It is returnless, however manifold vacuum is sensed as part of the control system--open the hood and take a peek...

The FPDM (Fuel Pump Driver Module) just drives the pump, as its name implies, it does not control the pump.

The pump is controlled, and thereby the fuel pressure) by the PCM operating in a closed loop¹ direct feedback loop that uses input from the FRPS (Fuel Rail Pressure Sensor) as the feedback device.

The FRPS does have a vacuum connection so that it can be aware of manifold vacuum and signal the PCM as to what rail pressure is needed to maintain a 40 psi drop across the injectors.

What manual is saying 40-70 psi? At idle the actual rail pressure should be as you observed, the target 40 psi drop plus the manifold vacuum of 18-22 inHg (which is a negative "pressure" of -8.8 to -10.8 psi).

40 + (-10) = 30...

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¹ - Not to be confused with, and only indirectly related to, closed or open loop engine control.

 

ok cliffy my pressure with vacuum connected was floating between 30-32, so does that rule out the fuel system causing the codes? how can i test for a vacuum leak with using the brake cleaner method?

 

you spray the brake cleaner around areas that would be suspect to leaking and youll hear a change in the way the motor runs if it sucks in the brake cleaner. It can also be done with propane or starting fluid. If the motor changes thats where your suckin air in.

 

It means that the FRPS and fuel pump control circuits are doing what they should--at idle, You need to monitor the fuel pressure at high rpm and WOT (should be 40+) to make sure the pump is delivering enough fuel. If it cannot keep the f/p up then the pump motor may be weak.

This is much more easily done with a data logger; as opposed to running beside the car or jamming yourself in under the hood.

In the past I have used a length of fuel line and gaffer tape to tape a mechanical gauge to the windscreen--but do NOT run a hose into the car, that would be unwise...

 

Well i have an sct xcal 2 that I can use to datalog runs. The thing is the codes only come when idling more than a minute. Driving and wot and all never show codes. I can jump on the highway drive an hour get on it and not codes. I'll stop idle for a min or two and scan and then the codes appear. The problem is only at idle

 

Well I just checked the gauge again while idling and it was bouncing between 22-24 psi and when vac line is disconnected it goes to 32 not 40. When I unplug frps it goes to 22 psi. Could this be a bad frps

 

So after a long battle with the lean codes Im 99.999% sure its fixed. I had a tune for my car before I put the LTs in and never got a new tune. Ive read sometimes youll throw codes due to the o2 sensors being farther back than the stock posistion and you need a tune to correct it. Well I contacted the great people at Bama tuning and they hooked me up with 3 tunes (Race, street, and emissions) and the car has not thrown the codes for 4 days now. Hopefully it never does again and now my mpg have gone up 2-3 since then

 

With long tubes, we make changes so that the front o2 sensors are know to be located further away and their temperature is compensated for. It was probably just in your tune file. I'm glad that our Bama tune was able to take care of the situation for you!

 

i'm having the same problem with mine, throwing code 1131 and 1151 with my LT's. I have a XCAL2, is there anything I can do?

 

yeah call bama tuning and tell them and theyll hook you up with 3 tunes for 50 bucks or 150 for tunes for life