I can not figure out why my fuel pressure drops.

Here is the symptoms....

When the car is cool I have normal fuel pressure. It idles around 38-40lbs, and at boost is will go to 50-53lbs.

After driving for a while it will still hold 40lbs at idle, but in boost the pressure will drop for a quick moment at transition.

After driving the car for around 45-60 min the fuel pressure will not change. It stays at 38-40lbs regardless if it's idle or in boost.

My fuel pump was rewired. I have checked voltage after the relay and it is pretty consistent. 13.7-14 volts. I've tested this while driving the car.

My supra pump, and fuel filter has around 5,000KM.

I am thinking of replacing the fuel pressure regulator, but after that I am completely out of ideas.


Do you guys have any???

 

Where do you source the vacuum for the fuel pressure regulator? In other words what nipple are you using for the vacuum line to FPR?

 

I have a stock FPR now. I was going to replace it with another stock unit.....

 

That is not answering the question. Where do you have your vacuum source connected? Do you have it routed to the solenoid still? If you do then eliminate the solenoid and route the line directly to the nipple on the LIM that was the original vacuum source.

 

FWIW: I just had a similar issue with my FD while tuning. It ended up being my BRAND NEW supra fuel pump. It has something wrong internally. It will hold fuel pressure until about 5 PSI then it dropped off. The tuner removed it to check it out and it was too hot to touch.

Walbro 255 in there now and running good. Loud as all hell though.

Just a thought. My Supra pump was the new part # ending 1270.

Dave

 

There are two relays. The first relay output voltage is on a blue wire with red stripe. That voltage feeds the second F/P relay which is called (on page Z-28 of the 1994 workshop manual) the "Fuel Pump Relay (Speed)." The second relay closes only when terminal 1K of the PCME (ECU) goes to a low state, which feeds pump power through to the pump itself. If the second relay contacts do not close, the pump receives power through a "Fuel Pump Resistor," in parallel with the second relay, which will drop that 13.7 - 14 volts before getting to the pump. (I don't know what causes terminal 1K to go low, but it might be the TPS reporting "wide open throttle" to the PCME.) Obviously Mazda thought that the pump needs more voltage to hold up fuel pressure when the engine is under full load.

The bottom line is that you should be measuring the voltage at the pump, or at least at the output of the second relay, on the white wire with red stripe that runs to the pump. The second relay is in the long narrow "relay box" under the air intake guide at the front passenger side corner of the engine compartment. It's the end relay farthest from the two small side-by-side relays.

 

if it was frp you would have bad idle to....yours seems to hold up pressure

i do not think its bad pump as you do build up 38 to 40psi at idle

i think Banzai-Racing is right on money

 

Forgive my newbiness but are you guys talking about the fuel pressure solenoid? I thought it was for hot starts....

https://www.rx7club.com/power-fc-forum-47/fpr-solenoid-question-899959/

 

Not me. You are right; the extra pressure is for hot starts.

 

yes fuel pressure solenoid

i run a single turbo...what we all do is eliminat that whole rats nest ...and run aftermarket FRP....aeromotive...and bigger fuel pumps...

so all you need to do is jut eliminat that one solenoid and run the vacum line from LMI to your FRP

 

You are saying that:

"After driving the car for around 45-60 min the fuel pressure will not change. It stays at 38-40lbs regardless if it's idle or in boost."

Let me try to make this as simple as possible.

The fuel pressure solenoid is not doing it's job. It routes the vacuum/pressure from the LIM to the FPR. The vacuum/pressure is what raises or lowers your fuel pressure. Obviously the FPR is not seeing any vacuum/pressure once the solenoid gets hot after you are driving around for 45-60min. The only thing that can cause this is the solenoid venting the vacuum/pressure that the FPR should be seeing.

This is very dangerous. The fuel pressure needs to raise when you go into boost, otherwise you will run lean and blow your engine.

Solenoid A, light blue line, back of the rack