Hot Start Problem - 911 CIS

[Charlie V]

Any engine management experts out there?? I need your help...........

My 82 911SC will not start when it is hot. When starting it hot, the engine will flash up and immediately die. What I have done to date....
1. replaced the fuel accumulator - no improvement
2. pressure tested fuel system - within spec. Check valve on fuel pump OK.
3. replaced fuel pump relay (it would get very warm) and cleaned all electrical connections on the fuse panel

When I remove the wire connection on the senor plate contact (connected on the mixture control unit housing - located behind the air flow sensor plate) the fuel pump is on continuously and the car starts and runs. However, idles higher and fuel
consumption is up.

So - my questions are:
- What does the sensor plate contact control?
- How does it control?

My Haynes manual does not have any information on this control. And, the Bentley manual does not get into any detail.

Taking it to a shop is way too easy - I'm one of those DIY guys. Besides, I would rather spend the money on tires and upgrades.

[ewave]

"So - my questions are:
- What does the sensor plate contact control?
- How does it control?"

It just a safety feature. If no air is passing through the intake plenum (motor has stopped) the fuel pump is turned off. - Imagine getting in a wreck, rolling the car. You are unconscious, the engine is stopped, the fuel line damaged, and the ignition of your car is still on. Without the sensor plate contact the fuel pump would continue to run, pumping fuel all over the place.

[Charlie V]

Thanks. This solves one more thing as I understand how the system works.

[Jim Williams]

Charlie,

The switch on the air flow sensor is a ground contact in the fuel pump relay circuit. It's operation is described here .

The ground connection is broken when either a) the air flow sensor plate rises off it's stop due to the engine ingesting air, or b) when you disconnect it.

So when you disconnect the switch you simply force the fuel pump to run continuously whenever the ignition is in "Run". It's not apparent (to me) how disabling this safety feature would affect your idle speed or fuel consumption.

[Charlie V]

Thanks again. I stubbled onto the site you linked through other topics on CIS. I will study the contents.

[Charlie V]

I found the problem. Car will now start hot, cold and everything in between.

After 5 months of frustration.

Some more history of the challenge. I also replaced the electrical switch portion of the ignition switch and cleaned all the major ground points in the car. Nothing.

Pulled the connectors off the "brain" under the passenger seat (aka. oxygen senor control unit, cold acceleration control unit and relay), cleaned then and BINGO. Car starts just fine and runs smoother too.

Now I can drive anywhere and not have the sick feeling when I try to start it hot. Or, wait 30 to 45 minutes to let it "cool down".

Hooray!!