Surging idle after installing fore L1 return fuel system

Car idled fine before changing to return fuel system. Now the idle surges between 800 to 400 rpms up and down. Changed IAC valve just to see but still the same. My tuner said it could be a vacuum leak the only vacuum line I took off was for frps that I put a t on for the regulator vacuum. Just seeing if anyone else has had this problem?

Did you get a tune for the new system?

Yes it has been retuned for the return fuel system.

What did you set the base pressure to? I didn't have any issues when I went return style.

Did you keep the FPDM in place? And if so did you tell the tuner? I only ask because I set a buddies car up with the FPDM driving the relays (per Malcolm's thread) and the tuner he used didn't know and loaded up his basic return system tune. The car would surge at idle. The tuner ended up rewiring the pumps to be on all the time and the idle is fine.

The base fuel pressure is set at 40psi. Yes still using the frps and the pumps run full time. All vacuum lines are hooked up and in good condition. It only does it at idle other than that the car runs good at wot.

The FPDM is the part in the trunk. The FRPS is the part on the fuel rail. If everything is good mechanically then it's your tune.

What size fuel pumps are u running? I know, crazy question, but went through weird issues trying to run too much fuel pump through stock feed line and rails. Long story short, you can absolutely 100% have too much fuel pump. Bigger and room to grow isn’t always the case, ask me how I know. Haha

I’m not using the fpdm and the tuner does know it’s not in use. The fuel pumps are Walbro 342 that come with the fore L1 return fuel system.

We’re you able to correctly set the fuel pressure, using the regulator? I know this may sound weird. I only ask because I ran into an issue trying to run dual Walbro 465’s through stock feed line and rails. Long story short, it overwhelmed them and I couldn’t get my fuel pressure set correctly. Had all sorts of issues. I pulled one of the relays to one of the pumps, every worked fine.( It was a fuel volume issue. Trying to stuff 10lbs of shit in a 5lb bag essentially.) Car ran perfectly.
In your case, I don’t think this is the issue. Just throwing out ideas.

Yes fuel pressure is set at 40 psi with key on engine off. The car idled fine before I switched to the return fuel system and new tune. That’s the only thing I changed on it.

I still says it's the tune. A vacuum leak would cause other issues.