fuel pressure question
fuel pressure question Ok so I have seen the posts for testing the fuel pressure and that I should be at 40 to 43 # with vac line off. now I have to ask what should I see for fuel pressure with the vac line on and driving around.
I had to change a leaking fuel pressure regulator yesterday and I tested it with vac line off and I have 42# at idle. Thats all good. Now I want to make sure it's right with the vac line on and off. While driving around I had about 35 to 38. When I jumped on it it would pick up from 35 or so to almost 40. and after I let off it would drop to 33 to 36#. kind of dark so I could not see exact numbers. I am going to test it again as soon as the rain stops. so what should it be reading? This is a BWM replacement not a GM so I want to make sure it's right. I am a 383 with LE2 haeds and cam with ford 30# injectors. thanks
I had to change a leaking fuel pressure regulator yesterday and I tested it with vac line off and I have 42# at idle. Thats all good. Now I want to make sure it's right with the vac line on and off. While driving around I had about 35 to 38. When I jumped on it it would pick up from 35 or so to almost 40. and after I let off it would drop to 33 to 36#. kind of dark so I could not see exact numbers. I am going to test it again as soon as the rain stops. so what should it be reading? This is a BWM replacement not a GM so I want to make sure it's right. I am a 383 with LE2 haeds and cam with ford 30# injectors. thanks
Last edited by jaycenk; Mar 11, 2010 at 10:29 AM.
Ok so I have seen the posts for testing the fuel pressure and that I should be at 40 to 43 # with vac line off. now I have to ask what should I see for fuel pressure with the vac line on and driving around.
I had to change a leaking fuel pressure regulator yesterday and I tested it with vac line off and I have 42# while driving around. Thats all good. Now I want to make sure it's right with the vac line on. While driving around I had about 35 to 38. When I jumped on it it would pick up from 35 or so to almost 40. and after I let off it would drop to 33 to 36#. kind of dark so I could not see exact numbers. I am going to test it again as soon as the rain stops. so what should it be reading? This is a BWM replacement not a GM so I want to make sure it's right. I am a 383 with LE2 haeds and cam with ford 30# injectors. thanks
I had to change a leaking fuel pressure regulator yesterday and I tested it with vac line off and I have 42# while driving around. Thats all good. Now I want to make sure it's right with the vac line on. While driving around I had about 35 to 38. When I jumped on it it would pick up from 35 or so to almost 40. and after I let off it would drop to 33 to 36#. kind of dark so I could not see exact numbers. I am going to test it again as soon as the rain stops. so what should it be reading? This is a BWM replacement not a GM so I want to make sure it's right. I am a 383 with LE2 haeds and cam with ford 30# injectors. thanks
Your FP is too low while driving.
You mean it's to low with the vac line on? I still have to pull the vac line off and run it to see what it does. sorry I had to edit I miss spoke on driving around with vac line off. It was on but when I pulled it in the driveway it was 42 or 43. Soi am to low then? I need to go to a racetronix pump?
You mean it's to low with the vac line on? I still have to pull the vac line off and run it to see what it does. sorry I had to edit I miss spoke on driving around with vac line off. It was on but when I pulled it in the driveway it was 42 or 43. Soi am to low then? I need to go to a racetronix pump?
Just went out and checked and I think I understand what you are saying now. let me just verify this. At idel with vac line on I should be at 43.5 and not over 45, and when I take the vac line off and plug the line to the intake I should be at 47ish which is a simulaton of WOT. let me know if I am understanding this correct.
Thankyou so much Dave!!! I am going to go ahead and leave it parked and order a new pump from racetronixs or something. Any other suggestions on pumps? I am guessing the old 98k mile stocker is not supplying enough because even with the old regulator on it, it was about the same 38# vac line on to 42# with vac line pulled. pump is the only thing left.
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Thankyou so much Dave!!! I am going to go ahead and leave it parked and order a new pump from racetronixs or something. Any other suggestions on pumps? I am guessing the old 98k mile stocker is not supplying enough because even with the old regulator on it, it was about the same 38# vac line on to 42# with vac line pulled. pump is the only thing left.
BTW - I am running a pretty similar setup to you and mine holds steady even at WOT at 43.5 on the stock pump. Granted mine only has 45K miles, but should be similar.
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lol yea it ended up being easy when I took the rail off but I have to try it the hard way first and do it with the rail still in the car. lol I'm kinda at a loss as to should I do the pump and hope thats it or is it the regulator?
Just pick up a stock replacement at your local AP store and swap it out to see if that is the issue. If not then return it and proceed with the fuel pump. Always easier to do the reg instead of cutting the trap door or droping the tank IMO.
Some inaccurate info in this thread...
GM spec for fuel pressure with key on, engine off, pressure should be between 41 and 47psi (43.5psi is ideal). Once the engine is running, pressure should DROP 3-10psi and slowly increase with throttle position.
When the engine is running, vacuum is applied to the fuel pressure regulator, which offsets the spring pressure in the regulator, resulting in lower fuel pressure. Fuel pressure at idle will vary somewhat depending on barometric pressure but should ALWAYS be less than KOEO.
OP, make sure the vacuum line for the regulator isn't plugged. In your situation though, it is most likely the fuel pump. Not all fuel pumps die instantly.
GM spec for fuel pressure with key on, engine off, pressure should be between 41 and 47psi (43.5psi is ideal). Once the engine is running, pressure should DROP 3-10psi and slowly increase with throttle position.
When the engine is running, vacuum is applied to the fuel pressure regulator, which offsets the spring pressure in the regulator, resulting in lower fuel pressure. Fuel pressure at idle will vary somewhat depending on barometric pressure but should ALWAYS be less than KOEO.
OP, make sure the vacuum line for the regulator isn't plugged. In your situation though, it is most likely the fuel pump. Not all fuel pumps die instantly.
Last edited by RamAir95TA; Mar 11, 2010 at 11:45 AM.
Some inaccurate info in this thread...
GM spec for fuel pressure with key on, engine off, pressure should be between 41 and 47psi (43.5psi is ideal). Once the engine is running, pressure should DROP 3-10psi and slowly increase with throttle position.
When the engine is running, vacuum is applied to the fuel pressure regulator, which offsets the spring pressure in the regulator, resulting in lower fuel pressure. Fuel pressure at idle will vary somewhat depending on barometric pressure but should ALWAYS be less than KOEO.
OP, make sure the vacuum line for the regulator isn't plugged. In your situation though, it is most likely the fuel pump. Not all fuel pumps die instantly.
GM spec for fuel pressure with key on, engine off, pressure should be between 41 and 47psi (43.5psi is ideal). Once the engine is running, pressure should DROP 3-10psi and slowly increase with throttle position.
When the engine is running, vacuum is applied to the fuel pressure regulator, which offsets the spring pressure in the regulator, resulting in lower fuel pressure. Fuel pressure at idle will vary somewhat depending on barometric pressure but should ALWAYS be less than KOEO.
OP, make sure the vacuum line for the regulator isn't plugged. In your situation though, it is most likely the fuel pump. Not all fuel pumps die instantly.
Some inaccurate info in this thread...
GM spec for fuel pressure with key on, engine off, pressure should be between 41 and 47psi (43.5psi is ideal). Once the engine is running, pressure should DROP 3-10psi and slowly increase with throttle position.
When the engine is running, vacuum is applied to the fuel pressure regulator, which offsets the spring pressure in the regulator, resulting in lower fuel pressure. Fuel pressure at idle will vary somewhat depending on barometric pressure but should ALWAYS be less than KOEO.
OP, make sure the vacuum line for the regulator isn't plugged. In your situation though, it is most likely the fuel pump. Not all fuel pumps die instantly.
GM spec for fuel pressure with key on, engine off, pressure should be between 41 and 47psi (43.5psi is ideal). Once the engine is running, pressure should DROP 3-10psi and slowly increase with throttle position.
When the engine is running, vacuum is applied to the fuel pressure regulator, which offsets the spring pressure in the regulator, resulting in lower fuel pressure. Fuel pressure at idle will vary somewhat depending on barometric pressure but should ALWAYS be less than KOEO.
OP, make sure the vacuum line for the regulator isn't plugged. In your situation though, it is most likely the fuel pump. Not all fuel pumps die instantly.
I never said fuel pressure should be below 40+ at WOT (and I certainly don't deny that). Where did I say that? 
I'm just trying to convey the correct info, that's all.
At hot start idle, between 31 and 44psi is fine. Pressure should smoothly increase from there at part throttle position.
At WOT the pressure should be between 41 and 47psi (not higher, not lower). Over 47psi for a prolonged period of time can set DTC 45/65.
OP, from the info in your very last post, your fuel pressure is fine.
I'm just trying to convey the correct info, that's all.At hot start idle, between 31 and 44psi is fine. Pressure should smoothly increase from there at part throttle position.
At WOT the pressure should be between 41 and 47psi (not higher, not lower). Over 47psi for a prolonged period of time can set DTC 45/65.
OP, from the info in your very last post, your fuel pressure is fine.