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Fuel pressure regulator vacuum check

iCasper

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97 SOHC Explorer II, 2WD
Truck runs lean. I am checking everything and put a vacuum pump on the fuel pressure regulator to check if it holds vacuum.
It goes from 15 hg to 9,5 hg in 60 seconds!

Is that normal?
There is no fuel in the vacuum hose...
 


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J_C

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It should hold vac without loss, but I would also wonder if the vac pump is losing vac instead and would check the fuel rail pressure to see if it stays within the required range, which for a '97 SOHC should be around 30PSI to 45PSI.

If the fuel pressure isn't dropping below that, you could need a new FPR but it might not be the primary problem yet.
 




iCasper

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97 SOHC Explorer II, 2WD
It should hold vac without loss, but I would also wonder if the vac pump is losing vac instead and would check the fuel rail pressure to see if it stays within the required range, which for a '97 SOHC should be around 30PSI to 45PSI.

If the fuel pressure isn't dropping below that, you could need a new FPR but it might not be the primary problem yet.

VAC pump? There is no VAC pump. Vacuum is provided by engine. A hose connects upper intake manifold directly to the FPR.

Have to buy a gauge to check fuel pressure.

I changed fuel pump two years ago with an Airtex part and later I read that Airtex pumps fail quite often.
 




koda2000

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VAC pump? There is no VAC pump. Vacuum is provided by engine. A hose connects upper intake manifold directly to the FPR.

Have to buy a gauge to check fuel pressure.

I changed fuel pump two years ago with an Airtex part and later I read that Airtex pumps fail quite often.

He means the vacuum pump you are using to test the FPR. You can borrow a fuel pressure tester from any auto parts store with a deposit. And yes, Airtex pump are garbage in my experience.
 




J_C

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Yeah I was wondering if the pump used to test was leaking. Seems like if the FPR was leaking that slowly, it should still be pretty near if not functional enough to run the engine, especially on a design with the fuel return where it's pumping more fuel than needed and only has to reach 30-ish PSI.

*I could be wrong* but the fuel pressure test should tell you, and I assume when you replaced the pump that you replaced the fuel filter too. If you didn't then I'd replace the filter before pulling the tank down IF your pressure is low.

Am I recalling correctly that the FPR that uses has gotten really hard to find? Thought I read about that a year or two ago in a topic here, or maybe it was just for my '98? Don't recall when the FRP changed from year to year, thought there were at least a couple years of Explorers affected. [Edit]Yeah the SOHC was only in '97 and '98 with the return fuel system so it looks like only those two years have the different FPR? [/Edit]

When I lookup FRP for your '97, or my '98 4.0L SOHC on Rock Auto, they only show the sending units in error, were only for '99 and newer which had the FPR on the sending unit.
 




koda2000

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OP - In a pinch, it is possible to test your fuel pressure using a dial-style tire pressure gauge. Of course doing this will only give you a snap-shot idea of the pressure and not the continuous reading an actual fuel pressure gauge will show you.
 




J_C

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^ Good point, I just looked up auto parts stores on the Canary Islands and didn't see any I'm familiar with, places I'd go in the US to get loaner tools... though I suppose they have phones there ;) and you could call around to check on loaner tool programs.
 








CDW6212R

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Truck runs lean. I am checking everything and put a vacuum pump on the fuel pressure regulator to check if it holds vacuum.
It goes from 15 hg to 9,5 hg in 60 seconds!

Is that normal?
There is no fuel in the vacuum hose...

Check the fuel pressure with a tester, there should be two pressures measured(with and without vacuum at the FPR). The pressure should be above 32psi normally, anything below that will make the engine run lean. With the vacuum line off of the FPR, the fuel pressure should be noticeably higher, more like 38-42psi. If the pressure doesn't go up or very very little, then the FPR is bad.
 




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