I have a s4 t2 with megasquirt ecu, and it is running real rough on closed loop, real lean. Long story short,I misdiagnosed a bad tps and had a buddy retune the ecu with the bad tps. After replacing the tps, the car has a super lean tune.

Anyways, I noticed the fuel pressure goes super duper low on my closed loop idle, like 25psi. Open loop idle when the engine has a decent tune I am at 40psi, and on priming my engine. Would you think the bad vacuum, horrible idle will make my fuel pressure drop that low? The car has sat for a while with this low mileage 255lph walbro in it, but the fuel pressure is great when the engine is also running well.

 

unless you have some odd hardware, the fuel pressure regulator is just hooked to the intake, so it just sees intake vacuum/boost (if you hooked it to the right place, there are a couple that only see vacuum)

we'll just use the stock stuff as an example.

so with the engine off, intake vacuum is 0, and fuel pressure is 40psi (give or take Mazda's spec). notice there is a 40psi difference between the fuel pressure and the intake pressure.

at -10psi, the regulator should be around 30psi, notice its still a 40psi difference.

and at 10psi, the regulator should deliver about 50psi.

so 25psi at idle is about right.

 

25 still seems low, wouldn't you need to be pulling 30in of vacuum to see that? A healthy engine pulling 18in of vac at idle with 40psi base fuel pressure should still be over 30psi right?

 

Oh ok, I was scared. My car is running horrible warmed up. I didn't know it was ok to drop that much lower than 40