39 psi at idle to around 36-37 psi at 4000 rpm. Im going to check it again after i put some hrs on and check it again. Still not sure why it doesnt change when i put a vacuum on it.
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Old 05-19-2019, 05:03 AM
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Unplug vacuum while gauge hooked up, itll change
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Old 05-19-2019, 10:42 AM
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That sounds like a good idea, but i think ill get similar results as i did with the vacuum pump attached, would you agree?
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Old 05-19-2019, 10:57 AM
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The regulator should hold vacuum, it’s bad if it doesn’t.
Vacuum applied , the pressure should go down. Removed , up.
If I'm remembering right.
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Old 05-19-2019, 04:26 PM
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I'll test it again this week and post my findings, thank you.
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Old 05-20-2019, 02:36 PM
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If it doesn’t hold vacuum , replace it.
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Old 05-22-2019, 05:44 PM
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I checked the fuel pressure again today, it was 38 psi, i then removed the vacuum hose it maybe dropped 1 psi to 37.
i also was wondering what the right top end reading should be fot the tps, throttle position sensor. Its set for .51v with throttlle closed, and goes up to 4.6v with the throttle fully opened. When i reconnect the linkage the full throttle voltage is abour 3.65v. Ive read a post that said this was normal. The red-orn voltage is 5.0v.
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Old 05-22-2019, 06:23 PM
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The procedure is to install a fuel pressure gauge to the fuel system and a vacuum pump to the regulator.

Start the motor. The fuel pressure will be X psi. Reduce the pressure to the pressure regulator. The fuel pressure should lower in concert with the regulator pressure being lowered. That is, if the pressure to the regulator is lowered by 4 psi the fuel pressure output should be lowered by 4 psi.

Both the fuel pressure and the regulator pressure should hold constant.
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Old 05-23-2019, 04:54 PM
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Alloyboy- so i guess mine is not working right, i did attach both and tried to apply vacuum, but the vacuum woul not hold, as soon as i stopped pumping the vacuum pump the vacuum went back to 0. When testing the fuel pressure under load up to about 4000 rpms it stayed pretty steady around 38. I might think it was ok to run it like that but im not sure how critical the fuel pressure spec is. Its running a little high.
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Old 05-23-2019, 06:30 PM
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My guess would be that the diaphragm in the regulator has a hole in it.

If the pressure in the regulator is higher than it is supposed to be it will deliver excess gasoline to the motor. Better that than too little.

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Old 05-25-2019, 11:41 AM
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Originally Posted by plumbob101 View Post
Alloyboy- so i guess mine is not working right, i did attach both and tried to apply vacuum, but the vacuum woul not hold, as soon as i stopped pumping the vacuum pump the vacuum went back to 0. When testing the fuel pressure under load up to about 4000 rpms it stayed pretty steady around 38. I might think it was ok to run it like that but im not sure how critical the fuel pressure spec is. Its running a little high.
Replace it. It’s torn. As alloyboy stated , it will probably just overfuel the cylinders a bit , won’t hurt the engine but may idle rich and run rougher than normal at idle and low load situations.
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Old 05-25-2019, 05:29 PM
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Ohh that may be why my o2 sensor was fouled when i checked it this spring, only had about 25 hrs on it.
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