1977 911 more fuel problems

dm36415 · dm36415's Garage

Replaced the fuel pump and fuel line between the warm air regulator and distributor. Tried to bleed the system, however when I had the ignition on and lifted the sensor plate, all I heard was a brief sound of air, did not hear the fuel pump kick on, nor did I hear the injectors.

Jumped pins 30 and 87a, and voila, could hear the pump and injectors. Tried replacing the relay and lifting the sensor, but still no pump or injector sound (both relays tried were new). Car briefly starts, then stops immediately, like it's fuel starved.

Pump worked prior to replacing the fuel line, the stream of gas out the bad line proved that. Could it be I keep flooding the engine when I'm trying to bleed it?

Help please.

ianc · 09:50 PM

Quote:

Car briefly starts, then stops immediately, like it's fuel starved

Had the engine out recently or done any CIS work?

I had this symptom once when I reversed the cold start valve and frequency valve connectors,

ianc

Edit: My bad, '77 didn't have a freq valve. I doubt you are flooding the engine however or it wouldn't kick at all.

2.7RACER · 09:33 PM

At the fuel pump relay 86 is 12 volts when the ignition is on.
85 is ground only when the air sensor plate is lifted.
With the ignition on, jumper 85 to a good known ground.
If the pump runs, the trouble is in the switch or wiring from the air sensor.
While the ignition is on, 86 should read 12 volts, if not chase that wire.
86 is the positive side of the fuel pump relay coil and 85 is the ground side of the coil.
Since you can make the pump run by jumping 87a and 30, I'm betting the problem is at 85 through the air sensor switch to ground.

Edit.
Ignore this: I have a better troubleshooting guide later into this post, 2.7Racer

dm36415 · dm36415's Garage

Thanks Tracer, I'll check the voltage tonight.

As an aside, can I start the car while jumpering 30 and 87a to see if it runs that way, to see if the problem lies therein or if there are other issues?

dm36415 · dm36415's Garage

Sorry Doug, I got your screen name wrong (2.7Racer), old, tired eyes this morning.

2.7RACER · 09-29-2006, 01:00 PM

dm36415,
You run the risk of flooding the engine with fuel. Could work. I wouldn't try to drive around that way.

dm36415 · dm36415's Garage

I'm learning, so please bear with me. So under normal operating conditions, the fuel pump turns on and off to regulate flow? Does the sensor raising and lowering regulate the on-off? Hence the need for tracing the voltage to the relay pins to ensure constant, correct flow?

Thanks,

Dave

ianc · 09-29-2006, 01:21 PM

Dave, there is a safety switch on the back side of the sensor plate housing that turns the fuel pump on and off. If the sensor plate is not deflected from rest position by airflow, the fuel pump will not run. This is to prevent spewing fuel all over in an accident where the engine might have stalled. As long as the sensor plate is raised, the fuel pump continues to run, there is no on\off while running. HTH,

ianc

ruf-porsche · ruf-porsche's Garage

Quote:

Originally posted by dm36415
Does the sensor raising and lowering regulate the on-off? Hence the need for tracing the voltage to the relay pins to ensure constant, correct flow?

Thanks,

Dave

Nope

The rising and lowering of the sensor plate which in turn raise and lower the plunger in the FD determine the amount of gas that is sent to the injectors. The higher the sensor plate rise, the more gas is sent to the injectors. When the the sensor plate is at rest such as when the engine is turn off, no gas is delivered to the injector.

dm36415 · dm36415's Garage

OK, 86 to ground is 12 volts with the ignition on. Jumpered 85 to ground with ignition on and pump did not run (even when I lifted the sensor). Pump does still run when jumpering 30 and 87A, however car is not even firing now. Seems more I fix, the worse it gets. I'm stumped.

ianc · 09-29-2006, 03:18 PM

I'm not sure I understand exactly what you've done, but:

86 is supply current for the control circuit in the relay, and 85 is ground. If you have current at 86 with ignit on, that's good. If you're sure 85 is grounding properly, but fuel pump is not running with ignit on and sensor plate lifted, likely the relay is bad. Try swapping it with another.

ianc

dm36415 · dm36415's Garage

ianc - tried two new relays, any way I can test them to see if they are any good?

ianc · 09-29-2006, 05:13 PM

Quote:

any way I can test them to see if they are any good?

Sure, apply voltage across the control circuit (85 & 86), and you should have continuity between 30 and 87. 30 and 87a will have continuity when the control circuit is not energized.

ianc

dm36415 · dm36415's Garage

I'm making some headway now, questions while my battery recharges

1. I have continuity between 30 and 87a. Do I jumber 30 and 87 directly to the battery to check for continuity between 85 & 86 under load, of do you have a better suggestion for applying a load?

2. Replaced fuel filter (again, just in case). Jumped 30 and 87A to start fuel pump, this time when I did that, I raised the sensor while the pump was running and heard a distinctly different sound from the fuel distributor and in the injectors. After 5 seconds or so, released the sensor and replaced the fuel relay. Car started right up, then died after a few seconds. Does sound like a relay problem, doesn't it?

dm36415 · dm36415's Garage

One other thing to consider. When I acquired the car (not having run for at least a year), it had a black relay 911.615.109.01 in the fuel relay position, rather than the red relay 911.615.108.01. Could this have caused damage? Should I retry with the black relay (as someone may have done something to compensate in using it)?

I was told the car was running when brought to the dealership I acquired it from.

ianc · 09-30-2006, 09:48 AM

Regarding testing the relay: I don't really understand what you mean by #1 above.

The releay is a simple device. 85 & 86 are connected via a coil of wire. They should always have continuity. When current runs through this coil, a magnetic field is created causing a contact to be made which permits current to flow through the 30 -> 87 path. At rest, 30-87a always has continuity.

There are many ways to test this; a continuity checker would be the easiest, and most VOM's (volt ohm meters) have this. just apply 12v across 85-86 and check continuity across 30-87. If there is none, the relay is bad.

Regarding the red vs. black relay, the red one has a diode in it which does not permit current to flow back through the relay from another circuit. I don't think you've damaged anything by using a black one; I used one for years with no ill effects until I finally switched over to a red one I came across.

As for the specifics of the fuel pump circuit, I'm afraid I cannot recall the exact specifics of it right now, so I cannot lay out a clear test plan for you. I know that its operation has been explained here before, so a search will turn up some good information, or a simple examination of the wiring diagram may prove enlightening.

Good luck with it,

ianc

dm36415 · dm36415's Garage

Thanks for the patience. What I was asking in #1 above was what do I use for the 12V to put the relay under a load? Should I jump pins 85-86 to the battery terminals, then test the continuity of 30-87, or is there a better way to put a load on pins 85-86?

ianc · 09-30-2006, 11:43 AM

Quote:

Should I jump pins 85-86 to the battery terminals, then test the continuity of 30-87

That should work fine. Any 12V source will be adequate.

ianc

dm36415 · dm36415's Garage

The relays are good, I have continuity between 30&87 while 85&86 are under load. No continuity when not under load. 30&87a have continuity when not under load.

I went ahead and tried to start the car while jumpering 30&87a. Lifted the sensor for a few seconds, cranked it over, the car started up and continued running fine.

I'm stumped. Is there a switch in the sensor that might be bad, or would that have identified itself when I did this:

"86 to ground is 12 volts with the ignition on. Jumpered 85 to ground with ignition on and pump did not run (even when I lifted the sensor)."

I've got to be overlooking something.

2.7RACER · 10-01-2006, 09:41 AM

Dave,
I've been thinking about your problem and it has led me to a better understanding of the circuit that controls the fuel pump in CIS motors.
First, the motor normally runs with the fuel pump relay is not energized. Yes, not energized.
You proved that, when you jumped 87a and 30 and started the motor.
So what is the purpose of the relay, if it just sits there when the motor is running?
If the motor is stopped for any reason, with the ignition on and the air sensor plate is in the down position, the switch at the air sensor is closed, continuity.
This energizes the fuel pump relay and blocks the power to the fuel pump.
This effectively stops the fuel pump.
It's a safety feature.
The fuel pump relay is a safety feature.
The fuel pump relay blocks power to the fuel pump when it is energized.
The fuel pump is energized when the fuel pump relay is not energized.
So whats with the fuel pump relay and that little switch on the air sensor?
That switch is normally closed, (continuity), when the sensor plate is down.
This energizes the fuel pump relay and blocks power to the fuel pump, until the ignition switch is rotated into the full start position.
Thats why the fuel pump doesn't run, unless the air sensor plate is lifted, unless the ignition switch is in the full start position.
Lifting the air sensor plate opens, (no continuity), the switch and de-energizes the fuel pump relay.
Lifting the air sensor plate with the ignition on thus causes the fuel pump to run.
When the ignition switch is cranking the starter it also sends 12 volts to pin 87 of the fuel pump relay.
From pin 87 the 12 volts goes to pin 30 of the fuel pump relay and then to the fuel pump.
So the injector fuel pressure is at the injector, ready to flow as soon as the air sensor plate is lifted by engine vacuum.
And as soon as the sensor plate is lifted the fuel pump relay is de-energized, which sends power to the fuel pump and the injectors keep flowing fuel.
Yea I know this sounds complicated.
The idea is to get the engine cranking and vacuum going before the fuel starts flowing through the injectors.
Now back to your problem, I would dis-connect the air sensor switch. This will keep the fuel pump relay off.
The motor should start with the relay in place, no need for a jumper.
If the motor doesn't start, likely, this means the wire to the air sensor is grounded somewhere, or the air sensor switch is stuck closed.
Simple check, lift the sensor plate, and check pin 85 at the fuel pump relay for ground. There should be no ground.
If you get a ground with the air sensor plate lifted, the wire is gounding somewhere, or the air sensor switch is stuck closed.
So, ground at pin 85 with the plate down, no ground with the plate up.
I know this is long but it should be enough info to fix this problem.
Good luck,