question about fuel pressure test
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Tuned
Joined: Feb 2003
Posts: 425
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question about fuel pressure test
I've got an '87 F250, 300 Six. This is the dual tank model with the low pressure fuel tank pumps and the high pressure on-the-frame pump. When I hooked up a fuel pressure gauge to check the pressure and did the key on, engine off test, fuel pressure came up and held after the key was turned off. But, when I started the engine, the pressure came up, engine ran, but, when I turned off the engine, the pressure dropped to 0 within a few seconds. I repeated both tests several times to make sure of what I was seeing.
Can anybody give me a hint of what is going on here? Why the near-instant drop after the engine is shut down but little, if any, drop after turning off the fuel pump when the engine is not running.
Thanks.
Can anybody give me a hint of what is going on here? Why the near-instant drop after the engine is shut down but little, if any, drop after turning off the fuel pump when the engine is not running.
Thanks.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
Not sure what the fuel pressure got to on the KOEO test but maybe the pressure did not get high enough to open the return ball valve in the FPR (Fuel Pressure Regulator) and when you run the engine the ball valve may be slow to close.
The three items that holds the pressure are FPR, chack valve in the high pressure pump and the fuel injectors (6).
The three items that holds the pressure are FPR, chack valve in the high pressure pump and the fuel injectors (6).
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Tuned
Joined: Feb 2003
Posts: 425
Likes: 4
The fuel pressure on the KOEO test was 50+ psi. This isn't quite as high as when the engine was running, but close. I don't think it's a fuel injector leak down because of how fast the leakdown occurs and the fact that I have no problem with a "rich" startup or dark plugs from flooding a cylinder. I'll dig further into the FPR and the hi pressure pump.
Thank you for your quick response.
Thank you for your quick response.
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