1976 911S 2.7 Fuel pump start problem

[thejet95]

Hello, so I have an issue with starting my car that has me puzzled. First off, its EFI converted. Turn ignition on and can hear my fuel pump run for a few seconds then shuts off. Turn to crank and it just cranks and cranks. I found if I squirt a small spray of starting fluid into intake before cranking it fires up immediately, and runs perfectly after that. Which led me to look at fuel pump relay. Tried swapping the red, and the black relay no difference. I followed an old thread on checking the pins on the fuel pump wiring, and it all checks out as normal. The pump gets power at ignition as expected and shuts off after a few seconds which is normal for the Bitz Racing EFI. It also is clearly running after it starts. I verified the fuel pump is not running while cranking. Had someone crank while under the car with multimeter. I cannot figure out what the issue could be. As I mentioned the car starts and runs great once I squirt some starting fluid. I took it for a 100 mile drive up the coast and it ran great, and it starts right up when warm. I went to lunch for an hour it cooled down and wouldn't start until I used the starting fluid. Any ideas of where to look?

[PeteKz]

On most EFI systems, the fuel pump runs for a few seconds to pressurize the system. It then shuts off the fuel pump until the engine starts and runs. "Safety feature." I'll guess that your fuel pump should also be powered by the current from the ignition switch when you turn it to "start"(like on the original CIS) and it sounds like that is not happening, because it does keep running after you get it started.

Does the Bitz EFI have some kind of fuel pressure accumulator? If so, also check that.

[thejet95]

Originally Posted by PeteKz

On most EFI systems, the fuel pump runs for a few seconds to pressurize the system. It then shuts off the fuel pump until the engine starts and runs. "Safety feature." I'll guess that your fuel pump should also be powered by the current from the ignition switch when you turn it to "start"(like on the original CIS) and it sounds like that is not happening, because it does keep running after you get it started.
Yes it uses the same wiring for CIS air safety switch.

Does the Bitz EFI have some kind of fuel pressure accumulator? If so, also check that.

Doesn't require the accumulator, so I removed it. Wondering if the actual ignition key wiring sends power to the fuel pump when cranking. Can't find information.

[PeteKz]

On SC's, the fuel pump relay powers the pump during cranking. On the 1974, the fuel pump does not have a relay--it runs all the time when the ignition is on. I'm not sure when that change occurred, probably with the SC series. You will have to study your 1976 wiring diagram.

[PeteKz]

By the way, is this a recent problem, and it was starting and running with the Bitz EFI before this problem occurred? Or does it correspond to installing the EFI?

[wazzz]

Originally Posted by PeteKz

On SC's, the fuel pump relay powers the pump during cranking. On the 1974, the fuel pump does not have a relay--it runs all the time when the ignition is on. I'm not sure when that change occurred, probably with the SC series. You will have to study your 1976 wiring diagram.

This safety relay with air sensor plate micro-switch was introduced on 1976 2.7 models. I don't see it on 74 and 75 diagrams. First appearance is on 76 diagrams.

[thejet95]

Originally Posted by PeteKz

By the way, is this a recent problem, and it was starting and running with the Bitz EFI before this problem occurred? Or does it correspond to installing the EFI?

The EFI was installed and had zero issues with starting for several weeks then one day it just wouldn’t start. I thought it must be a bad check valve or the fuel accumulator. The fuel pressure gauge drops to zero when it shuts off. Then I read that EFI doesn’t require an accumulator so I removed it. No change. That’s when I looked at the relay wiring and figured out no power gettting to the pump while cranking. For now I just drive around with starting fluid for when it gets cold, but I’d love to figure it out. Thought about installing a standard relay bypassing the oem relay circuit if I can find the wire that is hot during cranking?

[boyt911sc]

Originally Posted by wazzz

This safety relay with air sensor plate micro-switch was introduced on 1976 2.7 models. I don't see it on 74 and 75 diagrams. First appearance is on 76 diagrams.

Correct. The CIS airflow meter sensor switch for 911 was first introduced in 1976 CIS and lasted up to 1983.

Tony

[al lkosmal]

Greetings.....I just reviewed the Tbitz install manual and tony has the fuel pump relay wired as shown below. looks like power, while cranking is provided by the starter yellow wire......once the engine starts, the pump disable (M'squirt pink wire) changes state and the pump relay switches to the red power wire (constant 12V)

I would verify that the yellow wire connection is intact and providing 12V to the relay, while cranking the engine over with the starter.



regards,
al

[thejet95]

Originally Posted by al lkosmal

Greetings.....I just reviewed the Tbitz install manual and tony has the fuel pump relay wired as shown below. looks like power, while cranking is provided by the starter yellow wire......once the engine starts, the pump disable (M'squirt pink wire) changes state and the pump relay switches to the red power wire (constant 12V)

I would verify that the yellow wire connection is intact and providing 12V to the relay, while cranking the engine over with the starter.



regards,
al

Thanks for that! I reviewed the manual several times and I saw the yellow wire and my eyes only read the “from starter” and didn’t notice it says hot while cranking! I’m going to check that, thanks alot!

[TwoBluPorsches]

Here's the fun thing about that circuit. It kind of works backwards from what most people would expect.
If you expect that the relay energizes to make the pump run, you'd be mistaken.
The pump will run if the relay is Not energized and the ignition is on.
The CIS micro switch contact would energize the relay only when the flow plate was closed.

in the original configuration the pump does not run when the ignition is on and the CIS flapper is at the bottom. It was possible to push up on the CIS plate and hear the pump start and the injectors start to flow fuel.

When I modified my car to run with an EFI, I discovered that the pump runs all the time because the CIS switch wiring is open.

Most electric fuel pump relay wiring and function work the opposite.
When I added a GM TBI to an old truck I discovered that it would provide an energizing signal to a fuel relay for a few seconds and then remove the signal if the computer did not see a tach signal. When the tach signal was gone for a short period of time, the fuel pump would stop.

[thejet95]

Yeah that circuit running backwards doesn’t make it fun to troubleshoot. Tony modified the megasquirt to work with the factory wiring. That pink wire in the diagram above is connected where the plate used to be I think. My car when running CIS had the fuel pump run constantly when the key was turned. I finally fixed it, somehow a connector was undone. I reached out to Tony at Bitz racing and he said if it is shutting the pump off after a few seconds of ignition on then it’s working correctly.

So I just finished checking the relay wiring. The yellow wire is indeed on pin 87. I unplugged the connector in the photo I attached and checked continuity and it’s good. I then checked for voltage at the yellow wire when cranking and it does indeed. However it is only showing 10.4 volts while cranking. Wondering if that is why the pump isn’t running while cranking? If so not sure why the voltage would be low. Everything else is 12.4 or so.

[thejet95]

[TwoBluPorsches]

My EFI doesn't mess around with controlling the relay.

Car turns on the pump with the key switch and it goes off when switch is off.
Just like yours did when the CIS switch wiring was open.

I hear pump running when the key is on and engine not running, so it's a pretty good reminder to turn the key all the way off.

Occasionally the car will over fuel on a hot start so I'll have to pull the relay to be able to clear the flooding.

If I had that part of my tune figured out, I wouldn't care that the pump runs all the time.
Being well aware that there are safety implications I'm ignoring and it would be the smart thing to have the pump off when the engine is off.

[thejet95]

I’m going to try and ground the pink wire to “bypass” the safety switch and see if that makes a difference.

[PeteKz]

Al, thanks for that wiring diagram.
Let me add to what TwoBluPorsches said. The relay socket for the fuel pump relay has 5 pins in the socket. Although all the relays have 5 pins, the other relay sockets in the car only have 4 pins (the 87a are not used). If you need to replace that socket, be aware of that difference. I just bought a new socket for my 1980SC engine into my 1973.5T rewiring project (to add the fuel pump shutoff safety feature) and had to make sure it was a 5-pin version.
Also, FYI, the normal current routing through the other relays is power on pin 30, and power out pin 87. This is opposite on the FP relay. Be aware if you start fiddling with the wiring.

[PeteKz]

Originally Posted by thejet95

I’m going to try and ground the pink wire to “bypass” the safety switch and see if that makes a difference.

Don't do that. If I understand you correctly, you will only have power to the pump when cranking--the opposite of what's happening now.

Instead, for testing, jumper the pins 87a to 30 so the pump runs all the time when the ignition is on.

[thejet95]

Thanks for the information! I didn’t know they mixed 4 pin and 5 pin relays. I installed a temporary relay between the yellow wire connector going to pin 87 because of the low 10 volts it was getting. It increased it to 11.5 volts but car still won’t start up unfortunately. Now I’m back to wondering about the fuel side, check valve?

[PeteKz]

Re the low voltage: If it drops to 11 or 10.5 volts while you are cranking the engine, that's normal. The battery is putting out around 80-100 amps to crank the engine, so the voltage from the battery will drop with that much load. The voltage at the relay should be at either battery voltage (12.5-ish) with the ignition on and engine not running, or at 14-ish when running and charging. The fuel pump will run sufficiently on 10 volts or even less.

[TwoBluPorsches]

Originally Posted by PeteKz

Don't do that. If I understand you correctly, you will only have power to the pump when cranking--the opposite of what's happening now.

Instead, for testing, jumper the pins 87a to 30 so the pump runs all the time when the ignition is on.

PeteK is correct, no need to ground the pink wire as it will do the opposite of what you want.

You could jumper from 30 to 87A
Or, realize that the relay is a jumper from 30 to 87A and those terminals are normally connected when the relay is not energized. (BTW, this is what it shows schematically)

I'd start by removing the pink wire connection from the relay and see if the pump comes on with the ignition switch - it should.

With the 85 connection disconnected you should have a running fuel pump all the time when the ignition switch is on and in run or start.
That's how it is on my '76