Troubleshooting fuel pressure issue

[smith876]

Hi all, troubleshooting a problem and looking for some help.


My cobra was tuned in 2017 and has been running perfect till about 4 weeks ago. The problem initially showed with an SES light, I pulled the code and replaced the FRPS. Since then I am showing a dangerous AFR anytime I am in boost. If I ease into it there is no problem, but once I am over ~4PSI I start am over 12.5 on the AFR gauge.

The car is setup with a return fuel system; 3 Walbro 342’s through a set of Fore rails and Fore regulator. Data logging I found the fuel pressure drops dramatically once I am in boost. Just stabbing the throttle for 2nd gear hit yesterday I found boost went to 18PSI, fuel pressure dropped to 16PSI (from 55# base) while AFR shot up north of 16.

I checked the in-line filter and it looked perfect. No obstruction in the 100 micron filter, I did blow it clean just in case but nothing came out and it looks perfect when held to a light. As well I tested each of the fuel pumps individually and found each holds 55PSI at idle. But I’m struggling to figure out what failed, fuel pressure regulator, fuel pump(s)?


I am leaning toward replacing the fuel pressure regulator as a first step…is it possible something happened to the FPR after the FRPS failure (the share the same boost line reference)? Or is it worth focusing on the fuel pumps first? Sure appreciate any input/insight to help diagnose and focus on the problem.


Thank you in advance!
-josh

 

[olympic]

Sounds like a FPR problem to me. If you can buy/borrow a radiator pressure tester you could manually apply air pressure to the FPR while the car idles and see what happens to the fuel pressure.

 

[roy_1031]

I’d take the regulator apart and inspect it before replacing it. To me sounds like the pumps aren’t keeping up. Can you data log each one individually and see if ones really falling short?


Sent from my iPhone using Tapatalk Pro

 

[smith876]

Great ideas, thank you both! I'll try the FPR test and put a little air pressure on it. I never would have thought of that, but what a simple test.

As well I can absolutely can pull a fuse for each pump individually and will double-check that as well. Will follow up once I have a chance to test and validate both.

thank you again!!
-josh