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How many psi should the fuel pump give?


Axelshei

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99 Ford Explorer 4.0 SOHC
My 1999 Explorer runs rough at idle when could. The mechanic that looked at the problem thought the fuel pump was the problem. He tested the pump and it only gave around 13-15 psi when running at idle, and he ment it was supposed to give around 30psi. He changed the pump, and it did not help at all.
Does anyone know if the pump maybe is supposed to give just 15 psi at idle? And maybe increase the pressure when higher rpm?

PS: It is the SOHC 4.0 engine.

I hope someone can help me :)
 


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1998Exp

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Looks like your mechanic is a bit off with his numbers. If he tested the pressure at the Schrader valve (the usual test point), that pressure is regulated AGAINST the manifold negative pressure. In other words, you have to add the manifold vacuum reading to the measured pressure. At idle you have around 20 in Hg (or around 9psi) of vacuum in your manifold, for a total of around 24. This is still low vs. the normal 35, but not the same as 15. You can confirm by rapidly opening the throttle, which drops the manifold vacuum to almost zero. You may have a bad regulator, or possibly a clogged fuel filter causing the insufficient pressure. His gauge may also be inaccurate.. However, if the problem goes away when the engine warms up, the most likely culprit of poor idling is the classic 4.0 SOHC problem with leaky intake manifold gaskets. Is the Check Engine light on? If so, has your mechanic read the trouble codes?


My 1999 Explorer runs rough at idle when could. The mechanic that looked at the problem thought the fuel pump was the problem. He tested the pump and it only gave around 13-15 psi when running at idle, and he ment it was supposed to give around 30psi. He changed the pump, and it did not help at all.
Does anyone know if the pump maybe is supposed to give just 15 psi at idle? And maybe increase the pressure when higher rpm?

PS: It is the SOHC 4.0 engine.

I hope someone can help me :)
 
Last edited:




J_Train

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Yup what he said we see them here at the dealership a lot.
 




Axelshei

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Looks like your mechanic is a bit off with his numbers. If he tested the pressure at the Schrader valve (the usual test point), that pressure is regulated AGAINST the manifold negative pressure. In other words, you have to add the manifold vacuum reading to the measured pressure. At idle you have around 20 in Hg (or around 9psi) of vacuum in your manifold, for a total of around 24. This is still low vs. the normal 35, but not the same as 15. You can confirm by rapidly opening the throttle, which drops the manifold vacuum to almost zero. You may have a bad regulator, or possibly a clogged fuel filter causing the insufficient pressure. His gauge may also be inaccurate.. However, if the problem goes away when the engine warms up, the most likely culprit of poor idling is the classic 4.0 SOHC problem with leaky intake manifold gaskets. Is the Check Engine light on? If so, has your mechanic read the trouble codes?
Thank you for your reply! Yes, the Check Engine light in on all the time, and I have checked the oil level. I actually don't know if the mechanic has read the trouble codes. But it sounds like I have to talk to him tomorrow.
Do you think the IAC (idle air controller) can be the problem?
 




1998Exp

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Any one of a about 100 different things can be the problem. That's why these computer-controlled cars have trouble codes. These codes, in addition to some knowledge and other people's experience lead you to the most probable diagnosis. A basic scanner is so cheap (at least here in the US) that it definitely makes no sense, even for the do-it-yourself mechanic to do anything before reading the codes. Your 4.0 SOHC is known to have failing intake manifold gaskets. If you get 'lean' codes, that's most likely because of vacuum leaks there. Good Luck!

Thank you for your reply! Yes, the Check Engine light in on all the time, and I have checked the oil level. I actually don't know if the mechanic has read the trouble codes. But it sounds like I have to talk to him tomorrow.
Do you think the IAC (idle air controller) can be the problem?
 




Axelshei

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Any one of a about 100 different things can be the problem. That's why these computer-controlled cars have trouble codes. These codes, in addition to some knowledge and other people's experience lead you to the most probable diagnosis. A basic scanner is so cheap (at least here in the US) that it definitely makes no sense, even for the do-it-yourself mechanic to do anything before reading the codes. Your 4.0 SOHC is known to have failing intake manifold gaskets. If you get 'lean' codes, that's most likely because of vacuum leaks there. Good Luck!
I have been to the workshop today and read the trouble codes. The trouble code was P0174. I do not know exactly what it means, but I think it has something to do with vacuum or air control.
Any ideas? :)
 




koda2000

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I have been to the workshop today and read the trouble codes. The trouble code was P0174. I do not know exactly what it means, but I think it has something to do with vacuum or air control.
Any ideas? :)
if your mechanic read the codes and didn't tell you what they indicated, you need to find another mechanic. P0174 is a "lean condition" code. on the 4.0 SOHC motor this is usually caused by leaking intake manifold gaskets/o-rings or other vacuum leaks. a qualified mechanic wouldn't need you to tell him how to repair your vehicle.
 




Axelshei

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if your mechanic read the codes and didn't tell you what they indicated, you need to find another mechanic. P0174 is a "lean condition" code. on the 4.0 SOHC motor this is usually caused by leaking intake manifold gaskets/o-rings or other vacuum leaks. a qualified mechanic wouldn't need you to tell him how to repair your vehicle.
You have a good point there. However, it definitely looks to me like it is the Intake manifold gaskets which is causing the problem. The bad idling is almost gone when the engine is warm, and the idling is totally useless when cold.
 




koda2000

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You have a good point there. However, it definitely looks to me like it is the Intake manifold gaskets which is causing the problem. The bad idling is almost gone when the engine is warm, and the idling is totally useless when cold.
i agree that it's probably the intake o-ring/gaskets. once the engine gets hot they tend to swell a bit and that lessens or stops the air leak.
 




deweyville65

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try spraying starting fluid around the suspected bad gasket area. If there is a leak, the engine will smooth out for a few seconds. Did this on my daughters 05 Taurus, found the leak in just a few seconds. End of guesswork
 




Axelshei

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try spraying starting fluid around the suspected bad gasket area. If there is a leak, the engine will smooth out for a few seconds. Did this on my daughters 05 Taurus, found the leak in just a few seconds. End of guesswork
I tried this, and guess what - the engined smoothed out just like you said :) So the intake manifold gasket was the problem.

Thank you all for great help :)
 




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