S5 NA fuel issue (detailed trouble shooting)
 

1991 RX7 S5 non turbo 5spd.

Bought from a member on here who did a lot of work. He was working up to a turbo swap but never got around to it.

Trying to track down a fuel issue at the moment. The car will run on starter fluid so I am under the assumption the motor and ignition system are good. Car idles at 2000 RPM and seems to run smooth until it runs out of starter fluid.

The car will not start on its own however. Have been able to check the secondary injectors as they are the easiest to reach. Resistance is at 14.1 so they are within spec. 12v is going into and out of the injector. 12v has been verified right at the ECU on the light green wires for both Primary and Secondary injectors. All 4 ground wires from the ECU are checking out good as well.

Since the primary is so difficult to reach, I had an Idea of swapping the signal wire at the ECU plug for the Primary and Secondary injectors. Since we can't get the motor over the swap RPM ~35/3800 (3,688-FSM) RPM and don't know if the primary injectors are good, I think this could work to make sure the injector driver is at least working on the ECU side. I understand the secondary injectors are 850 but this would solely be for testing the operation. If they fire and the car starts on its own then the primary injectors are bad and will be swapped out. If it doesn't work then it must be the ECU?

The pump will operate at 12v when used with a battery box and also when using the jumper wire. When using the jumper wire the pump receives 12v and does not change when the pedal is pressed all the way.

When cranking however the pump only receives 7v. We have the power and ground wire cut to test for voltage. When they are reconnected the pump acts like it isn't receiving voltage. Perhaps the pump is bad and will only operate at 12v and nothing lower?

We also know the secondary rail is receiving fuel by disconnecting the supply line and powering the pump and getting fuel.

Also have a question for the FPR on the secondary rail. I am under the assumption the FPR retracts under high vacuum (idle) and allows more fuel to return to the tank. Under more throttle it closes back up keeping more pressure to the system?

I understand the AFM will not allow the pump to operate if it isn't working.

Found this image and believe it shows how the system works electrically. When doing the jumper test, we are bypassing only the AFM or just putting direct power to the pump and ignoring the relays as well?

As I dig deeper into the depths of the interwebs.

If the circuit opening relay is bad only during cranking it will give me low voltage (7v) when it should be 12 volts. Then when the key is brought back to ON position and I am getting around .66 volts because the car sees that it is low speed (idle) and it is now passing through both resistors?

FYI: The factory service manual has a lot of great testing info for the fuel system.

Reading your post, I see that you've checked the injectors and the fuel pump operation. Have you checked the operation of the Fuel Pump relay located under the dash next to the steering column? I've had one of these fail and give the same symptoms as you described.

Which wire are you measuring? The Blue wire powers the pump and the voltage drops because the starter requires many amps. Not sure it should drop all the way down to 7 volts, though (9 volts seems more normal).

The two B/W wires measure differently depending on the situation. Top left B/W wire has voltage w/key to the start position only and then voltage stops once the key is let go from the start position. The B/W wire to the right of it has voltage w/key to on (either w/the engine not running or running).

this isn't an FD so you have 440(same size as the primaries) secondaries, not 850's.

there's no mention of fuel pressure, just because the pump makes noise doesn't mean it's actually making pressure. the low voltage during cranking is rather normal.

start by checking the fuel pressure to the rails after the fuel filter. if the pressure tests ok then move onto testing the injectors. you can swap the pins at the ECU on the harness as a quick test to see if the secondaries will fire the engine, otherwise you can depin the injector wires at the ECU and momentarily ground them(key on) and have a helper listen for an audible click from the top of the engine telling you that they are physically moving. for a car that has been sitting a long time, it is not uncommon for the injectors to be stuck closed, sometimes grounding them in this way will shock them free and open(just do not hold them grounded, it will overheat the injectors after several minutes without any fuel flowing through them).

HAve you tried starting the car with the fuel pump jumped with 12v? That will eliminate any issues with the pump receiving power.

What do you mean "when cramking the pump only receives 7v"? You have it jumped with 12v and the voltage goes down to 7v when youre cranking, with the 12v still hooked up?

Secondary injectors are not used to start or idle the car.

You could isolate the Light Green wires at the ECU for the primary injectors. You then take an LED light and place one wire to the primary injector wire and the other wire to pin 1B (Black/White wire) and w/key to start the light should flicker indicating the ECU is attempting to fire the injector.

That is actually a damn good idea. I have led's installed in the check connectors in the engine bay to diagnose issues in the rare time they arise, but have been wanting to relocate them to the blank spot below my radio...hmm, next weeks project . :p

Edit: you can actually monitor all the sensors on the car with LED's...other than the oxygen sensor. You can even monitor the ignitors on the coils, injectors, fuel pump resistor relay, the tps, the afm, you can get as creative as you want.

Sorry for the delay guys. Before I address your comments I have an update. Replaced the fuel pump and battery, and the car started on its own! Drove it to the gas station to refill fuel and air. In its current parking spot I started and ran it for 5 minutes each day without issue until a massive rain storm came, and I decided not to start it for a couple days. Lo and behold, I attempted to start it on the first clear day and the original symptoms returned. New battery works fine of course, but the car is acting like the old fuel pump is back in there. It will turn over, rev up to 3500rpm, and die again, so I don't think the plugs are fouled (yet). It seems like we have an electric connection issue with the fuel pump.

@wozzoom, I like the idea of checking the fuel pump relay. Let you know how that goes.

@RotaryEvolution, I've seen fuel flow at the rail (from the supply line) but I'll verify the pressure to be sure.

@fuhnortoner yes before I replaced the battery I had my DD powering the battery terminals and a portable jump starter powering the fuel pump. This actually worked in getting the engine to turn over and run, which is another reason I think the car's power supply to the pump is messed up somewhere.

@satch thanks for the awesome idea, I'll try that too.

@fuhnortoner as for your question, it only dropped down to 7v when the pump is not jumped, only when the battery was jumped. This is what prompted me to jump the pump itself. I haven't tested this since I replaced the battery and pump, however.

Just trying to rule out the fuel pump variable. Satch should chime in soon on the wiring. I hate electrical.

depends where you are checking the voltage from. from the cut relay under the steering column the wires to the pump are blue, at the LR strut tower where the actual pump connector is the wire is black with a white tracer. use the B/W wire at the back of the car to measure the voltage. there is also the factor of ground to the pump through the pump housing through the fuel tank bulkhead and then to the chassis, even if there is voltage the ground needs to be good.

even if there is voltage though it doesn't mean there is pressure, which is why i start with a pressure test and if there is none, then move on to voltages and mechanical parts testing.

When he jumped the pump with a battery pack the car started and idled fine. When he didnt jump it, it wouldnt start. Doesnt that point to an electrical problem?

it could, it could also just be coincidental. if the engine is turning extremely slow without a jumper battery then it could simply be his battery is toast.

I mean he used the battery pack to jump the fuel pump directly, not the car battery.

if it isn't running with 7v and only when 12v is going to it then the pump may be dying

i figure this wasn't an issue since he said the pump gets 12v with the fuel pump jumper connector in the engine bay jumped, does the car not run in that mode? the 2 step pump relay isn't a necessity, if it is having issues with the lower pump step voltage, just bypass it.

Well it could be the fuel pump I suppose. It just got replaced, but 1) it sat on the shelf for months beforehand and 2) it's possible there is still corrosive material in the tank (previous owner had replaced the fuel tank and pump himself due to rust).

I do think it is electrical but RotaryEvolution is bringing up some good options for eliminating the fuel pressure issue. At this point the engine has reverted to starting only with starting fluid. Before that rainstorm it was starting by itself. It also could be air because the first time I got it to work, I needed a friend to constantly adjust the air intake plunger and throttle body valve to keep the engine alive, and then eventually it stayed running on its own (which I assume is a temperature thing).