How does the MFI temperature switch work

[MaRu]

Hello everyone,

I am working on a 72 2,4T where the cold start injection does not work. The solenoid works, but I am not sure about the relays and the temperature switch.
When I start the car cold I have + on 85 and 86. So does the temperature switch "send" the positive + signal from the starter to the relays, or does it ground the reais?

Many thanks, regards

Malte

[dicklague]

I had the same question when I got my 73 911 15 years ago. It was not working at all and the advice I got was to hold the throttle 1/2 open on start.

Well I started researching it and was not impressed with how they designed it. I installed a relay and a push button on the dash, and eliminated the cold start circuit.

It is like "tickling" the carb on an old motorcycle. You turn the key on. Let the pump build some pressure press the button for 2 seconds crank the motor, and it starts. Crack the throttle a bit. If the RPM goes down hit the button a couple of times. Works perfectly. At least you are in control and can be precise with the additional fuel.

Once I became a Pelican I found out this is a common modification.

I also modified the warm up regulator, but you did not ask about that.

[tharbert]

I think this will answer all your questions: MFI Cold Start Saga Continues. Did mine...

[tobluforu]

Yes it works great, mine is fully stock.

[MaRu]

Thank you very much! This answered all my questions!!! I will check tomorrow, but with these infos I am pretty sure the temperature switch is shot.

Regards Malte

[tharbert]

New cold start switches are available from our host (911-617-117-00-M100). You can occasionally nab one on the used parts thread. Mine tested just fine, but, if it ever breaks, I'll go with the push button on the dash.

I also replaced the fuel lines and cleaned up the squirters. The fuel lines are notorious for leaks. The little squirter nozzles have a check valve that gets gummed up allowing the system to lose it's prime. When the fuel can leak out, the system is almost useless as it take a few cranks to refill it with fuel by which time, the switch cuts off.

Be very careful with those squirters/nozzles. You almost can't find them anymore.