Have a Donzi 38 zx with factory fuel pressure gauges that only go to 15 psi. The sensor is at the TB. gauges read around 8 - 10 . Is this correct?

You have two fuel pumps, one low pressure and one high pressure. The low pressure would be around 8 psi and the high fuel pressure @ Idle should be 193 kPa (28 psi) & WOT should be 179 kPa (26 psi).

From the service manual,
Fuel Pressure Idle 193 kPa (28 psi) WOT 179 kPa (26 psi)
Min. Battery Requirements 750 cca, 950 mca, or 180Ah
Firing Order 1-8-4-3-6-5-7-2
Spark Plug Type
AC MR41T
Champion V4C
NGK R5673–8 (33–813421)
Spark Plug Gap 0.9 mm (.035 in.)
Timing (at Idle) 8° BTDC4
Thermostat 62 °C (143 °F)

Fuel pump item numbers 3 & 48,
http://www.mercruiserparts.com/Show_...nd+Fuel+Cooler

28 at idle good. Mine always read low 20s on the big end. A/F has always been fat.

Do you have a chiller? Where do you boat at?

The sensor is at throttle body so that should be high pressure

It could be that the sensor and gauge are mismatched.

All the 38 zx 575sci boats are the same fuel pressure guages. Mine is a 2001 year. Looked at pictures of a 2004 and same 0-15 fuel pressure guages.

Try posting a photo.

cant post a pic. must not have paid enough.
here is a link to a replacement set.
http://www.ebay.com/itm/Donzi-Boats-...item53f4f472ea

The sender sends a reading that controls how much the needle moves. I would guess you need fuel psi gauges that are the 0-45 or 0-60psi to get the correct reading. I have one that is 0-90psi, like this http://www.ebay.com/itm/Gaffrig-Fuel...r#ht_45wt_1159
but that would show you had 50psi.

You can test the one I have if you like and see what it reads.