MFI 1969 911S 2.0 starting problems?
 

Hello,

After reading a lot and spending 2 years on having my 911 through MFI CMA there's still something not solved yet.

The car starts, cold or hot, but if the car has been stopped for more than about 2 or 3 hours you have to wait at least 2 minutes until the car can be driven. You press the gas pedal and won't rev up 1500, smokes and don't want to rev up.
Once it rev's up, no problem, drives perfectly.

Cold start solenoids work as they should, any ideas???

THANKS!

Josep

Wow, that is a tough one. I am subscribing since I am curious as well.

Is the fuel pump on top of the driver side rear anti-sway bar mount for 1969s? My 1971 is like that. If so, perhaps it is a heat-soak issue in the fuel pump, causing it to get too hot, or perhaps a sort of vapor-lock problem?

How long does it take before it functions normally? Does it suddenly become drivable or is it a gradual improvement? When you go to start the car in this time period, it starts fine but just does not rev?

The fuel pump is under the gas tank in front. The fuel pump is also checked.

Yes, the car becomes drivable and you can run without problems then.

Thermo time switch?

I agree. It sounds like the enrichment device is working after the car is warmed up which is causing it to run very rich on start-up. Unplug the wire on top of the fuel filter canister and see if the problem continues.

When he said that the cold start solenoids were working, I figured he meant that the cold start squirters were squirting fuel once, then stopping. The thermo-time switch is activating the cold start system, but he does not mention if he has checked to see if the squirters are continually squirting fuel.

Does the cold start system work on a piston type mechanism with the solenoid squirting fuel or does the solenoid simply open a valve and allow pressure to flow to the squirters?

tangerine69s,

Does the car gradually become drivable or is it an abrupt change where it suddenly runs cleaner?

2-3 hours seems like enough to be a "cold start", so it seems that enrichment would be needed.

Also, if you post this on the Early 911S Registry, you may get the solution sooner.

Early 911S Registry

On a 69 the enrichment system is by the solenoid on the MFI pump.
I disconnected mine and ran a circuit to the unused windshield washer button on the dash, so I had complete control for the cold start circuit.
The fuel system is a restricted return type...so the front fuel pump provides proper pressure to the MFI pump, and the excess is bled off back to the tank.
I also agree that the cold start system sounds like the problem...so if the cold start relay is temporarily disconnected (for that no start 2 -3 minutes) does the car start and idle properly?
There are 2 sensors on the breather enclosure, I believe one of those is the trigger for temperature for control of the enrichment system.
Hope this helps.
Bob

It's abrupt, it suddenly runs cleaner.

I'm going to disconect both systems, the one on top of the fuel filter and the one in the mechanical pump and will tell you what happens.
I checked the pump solenoid with a tester and gets proper voltage only when crancking, not later.

You can peak under the air cleaner with engine running when you have the smoke/poor run condition and look for fuel coming out of the rails on the top inside of the air cleaner (obviously there should be none). Be careful if you chose to do this test as if there is fuel you do not want it to go anywhere but down the stack. This would eliminate possibilities of electrical problems and a sticky solenoid associated with your fuel filter assembly.
However, it would not eliminate a problem with the electricals or solenoid on the MFI pump.

I tried disconnecting both solenoids and the problem continues the same so the electric part seems discarted.

I opened the air filter housing and there's something in there, seems oil, not fuel.

Any suggestions?

By "both solenoids" I'm going to guess you mean the two on the back of the pump. One of these is the decel solenoid, which is used to shut off fuel delivery when you lift off the gas. The other, on a '69, is an electric solenoid that controls the cold start mix by pushing a plunger against the main rack. Disconnecting either of these will not solve the problem you describe.

The solenoid you need to disconnect is the one on top of the fuel filter console, in the forward left corner of the engine bay. This is the solenoid that is activated by the thermal time switch. Its function is to re-direct the pressurized fuel delivery from the electric fuel pump that normally goes to the MFI pump; it re-directs that flow directly into the six intake stacks themselves, providing a priming charge. Just like pumping the gas on a carbureted car to squirt fuel from the accelerator pumps.

It sounds like it is taking too long for the thermal time switch to heat up enough to trip, so this solenoid is staying energized too long. Disconnect the single wire at the very tippy top of the fuel filter console. Oh, and reconnect the two on the MFI pump itself. Let us know how that works.

If there is no fuel dispensed by the air cleaner when running, I would revisit (as you state you have been through he CMA) the thermal disks in the MFI pump. Verify free movement, clean, orientation, and number of disks.

That's what I was thinking to do next. I will do it.

I have already done so but no difference.

By the way, the smoke is white while the car is not running correctly.

I would also pull plugs immediately after running bad, without getting to the stage of running good. Look for differences in the plugs (ie wet, oiled etc.). Also check rotor resistance and cap integrity.

So it's white smoke! Not the over rich black smoke. The problem could be that the thermostat is sticking in the warmed up state to long and after your 2 to 3 hour cool down the sticking thermostat is still expanded and making the MFI pump to lean for a cool start, not cold enough for the cold start system to work yet. Just a few minutes of poor running in that sticking condistion, not being able to rev up past 1500 to 2000 rpm, then all of a sudden the vibrations of the engine free up the sticking thermostat and snap.......the engine gets the needed extra fuel enrichment, runs smooth and drives great. I have seen this problem on my test stand when testing pumps.

If the above suggestions do not help, look at your fuel pump. You should be able to hear it before you start the car. Believe it or not, these engines will start and run poorly with no fuel pump. The MFI will pull in the fuel.
I know this because as an anti theft, I disconnected the power on the pump only to find the car still ran well enough to move the car.

I can see this on the later exhaust heat activated warm up solenoids, but would the earlier electric warm up solenoids do the same thing?