fuel pressure stays the same

[dan2288]

I installed a fuel lab fpr a couple days ago. I set it at 43.5 psi with the vac line removed. Today I was looking at it and its at 43 psi with the line on. I then took the vac line off and the pressure did not change at all. So basically the pressure stays the same with the vac line on or off. How should I adjust it then?

 

[thatblondechik]

Don't worry about it not changing much. That vac line is connected to the throttle body and changes with the throttle opens. So it won't change much (if any) at idle.

 

[talonDSMerr]

It definitely should change taking the vacuum hose on or off at idle. Check if there are holes or leaks in that line, make sure it is sealed. Pressure should be lower at idle because of the vacuum. If it doesn't change with vacuum, it won't change with boost and you'll end up running incredibly lean under boost.

 

[daren_p]

Don't worry about it not changing much. That vac line is connected to the throttle body and changes with the throttle opens. So it won't change much (if any) at idle.

Incorrect, as Dennis posted below this is defently something to worry about.

What kind of vacuum do you see at idle? May be a dumb question but are you sure you hooked up the correct vacuum line to the fpr? (& can you install a gauge to make sure the line is seeing proper vacuum?) Is the pressure port installed correctly on the FPR (by any chance did it get blocked with teflon tape, or sealent, etc?). Do you have any method to apply pressure to the pressure port of the FPR to see if fuel pressure increases with pressure?

 

[thatblondechik]

It definitely should change taking the vacuum hose on or off at idle.

I don't think so. Unless you want to count the vibrating of the needle. My gauge makes no noticeable change at idle with the hose connected or not. But there's an OBVIOUS change when you open the throttle.

One sure way to check. Connect the hose, then open the throttle with your hand, watch the gauge. If you see no change, somethings messed up. Follow that hose back to the TB and make sure it's leak-free and connected.

 

[snowborder714]

Christi, what happens to yours if you open the throttle but don't build any boost?

As for fuel pressure at idle with the vacuum line connected, your fuel pressure will drop 1 psi for every 2 inHg of vacuum. For example, if your idle vacuum is 20 inHg, your fuel pressure will drop 10 psi to 33.5 psi once you re-attach the vacuum line.

 

[dan2288]

Okk, well when im in drive my vacuum is about 13-14 on my guage. When im in park its about 10. It seems alittle low for a stock motor, right?

 

[thatblondechik]

Christi, what happens to yours if you open the throttle but don't build any boost?

As for fuel pressure at idle with the vacuum line connected, your fuel pressure will drop 1 psi for every 2 inHg of vacuum. For example, if your idle vacuum is 20 inHg, your fuel pressure will drop 10 psi to 33.5 psi once you re-attach the vacuum line.

Well now you have ME scared! It's always jumped up 5-10 psi then dropped back to normal. And it's never changed when I connect the vac. And I was told by what I thought was a reliable source that this was normal.

But from what you're saying, I have no vacuum at idle, but I read vacuum on my boost gauge...It's not a huge concern as the car is currently parked anyways but I'm obviously very confused right now.

 

[snowborder714]

You said you're pulling your vacuum source from a throttle body nipple Christi? Try connecting it to the original spot - a nipple on the intake manifold - and see if it changes. I don't think I've heard of someone sourcing their vacuum from the throttle body.

 

[dan2288]

well I checked again and there was alittle difference this time. With the vac line off, I set it to 43.5. When I put the line back one, It was like 38-39. Is that still too much? I dont want to be running too rich.

 

[snowborder714]

What was your vacuum at idle? That's sounding much better.

 

[Calan]

Don't worry about it not changing much. That vac line is connected to the throttle body and changes with the throttle opens. So it won't change much (if any) at idle.

uhhh... what?

Not sure if this applies, but if that AFPR is like some of the Areomotive's, you may need to replace the spring with one rated for a different pressure. Mine actually came with two springs (which I had to swap), but I've heard of some arriving with the correct spring not included.

But like I said, it may not apply to the FuelLab you have.

 

[dan2288]

In park and just idleing, the vacuum was 14-15.

 

[ishnish]

Finally found a thread where someone was having the same issue as me so I'm reviving it. I have this fact same problem. Have base fuel pressure set to 43 psi but when I connect the vacuum line (at idle), the fuel pressure does not change. No point in starting a tuning process when little things like this make a big difference.

I have a vacuum line from my evo 3 intake manifold going to the boost gauge with reads -18inHg at idle. My fuel modifications are a walbro 450lph fuel pump, fuel lab 515 afpr, FIC 2150cc injectors, and a new, stock OEM fuel filter. The lines fuel rail are all stock other than the line off the return of the fuel rail which is a -6 AN going into the afpr.

My first check will be to see if there's a cut or something on the vacuum line that is connected to the afpr which goes to a T connecting to the vacuum line that goes from the intake to the boost gauge.

What other things could I check to diagnose this properly?

 

[1990TSIAWDTALON]

On a 1-1 rising AFPR, the vacuum on your motor at idle SHOULD pull the pressure down by whatever vacuum is at. Say 43 lbs on the gauge then with 18hg of vacuum it should pull the pressure down, I don't know if it ever really pulls it down 18 lbs (which would show 25 on the gauge) but it should pull it down like it raises it when you are under boost, say 15 lbs of boost plus the 43 lbs of fuel should be close to 58 lbs.
My car has about 13hg of vacuum and when I have the vacuum line hooked up the pressure goes down to 35-38ish for a comparison.

 

[ishnish]

That's the problem Marty, after doing so much reading on the subject, I've found that after converting inHg to psi, 18inHg = 8.84psi or basically, 2inHg is roughly = 1psi. So by that, my idle at -18inHg of vacuum should be pulling almost 9psi from my base fuel pressure of 43psi, so I should be getting from 34-35psi at idle. But as I've said, no change whatsoever..

 

[Vegas Smith]

Then your afpr is broken.

 

[ishnish]

It's a brand new unit..how can it be broken? Is there a way to make sure?

When I use the adjuster screw, fuel pressure goes up or down depending on how much I screw or unscrew. I would think my afpr is good and not broken. Fuel pressure goes down gradually once j turn the car off to 30 psi and holds there pretty well for quite some time which the gauge represents. I just really feel like my afpr is not broken..

 

[1990TSIAWDTALON]

Add 20 lbs of compressed air to the nipple and see if the pressure rises 20 lbs Ishnish. Don't add 120 lbs, like straight out of an air compressor, or you WILL have a broken afpr.

 

[ishnish]

I don't have an air compressor Marty. But can I rev the engine and see if the fuel pressure rises on revs?

 

[1990TSIAWDTALON]

It won't go into boost. The AFPR needs to see pressure in order to rise.

 

[Tyeler18]

I don't have an air compressor Marty. But can I rev the engine and see if the fuel pressure rises on revs?

Do you have vacuum at the AFPR if you pull the hose off? Strong vacuum that is? If not you've found your problem. Without a compressor you can put a hose on the AFPR and suck it down with your mouth and watch pressure drop as well.

 

[ishnish]

I will try that and see if the pressure goes down. I'm at work so I'll have to do this later tonight when I get home to tell. I REALLY hope the problem is all just within the vacuum line. If not, I really don't know what else to check into. I've even searched other forums of different platforms just to see what else could cause this.

Again, I am pretty sure it's not the afpr. From all the other functions of it, I really think the problem lies somewhere else.

 

[Tyeler18]

They're a pretty simple design, if it has vacuum and doesn't drop pressure, but you can drop pressure by adjusting the bypass (means you're not overrunning it), then it's bad.

 

[ishnish]

But if I take off the line and just suck through the hose with my mouth and see a pressure drop, wouldn't that mean it's the line that's bad, not the afpr?