chris_stampe

First off my fuel system consists of a walbro pump, aeromotive fpr, 1600 secondaries and stock primaries, ran by a MT LT8.

I started having starting problems with my car and replaced plugs, adjusted cranking maps, replaced battery and what not. I know its getting spark and that it is not flooded.

While having someone else crank it I noticed that the fuel pressure drops off while cranking from 45 psi to 0 in only about 3-5 cranks. I also noticed that the car will only start when the pressure is high on the 2nd or so crank.

So I am wondering why in the hell my pressure is dropping so much during cranking. I assume this is not normal. I checked the walbro to see if maybe fuel was leaking back through it, but I dont think it is.

Anyone have any suggestions? I have the pump wired right off the battery (controlled by the MT and a relay). I dont know what else I can do.

RotaryEvolution

iTrader: (30)

how long does it take for the relay to energize during cranking? the aeromotive is an out of tank pump and like the stock system the microtech primes the system then turns it off until the engine is cranked then re energizes the relay. an out of tank pump will take longer to prime especially if it is far away from the fuel tank, i would suspect this has more to do with your fuel system setup than anything else. double check with aeromotive and be sure your pump is located properly and that your pickup tube is not plugged up or sucking air. you did not leave the stock pump in the tank by chance?

prjct87rx7

iTrader: (7)

i didnt read anywhere that his pump is no longer in the tank . but maybe im a tard..wich is a good posibility (sp?)

Aaron Cake

iTrader: (1)

The Walbro pump is in tank I would assume.

You can put a check valve in the fuel feed line from the pump to maintain pressure when the car is turned off. Aeromotive and most of the other fuel system companies sell them. Many production cars have this standard.

Edit...You can also add a fuel priming button which activates the pump, but that's kind of a hack...

chris_stampe

The check valve sounds like a good idea, but I was under the impression that most fuel pumps had check valves in them. I took the walbro out and tried to blow back through it to no avail.

The priming button doesnt really help because it only looses pressure when cranking even when I have my "jumper switch" turned on and it is getting constant voltage independent of the relay.

The relay seems to prime the system just fine, its just after a couple cranks that it has no more pressure left.

It would be interesting to see how the fuel pressure acts on a stock car, but none of my friends have a fuel pressure gauge to compare to.

EDIT: Maybe my fpr is to blame and is leaking a small amount of fuel, not enough to affect it when the pump is on, but enough to not keep enough pressure when it is momentarily off.

HAILERS

On a stock car there is no such thing as Prime.

The fuel pump does turn on any time the key is HELD to start or the engine actually starts and runs.

The FSM states the rail fuel pressure on a stock car should hold pressure after the key/engine is shut off. Ive done that test and the pressure held for over 30 minutes (memory).

A totally unpressurized fuel system will hit 40 psi in less than three seconds. Been there, done that and timed that. Reduced the pressure with a fuel cut switch to zero by reving to five grand and then holding the pedal to the floor as the fuel cut switch was CUT. Had a real pressure gauge on the fuel rail to observe ZERO pressure and then approx 37psi as the key was turned to Start.

That's all stock pump etc. The section of the FSM that describes the HOLD pressure for the fuel rail pressure is found in the FUEL section of the S4 or S5 manuals.

I know zip about aftermarket pumps.

RotaryEvolution

iTrader: (30)

yeah i mixed up the part about the aeromotive FPR not the fuel pump, the aeromotive fuel pumps are not in tank pumps which is why my post made no sense.

the microtech should be energizing the fuel pump relay when cranking so i would guess there is something wrong with the wiring of your relay, check the power source and ground for the relay as well check to see if the control circuit is being powered up during cranking to be sure the microtech is doing its job correctly, be sure you are getting spark and that the amber lights flash during cranking, when those lights are flashing you should have spark and it should energize the fuel pump relay which should instantly give you your idle set fuel pressure.

Aaron Cake

iTrader: (1)

Yes, the Microtech primes the pump when the key goes to IGN by running it for several seconds. The pump then remains off until the car starts.

If you are loosing pressure that fast it seems that there is a leak somewhere. Either at the FPR or maybe internal to the tank.

BASTARD

iTrader: (4)

I know you said you replaced the battery... but you may need a new battery... or you need to rewire your pump with heavier gauge wire?. Maybe you need new battery cables? Do you have the return lines connected to FPR correctly? Do you still have the factory FPR as part of the system? Can you volt meter (or test light...the test light will go dim) on the fuel pump lead while cranking?


Also.. All EFI fuel systems should create pressure when turned to the IGN on position (I.E. prime) and the fuel pump should be working and maintaining pressure while the starter is cranking

NZConvertible

The stock system does not do this, and I don't know why any system would need to. Even if it did, who turns the key to ON and waits for the fuel pump before cranking? Unless there's a problem there should be no need to do that. I put the key in and turn it straight to START, and it has no problem starting. A "prime" function wouldn't have time to do anything useful.

Not that this thread has anything to do with this, since the problem occurs during cranking, when the pump should definitely be working.

HAILERS

*********
Not that this thread has anything to do with this, since the problem occurs during cranking, when the pump should definitely be working*************8

It's relevant because the thread author quried about how the stock system worked.

Ignition key to ON............no pump action period. Read the fsm.

chris_stampe

well, the mt does use a prime, not that this has been debated. It turns out my fpr was leaking ever so slightly. I fixed it and now everything is happy! thanks for the help.

RETed

If it's losing pressure while cranking, something is definitely wrong.
The MT should be able to hold fuel pressure while cranking.

I would try and run the fuel pump full-time (use a switch to turn it on and off).
You should be able to splice the wires in pretty quickly.
If the system runs with the fuel pump on a separate circuit, then something is fucked with the MT (or upstream from it).

I think this thread really needs to be moved to the MT section?


-Ted

NZConvertible

Since he solved the problem (a leaky FPR), I think not...

RETed

Doh, sorry for the misread!
I'm freakin' tired!


-Ted