911SC starting troubles

[Selectbob]

HELP! I'm sure others have seen this problem, but can't find the answer by searching this forum.
My 1979 911SC has just had the engine refitted after rebuild (usual broken head stud problem). It was running/starting fine before rebuild. Now, when I turn the key the motor turns over OK and appears to fire if I hold the key in start position, but dies as soon as the key is released. Everything electrical is original. I'm not sure what to check next, all connections appear correct, but I do not know what voltages, resistance etc. should be present at the coil.
Any ideas?

[timmy2]

Have you bled the air from the injection system? You need to pull the injectors and bypass the pump relay and then lift the air metering plate until fuel flows from all the injectors. 6 baby food jars will contain the fuel.

[timmy2]

Then you will need to test your fuel pressures and mixture.

[boyt911sc]

Originally Posted by timmy2

Have you bled the air from the injection system? You need to pull the injectors and bypass the pump relay and then lift the air metering plate until fuel flows from all the injectors. 6 baby food jars will contain the fuel.

Dennis,

This is another myth . This may apply to carburated system (low operating pressure @ 5 psi.). Unlike fuel injection CIS, where the system fuel pressure is @ 70 psi. and injectors operate between 40 - 50 psi. So any air/gas present in the fuel line system would be expelled/displaced as soon as the FP and injectors start to run. Gasoline is an incompressible fluid. One thing many people forget is that a lot of fuel is recirculated back to the fuel tank in a CIS. But I completely agree with your suggestion to run the FP briefly to help pressurize the fuel system. No need to pull the injectors unless you want to inspect the spray pattern of each injector and check for drip.

This is what I usually do during a start-up of an engine either after a rebuild or simply a start-up. Check the following:
1. Ignition.
2. Fuel pressures.
3. Source of vacuum leak/s (only if it failed to start).
4. Battery fully charged.

Stay away from tinkering the mixture setting specially if the engine ran before. This is used for fine tuning and not for starting an engine unless the setting is way off the chart. This is where a lot of people get into problem with their CIS engines. And a simple mistake is magnified by this lack of understanding.

Tony

[timmy2]

Tony, my method was a suggestion to confirm all injectors are working and flowing. If he has fuel, then he needs spark.

[boyt911sc]

Originally Posted by timmy2

Tony, my method was a suggestion to confirm all injectors are working and flowing. If he has fuel, then he needs spark.

Dennis,

I agree with you and should be part of a regular procedure for start-ups.

Tony

[Selectbob]

Thanks for the tips, but I'm sure that the problem is electrical, it does fire when key is turned. However it seems as though the spark fails as soon as the key is released. Can anyone make any sense of this?

[1982911SCTarga]

Bob, if you suspect it's electrical, go back over every connection, particularly the two plug-in engine harness blocks. Use a mirror and a light to make sure the rear one by the firewall is seated properly. It's possible to miss those connectors by a row sideways or length-wise.

Brian

[boyt911sc]

Originally Posted by Selectbob

Thanks for the tips, but I'm sure that the problem is electrical, it does fire when key is turned. However it seems as though the spark fails as soon as the key is released. Can anyone make any sense of this?

Bob,

You could be correct that the problem could be electrical but you don't know that. Or it could be fuel related. You don't know that too. Test and confirm. If I were doing the troubleshooting:
1. I'll check the fuel pressures (system, control, and residual).
2. Hook up an inductive timing light and crank the engine. Look for signal. Determine and establish the presence of ignition sparks.
3. Provide a fully charged battery and fresh fuel supply.

Unless you have a big air leak in the system, the engine should turn and start. Keep us posted.

Tony

[Selectbob]

Well, it's sorted now. Having stripped and rebuilt, putting everything back as it was (or so I thought) I expected it to start!
However, I had swapped the cold start and fuel cut off terminals (at least I think that's what these 2 are for), meaning fuel pump wasn't working some of the time. These terminals both look the same and are hidden away at the front of the engine. changed these and fired up straight away.
Thanks for the help.
Bob

[Bob Kontak]

Good job.