Is it normal for the fuel pressure to change slightly when the grid heaters are cycling? I never noticed it happen with the stock fuel pump, but now that I'm running a FASS it changes slightly....It's only about 2psi, but you can tell it drops as the grid heaters clycle on and off. Anybody else notice this?

 

Yup, mine does too.
I'm running a Walbro 392 and I see the same.

Those grid heaters are a HUGE amp drain on the electrical system when the cycle on.

 

Perfectly normal.

 

More than likely the FP doesn't actually drop.
The gauge reading drops due to low voltage because of the grid draw.

 

ditto

 

As I am using a mechanical gauge, that's not the case on my truck.

 

Then my second guess is the low voltage is slowing down the lift pump.

 

must be electrical. i have a purely mechanical pump abd see no changes when they kick on/off.

 

That's the answer. The voltage drops and it slows down the pump.

 

BINGO!

 

Interesting....I just never noticed this happen when running the stock lift pump....So, you're saying that it's pulling power away from the FASS when the grid heaters cycle on? Does the FASS use more power than the stock pump did? I will have to hook up a mechanical gauge some time & see if it does the same thing as the electronic gauge...