I currenrtly have a carbed 355 that was switched from a TBI 305. When my engine starts up the fuel pressure hits 7.5 and the needle on the fuel pressure gauge rapidly pules back and forth between 7.5 and 8. After about 2 minutes the Fuel pressure gauge stops jumping and stays at 7.5 PSI ( when this happens it happens immediately like someone turns a switch). If I run my engine for about 20 minutes the fuel pressure gradually works its way down to 3 PSI. I haven't allowed it to go any lower when it hits 3 PSI I shut the car off.

Some background info:
I have the Mallory FPR 4309 3 way by pass
I have an intank TPI Pump
My fuel pressure gauge is hooked right to the regulator and is a summit liquid Filled gauge.

I can't figure out why the pressure will not remain steady

 

Get a new gauge. My Liquid Filled Summit gauge did the exact same thing leaving me to pull out hairs for days/weeks over why my TPI FI pump was bleeding off pressure once it got warm. I put an autometer phantom mech in the cowl and it bleeds off some but only 1/2 psi or so. I can see a difference between the autometer and the liquid filled one once it becomes heat soaked.

I'm also using the 4309 return regulator too. :-)

I doubt the bowls would even remain full at 3 psi in hard driving.

 

I guess I'm going to try the gauge however before I spend close to $100 on an autometer set up Do you think any of the following could be impacting the fuel pressure.

The mechanic that installed the engine drilled a hole in the gas cap to vent it, he also disconnected the line to the EVAP canister the line is not plugged.

Doesn't the TPI pump have something that makes it pulse? Could that be the problem?

 

I dunno if an ice cold towl would be enough to cool the liquid gauge back down to get a good reading.

I left the EVAP in place. It's just venting to air inside the passenger side splash guard area. I jumped the fuel relay with a run hot only wire so it runs all the time when the key is on. Without the jumper you will only get the 2 second pressure blast when the key is first turned. I used 180* turns off the hard lines into the OEM rubber lines to a connector to the 4309. I used 8 AN off the regular mounted by the old cruise control unit area on the driverside fender to the fuel log, which is where my gauge is mounted. Off that back of the lot I think it's 4 AN line it loops around the back of the distributor and up into the cowl area (4") where my 2 5/8" gauge is mounted in a cup.

The liquid gauge works great for checking FP cold and stuff but it wasn't reliable at all once it got heat soaked.