'74 1.8L LJet/fuel pump, Fuel Pump not staying on Options

[RKramden]

This is my first time starting a topic, I don't know if I am even typing in the correct location, but I am having an issue with my '74 LJet and fuel pump. The pump starts with the ignition switch, Pump also runs when Air sensor arm is manually moved. However when starting, it seems that the air sensor is somehow cutting off the fuel pump circuit. The PO had bent the arm so it never opened, but I have bent it back to move as designed. If I disconnect the electrical plug to the Air Sensor, the car starts and runs fine......any thoughts? BTW I have replaced the dual relay, but that did not fix the problem.

[ndfrigi]

do you mind also sharing pic of your car and engine? try checking also fuel lines below the fuel tank, maybe the rubber kink. you borrow fuel pressure test kit from local store and try checking of fuel flow and pressure. what city are you?

[RKramden]

do you mind also sharing pic of your car and engine? try checking also fuel lines below the fuel tank, maybe the rubber kink. you borrow fuel pressure test kit from local store and try checking of fuel flow and pressure. what city are you?

Too embarrassed to show any pix just yet. Stripped the wiring harness wrap off of the EFI harness looking for shorts, so imagine a pile of rust with a bunch of loose wires on top. Fuel flow seems fine as it will run when I unplug the air sensor. The car currently in intensive care in Long Beach CA.

[ClayPerrine]

Check where the engine harness plugs into the relay board. There should be a yellow wire with a spade connector that is separate from the 12 pin plug. That wire should hook to one of the two rear pins on the 4 pin connector located in the left rear of the relay board. Without that wire, the fuel pump won't receive power when cranking.


Clay

[RKramden]

I will check this evening, but I am getting fuel when cranking. If I load it up a little it will start, but it will die when that fuel runs out. With the ignition on, I can move the air flapper and hear the pump run as well. Something appears to be happening between initial start and air sensor opening that is not letting the fuel pump switch over voltage to occur.

[ClayPerrine]

I will check this evening, but I am getting fuel when cranking. If I load it up a little it will start, but it will die when that fuel runs out. With the ignition on, I can move the air flapper and hear the pump run as well. Something appears to be happening between initial start and air sensor opening that is not letting the fuel pump switch over voltage to occur.

Are all the vacuum hoses hooked up per the vacuum diagram? If you have an air leak, especially between the air flow meter and the throttle body, an L-Jet car won't run.

Also, check to see if the big black hose between the air flow meter and the throttle body is split. They are prone to doing that between the folds, and it is hard to see it. Symptoms will be that the car will try to start and immediately die when the engine moves ln the chassis, causing the split to open up.

Clay

[ClayPerrine]

I will check this evening, but I am getting fuel when cranking. If I load it up a little it will start, but it will die when that fuel runs out. With the ignition on, I can move the air flapper and hear the pump run as well. Something appears to be happening between initial start and air sensor opening that is not letting the fuel pump switch over voltage to occur.

Are all the vacuum hoses hooked up per the vacuum diagram? If you have an air leak, especially between the air flow meter and the throttle body, an L-Jet car won't run.

Also, check to see if the big black hose between the air flow meter and the throttle body is split. They are prone to doing that between the folds, and it is hard to see it. Symptoms will be that the car will try to start and immediately die when the engine moves ln the chassis, causing the split to open up.

Clay

Also, replace the oil cap seals. They will cause it to run badly or not at all.

[RKramden]

thanks Clay, I will check all those things tonight. Big hose to Throttle body absolutly has holes. I have temporarily taped it up with a new one on order. I think I have the oil cap seal in the box of other stuff I have previously bought. All other vacuum hoses are new silicone but I will double check routing. What is puzzling me is that when I unplug the Air Flow Sensor, the car starts and stays running.

[jcd914]

thanks Clay, I will check all those things tonight. Big hose to Throttle body absolutly has holes. I have temporarily taped it up with a new one on order. I think I have the oil cap seal in the box of other stuff I have previously bought. All other vacuum hoses are new silicone but I will double check routing. What is puzzling me is that when I unplug the Air Flow Sensor, the car starts and stays running.

There is a switch inside the Air Flow Sensor that until the flap moves the pump won't run. Disconnecting the Air Flow Sensor eliminates this fuel pump switch and the pump runs when the engine cranks over or the ECU sees an ignition pulse.
It sound like the flap in the Air Flow Sensor does not move enough while cranking to close and allow the ECU to turn on the fuel pump.

Jim

[dr914@autoatlanta.com]

with the key on and when you move the arm under the black plastic air flow meter top, the dual relay will click and the fuel pump will run. also when you crank the car and observe the air flow meter pointer, it will wiggle back and forth coming off the small set of points and letting the fuel pump run. If the flap/pointer does not move you have a massive air leak, could be the ribbed intake boot loose or with a crack.

This is my first time starting a topic, I don't know if I am even typing in the correct location, but I am having an issue with my '74 LJet and fuel pump. The pump starts with the ignition switch, Pump also runs when Air sensor arm is manually moved. However when starting, it seems that the air sensor is somehow cutting off the fuel pump circuit. The PO had bent the arm so it never opened, but I have bent it back to move as designed. If I disconnect the electrical plug to the Air Sensor, the car starts and runs fine......any thoughts? BTW I have replaced the dual relay, but that did not fix the problem.

[porschetub]

This is my first time starting a topic, I don't know if I am even typing in the correct location, but I am having an issue with my '74 LJet and fuel pump. The pump starts with the ignition switch, Pump also runs when Air sensor arm is manually moved. However when starting, it seems that the air sensor is somehow cutting off the fuel pump circuit. The PO had bent the arm so it never opened, but I have bent it back to move as designed. If I disconnect the electrical plug to the Air Sensor, the car starts and runs fine......any thoughts? BTW I have replaced the dual relay, but that did not fix the problem.

My understanding was that the pump only gets power from ignition switch and won't start until the AFM air gate moves off the cut off switch,this should happen when the engine is cranking before starting.
I think the switch is to stop the pump on throttle overrun,in the later L-jet version (bus) the ecu controls that function so they use a different AFM.
When you say the car runs when you disconnect the air sensor ?do you mean the AFM? if so I can't see how it would run with the plug off.
Whats the condition of the wiper board inside the AFM ?,if the wipers have bad contact it can cause the issue you have,can be cleaned or reset if tracks are worn.
Good luck.

[ndfrigi]

@rkramden I have spare AFM for Ljet, if you are around Stanton (anaheim/buena park) you can come and use my AFM to test it on your car.

[ClayPerrine]

[
My understanding was that the pump only gets power from ignition switch and won't start until the AFM air gate moves off the cut off switch,this should happen when the engine is cranking before starting.
I think the switch is to stop the pump on throttle overrun,in the later L-jet version (bus) the ecu controls that function so they use a different AFM.
When you say the car runs when you disconnect the air sensor ?do you mean the AFM? if so I can't see how it would run with the plug off.
Whats the condition of the wiper board inside the AFM ?,if the wipers have bad contact it can cause the issue you have,can be cleaned or reset if tracks are worn.
Good luck.

The fuel pump will run when cranking via the wire from the relay board to the double relay. There is not enough air flow during cranking to open the flap. Once it is running, the air going into the AFM causes the fuel pump contacts to close.

And, any air leak between the AFM and the throttle body will cause it not to run. The air bypasses the AFM, and goes directly into the throttle body. That keeps the AFM from opening enough to keep the engine running.


Definitely replace the big tube between the AFM and the throttle body. Then report back on how it runs.


FYI.. My wife has an L-Jet injected 914. I was trained by VW to work on the L-Jet systems in the 80s. And I have been keeping my wife's 914 alive for 34 years now.

[RKramden]

Thanks All!!
So I checked the Vac lines, all to the right spot and all new
Large intake rubber hose taped up temp for now. New one coming
Unplugged AFM or Air Flow Sensor (they seem to be used synonymously) and the engine will start and idle. The flapper moves freely and the pump contact closes when engine starts.
If I give it gas it will quit. Seems logical as the ECU is looking for an air signal, assume it will idle without one.
If I plug it in, it quits. My feeble mind thinks that maybe the fuel pump has been hotwired to bypass the AFM/Dual relay signal and somehow sending that signal is grounding out the fuel pump. As previously noted the PO had bent the arm in the AFM so that it would never make contact (I have bent it back to norm).
I will try to trace the fuel power this evening.
Could someone confirm that the ECU is out of the picture as far as fuel pump power goes? In the line diagrams I have seen, it looks like pump power is either Ignition switch to Dual Relay to pump, or AFM pump relay to dual relay to pump. Input #20 to the ECU looks to be monitoring the pump relay, but I am not sure what it is doing with that input.
thanks again.

[ClayPerrine]

Try these tests:


1. Disconnect the Yellow wire on the starter. Then turn the key to the start position. Does the fuel pump run when you do this?

2. Disconnect the big rubber hose between the air flow meter and the throttle body. Make sure the air flow meter is plugged in. Turn the key on. Open the air flow meter with your finger. Does the fuel pump run?


Clay

[RKramden]

Try these tests:


1. Disconnect the Yellow wire on the starter. Then turn the key to the start position. Does the fuel pump run when you do this?

2. Disconnect the big rubber hose between the air flow meter and the throttle body. Make sure the air flow meter is plugged in. Turn the key on. Open the air flow meter with your finger. Does the fuel pump run?


Clay

#1 pump will run in this scenario.
#2 I can turn the flapper from the top of the AFM with the cap off. The fuel pump will run.

[ClayPerrine]

Try these tests:


1. Disconnect the Yellow wire on the starter. Then turn the key to the start position. Does the fuel pump run when you do this?

2. Disconnect the big rubber hose between the air flow meter and the throttle body. Make sure the air flow meter is plugged in. Turn the key on. Open the air flow meter with your finger. Does the fuel pump run?


Clay

#1 pump will run in this scenario.
#2 I can turn the flapper from the top of the AFM with the cap off. The fuel pump will run.

Then the double relay is working as expected. Start by fixing the false air problem (the split hose between the throttle body and the AFM). If that doesn't fix it, then the AFM is probably bad.

[RKramden]

Try these tests:


1. Disconnect the Yellow wire on the starter. Then turn the key to the start position. Does the fuel pump run when you do this?

2. Disconnect the big rubber hose between the air flow meter and the throttle body. Make sure the air flow meter is plugged in. Turn the key on. Open the air flow meter with your finger. Does the fuel pump run?


Clay

#1 pump will run in this scenario.
#2 I can turn the flapper from the top of the AFM with the cap off. The fuel pump will run.

Then the double relay is working as expected. Start by fixing the false air problem (the split hose between the throttle body and the AFM). If that doesn't fix it, then the AFM is probably bad.

Clay, I am leaning towards the AFM as the issue as well, I will try another one once the new hose arrives. Do you have any idea why the car and pump will stay running with the AFM unplugged? Last night I got the RPMs up by slowly increasing the throttle. If I punched it, it would die.....this is all with the AFM unplugged!

[ClayPerrine]

Clay, I am leaning towards the AFM as the issue as well, I will try another one once the new hose arrives. Do you have any idea why the car and pump will stay running with the AFM unplugged? Last night I got the RPMs up by slowly increasing the throttle. If I punched it, it would die.....this is all with the AFM unplugged!

That is a sorta "limp mode" function. If the AFM dies, the L-Jet sees the ignition pulse that is over a certain RPM and will keep the engine running. Without a AFM signal, it will just barely run. Good for getting the car off the street into a parking lot, and not much more than that. It only knows how to make the car idle, nothing more.

[RKramden]

Interesting......just makes it harder for us novices to try to trouble shoot.
Picking up a loaner AFM this evening