How to check fuel pressure under boost????

[a1320eclipse]

OK, so ive been fighting a knock problem after about 5k to redline. my base fuel pressure is at 40lbs, tried from 38-45 and still the same effect. replaced the filter, tried adjusting the afc, and when i pull fuel it gets worse, but when i have it set to about +15% at about 5k, its the best it gets, and at 18psi of boost im getting about 12-15 counts of knock. i have a rewired walbro 255 in the tank and it was originaly out of a honda. so i just kinda made it work. so could i be losing pressure at the pump itself, is there a way to check for rising fuel pressure under boost. All i have is a inline fuel pressure gauge between the filter and rail. and a b&m fpr, which ive been told is a 1.1 rising regulator. Im on th stock injectors. should i be putting +15% fuel into it at 18psi. i think i remember reading it should only be like around +5-7% on the 450's. Also my injectors are commonly at 92-96% idc after 5500rpm.

 

[95-talontsi]

get some fittings ands run that gauge into your car would be the only way to tell. If you are talking about the b&m command flo fpr all that lets you do is raise full pressure and not lower it.

 

[dedlewamp]

Why not pressurize the line leading to the regulator, and watch to see if it rises at a 1:1 ratio?

 

[oldman]

1. First you need to get rid of the B&M and replace it with a real AFPR, the B&M unit is not a fuel pressure regulator, it's a fuel pressure modifier (to the stock unit).

2. To test the AFPR, power up the fuel pump via the check connector on the firewall, remove the intake manifold vacuum hose from the T (where it splits into afpr and boost gauge) and replace it with a hose leading to an air compressor.

3. Apply pressure in 5 psi increments (according to boost gauge). This is when you're suppose to realize the boost gauge is dropping slowly when it should be holding. Applying soapy water at this time will reveal that both the adjustment screw and vacuum fitting on the AFPR is leaking air, this is what is missing from most AFPR installation instructions.

4. Apply Tafflon tape on the vacuum fitting and adjustment screw (note to use liberal amount on the adjustment screw, the threads are huge and require a lot of tape to seal).

5. Set the base pressure (1g=37.5psi, 2g=43.5psi) and repeat #3, continue to test for leaks and redo the adjustment screw if need be (it took me 3 times to get enough tafflon on there to seal at 30psi). Compare the boost gauge with the fuel gauge at each increment and make sure 1:1 ratio is maintained along the way to at least your current boost level.

I strongly urge anyone who did not do this during their AFPR install to go back and follow this step by step. From the results of my test, a leaking adjustment screw will drop the fuel pressure by 3-4 psi at 30 psi of intake pressure, I'm sure it has the same effects in vacuum. Not be mention how close the afpr is to your gauge.

 

[slipstream808]

Just as a heads up, if you have the money and wanted them, if you were to get the Defi Link gagues and have the fuel gauge and boost gauge, you can monitor the pressure differential between them. There is a button on the control unit that allows for this. Personally I don't have the fuel pressure gauge yet, but soon I will.

 

[Auto RS T]

Take Oldmans advice and replace the "modifier" with a real 1:1 regulator and teflon the adjustment screw and fitting, his original post on this topic was very informative. Your IDCs are pretty high up top, it may be time for some new injectors but fix the current problem first. Then see where you stand.

 

[a1320eclipse]

So i pressurized the line goin to the fpr and watched the gauge and it only went to 45 and stopped solid, so i put on a stock fpr and got the same results. If i do this right shouldnt the gauge be able to go to at least 58 (40psi base pressure + 18psi of boost)?

 

[donmagicjuan]

If you're running a 255 lph pump with a stock regulator body, I wouldn't trust any of the pressures you're reading at idle. That regulator is being overrun by the pump at idle (or with the car off), which means you can't accurately set your base fuel pressure. I would do exactly as Bruce suggested and get a real AFPR.

 

[siceclipse]

yeah just to say it again
you need an adjustable fuel pressure regulator

i have the 255 and my stock FPR was being overrun and killing my gas as well as killing my o2 sensors.

get an adjustable one and i think, scratch that, WE KNOW your problem will be done with.

 

[Black95TSIawd]

I had a very similar problem with going lean no matter what psi I ran. Turned out to be a bad fuel pump.

 

[a1320eclipse]

IM pretty sure i figured it out. The outlet wasnt sealing on the end of the fuel pump so at a certain psi, the fuel was being pushed back out the rubber O-ring on the pump. So to solve the problem, i got a 1.75" peice of fuel line and attached it to the pump with a clamp, and cut the bell off the end of the line goin out to the engine, and then attached the fuel line to that with clamps. and WOW!!!! what a difference. stays rich in the top end now and pulls very very hard. went to the track and even though they werent running the clocks, im almost positive im at a 12.5 now, trapping at around 105 according to the speedo and its 3mph slow. I beat two different STi with decent drivers by 6-7 car lengths so its alot better.