1982 porsche 911sc starting issue

[Jowniofownio212]

Hello

I have seen some very helpful replies on here regarding starting issues but unfortunately have been in able to use to resolve my starting issue. I have done a number of tests and read lots of information and am now more confused as to what is going wrong!

Basically, my car never starts on first attempt. If I turn of then on again, 9/10 times it always goes on the 2nd go. I want to resolve this.

It happens when it’s hot or cold. It runs smooth on start up hot and cold.

I have tried swapping the fuel pump relay with a black one and it was the same fault.

I have bench tested the relay and it works.

I have followed a test procedure on here which was written by boyt911sc:

Here is the basic test for the FP and relay circuitry:
1). Pull out the FP relay and do the test (ignition switch @ OFF position) for the individual terminal at the socket: A simple 12-volt self-powered test light will work well for this test or a regular non-powered test light, or a volt meter.

87...........no power
87a.........no power
86...........no power (87a 86 are bridged)
85...........no power (should be grounded at this point)
30...........no power (has ground contact?)

2). Turn ignition switch @ ON position (not start):

87............no power
87a..........has power
86...........has power
85...........no power, presence of ground contact
30...........no power, presence of ground contact

3). Next, insert the FP relay into the socket and turn the ignition SW @ ON position (not start):

87...........has power
87a.........no power
86...........no power
85...........should have ground contact
30...........no power but exhibit ground contact.


For me, test 3 I have nothing on all pins except I have 12v at 86 and 87a. I would have thought in this state, there should be 12v at 86 as the relay is energised but the test about says different?

If I manage to run the fuel pump before trying to start the car it does start first time so it has to be something to do with the pump not running when cranking

Any help would be much appreciated

Thanks

Jon

[Joe Bob]

Check valve in the fuel pump neck? Cheap part.

[High Life]

Originally Posted by Jowniofownio212

Basically, my car never starts on first attempt. If I turn of then on again, 9/10 times it always goes on the 2nd go. I want to resolve this.

Not clear to what is happening when you say it never starts...Does the starter engage and the engine turns over - or do you get nothing (no click)?

If no click - I would think the ignition switch could be faulty and not completing the circuit (mine does something similar off and on). The switch seems to be a common issue:

https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/91-Ignition_Switch_Repl/91-Ignition_Switch_Repl.htm

[Jowniofownio212]

Originally Posted by High Life

Not clear to what is happening when you say it never starts...Does the starter engage and the engine turns over - or do you get nothing (no click)?

If no click - I would think the ignition switch could be faulty and not completing the circuit (mine does something similar off and on). The switch seems to be a common issue:

https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/91-Ignition_Switch_Repl/91-Ignition_Switch_Repl.htm

Yes, on first attempt, the starter turns the engine over and I can keep trying until the battery would finally die.

[Jowniofownio212]

Originally Posted by Joe Bob

Check valve in the fuel pump neck? Cheap part.

Possibly but how could this give an intermittent fault like this?

Thanks

[Joe Bob]

The check valve pulls residual pressure into the starting circuit. If it bleeds down car won’t start and Hass to build up pressure from the fuel pump.

[boyt911sc]

Originally Posted by Jowniofownio212

Hello

I have seen some very helpful replies on here regarding starting issues but unfortunately have been in able to use to resolve my starting issue. I have done a number of tests and read lots of information and am now more confused as to what is going wrong!

Basically, my car never starts on first attempt. If I turn of then on again, 9/10 times it always goes on the 2nd go. I want to resolve this.

It happens when it’s hot or cold. It runs smooth on start up hot and cold.

I have tried swapping the fuel pump relay with a black one and it was the same fault.

I have bench tested the relay and it works.

I have followed a test procedure on here which was written by boyt911sc:

Here is the basic test for the FP and relay circuitry:
1). Pull out the FP relay and do the test (ignition switch @ OFF position) for the individual terminal at the socket: A simple 12-volt self-powered test light will work well for this test or a regular non-powered test light, or a volt meter.

87...........no power
87a.........no power
86...........no power (87a 86 are bridged)
85...........no power (should be grounded at this point)
30...........no power (has ground contact?)

2). Turn ignition switch @ ON position (not start):

87............no power
87a..........has power
86...........has power
85...........no power, presence of ground contact
30...........no power, presence of ground contact

3). Next, insert the FP relay into the socket and turn the ignition SW @ ON position (not start):

87...........has power
87a.........no power
86...........no power
85...........should have ground contact
30...........no power but exhibit ground contact.


For me, test 3 I have nothing on all pins except I have 12v at 86 and 87a. I would have thought in this state, there should be 12v at 86 as the relay is energised but the test about says different?

If I manage to run the fuel pump before trying to start the car it does start first time so it has to be something to do with the pump not running when cranking

Any help would be much appreciated

Thanks

Jon

Jon,

I believe you have copied an old post of mine that had incorrect data. This was later identified to have incorrect information and corrected. See highlighted in bold letters.

Test #3: Should be like these (corrected version).
87................no power
87a..............has power
86................has power
85................presence of ground
30................no power/presence of ground

Later in the thread, I made the correction. See post #28 in DKLever48 thread where you quoted the above post.

Tony

[Jowniofownio212]

Originally Posted by boyt911sc

Jon,

I believe you have copied an old post of mine that had incorrect data. This was later identified to have incorrect information and corrected. See highlighted in bold letters.

Test #3: Should be like these (corrected version).
87................no power
87a..............has power
86................has power
85................presence of ground
30................no power/presence of ground

Later in the thread, I made the correction. See post #28 in DKLever48 thread where you quoted the above post.

Tony

Ahh makes sense now! So my tests are good then. I have put my tester on pin 30 as I crank it with the key in start and don’t get 12v there until the engine fires up and it’s running? Surely this is incorrect as how else would I get fuel pressure at start up?

[boyt911sc]

Originally Posted by Jowniofownio212

Ahh makes sense now! So my tests are good then. I have put my tester on pin 30 as I crank it with the key in start and don’t get 12v there until the engine fires up and it’s running? Surely this is incorect as how else would I get fuel pressure at start up?

Jon,

With the ignition @ ON (not START) position, the NC (normally closed) terminals #87a-#30 will switch to #87-#30 NO (normally open) terminals. So the presence of power @ terminal #30 coming from 87a will be lost momentarily when it switches to 87-30. Then as you switch to START position, the CSV and the starter are energized causing the motor to turn and create vacuum and a blast of fuel coming from the CSV (provided the TTS is grounded) followed by the 6 fuel injectors.

Once the the airflow sensor is lifted up by the engine vacuum, the AFS switch will open and energize the FP to run and move the FD plunger up. So there is an short period of time that the fuel supply is only coming from the CSV until the FP takes over.

Tony

[Jowniofownio212]

Originally Posted by boyt911sc

Jon,

With the ignition @ ON (not START) position, the NC (normally closed) terminals #87a-#30 will switch to #87-#30 NO (normally open) terminals. So the presence of power @ terminal #30 coming from 87a will be lost momentarily when it switches to 87-30. Then as you switch to START position, the CSV and the starter are energized causing the motor to turn and create vacuum and a blast of fuel coming from the CSV (provided the TTS is grounded) followed by the 6 fuel injectors.

Once the the airflow sensor is lifted up by the engine vacuum, the AFS switch will open and energize the FP to run and move the FD plunger up. So there is an short period of time that the fuel supply is only coming from the CSV until the FP takes over.

Tony

Ahh ok, so looks like relay and pump circuit is good. From the symptoms I have, what is likely the cause and what tests are carried out to diagnose the faulty part?

[timmy2]

Time to check your fuel pressures.

[stlrj]

Time to replace the fuel pump check valve and that's unanimous.

[timmy2]

I prefer testing prior to parts replacement
If the residual fuel pressures are crap, then yes, check valve or fuel accumulator.
Better to know what is going on before throwing parts at it.

[Gretz]

I agree with Dennis. With CIS there could be a combination of factors causing problems. If you don’t really dig in and test and understand what is happening you will end up wasting a bunch of time and money.

[boyt911sc]

Jon,

Do yourself a favor by checking your fuel pressures:

Cold control pressure
Warm control pressure
System pressure
Residual pressure

Do not replace CIS components without testing them. Anyone suggesting to replace any part/s without testing or verifying them does not fully understand the system.

The three (3) common culprits for poor residual pressure are:
1). Defective or leaking FA.
2). Defective or leaking FP check valve.
3). Defective or leaking FD primary valve.

Tony

[Jowniofownio212]

Been doing some tests. Correct me if this is wrong regarding the fuel pump check valve:

Lift the airflow sensor on the engine with the key in the run position and it will turn on the pump and pressurize the system and add fuel to the cylinder to start.
If the check valve is the problem, it should start,

I lift the air sensor behind the air filter while ignition is on, the fuel pump runs. I then go and start the car and it starts first time! Does this prove the check valve is not holding pressure and I should replace?

Thanks

[boyt911sc]

Originally Posted by Jowniofownio212

Been doing some tests. Correct me if this is wrong regarding the fuel pump check valve:

Lift the airflow sensor on the engine with the key in the run position and it will turn on the pump and pressurize the system and add fuel to the cylinder to start.
If the check valve is the problem, it should start,

I lift the air sensor behind the air filter while ignition is on, the fuel pump runs. I then go and start the car and it starts first time! Does this prove the check valve is not holding pressure and I should replace?

Thanks

Jon,

What you had demonstrated is your problem with residual fuel pressure. It could be the FP check valve but you don’t know that as a fact. You are just guessing and hoping to be lucky. Test and verify that the FP check is the culprit. Stop doing guess-work troubleshooting.

Test your residual fuel pressure. If you are losing pressure after 15~20 mins., you got to identify the culprit/s. Start with the fuel accumulator. This is the easiest to test among known culprits. Then go next to FP check valve. There is a third culprit but leave it alone for the moment. Do you have fuel pressure gauge kit?

Tony

[Jowniofownio212]

Originally Posted by boyt911sc

Jon,

What you had demonstrated is your problem with residual fuel pressure. It could be the FP check valve but you don’t know that as a fact. You are just guessing and hoping to be lucky. Test and verify that the FP check is the culprit. Stop doing guess-work troubleshooting.

Test your residual fuel pressure. If you are losing pressure after 15~20 mins., you got to identify the culprit/s. Start with the fuel accumulator. This is the easiest to test among known culprits. Then go next to FP check valve. There is a third culprit but leave it alone for the moment. Do you have fuel pressure gauge kit?

Tony

Tony,

Thanks for very helpful advice. All makes sense. I do not have the means to test the fuel pressures as don’t have the kit. That is why I’m doing a bit of guess work. You have confirmed to me that there are a number of reasons I may be losing pressure so will have to take it to get tested.

Thanks

Jon

[timmy2]

Jon, where are you located? Maybe someone nearby has a tester you can use.
The testers are relatively inexpensive, about the price of 1 hour shop labor or less in today’s market, and you have it when you need it.
Here is one for $94
https://www.tooldiscounter.com/ItemDisplay.cfm?lookup=SAG33800&source=froogle&kw=SAG33800&gclid=EAIaIQobChMI7tvu7Yic5AIV1h6tBh2CmgqYEAQYAiAB EgL9ePD_BwE

[gomezoneill]

Check the cold start circuit and the csv also. CSV could be NG or the thermo time switch could be faulty. These can easily be ruled out be simple testing with a VOM.