1973 MFI Question for MFI Gurus

[Terry Worick]

I have a 1973 911S w/MFI that has been completely rebuilt (engine and MFI system). I sent the MFI pump and injectors to Guss at Pacific Fuel Injection). Upon install I followed the Check Measure Adjust procedures and all other precautions as directed. The good news is the car runs fantastic!

I only have one small glitch. If the car is driven then parked the start up is slow. It will initially fire on a few cylinders and the the others will gradually join in.

It is my theory that the injectors leak out fuel from the lines (metal injector feed lines) when hot. Then upon trying to re-start it takes a few seconds to pressurize the lines where the injectors may have leaked out.

My question is, has anyone had experience with this condition?

I can buy the new injectors to try to eliminate the problem but it is an expensive experiment.........

As I said the system functions perfectly with this one exception, the problem can be lessened by stepping down on the gas pedal when the condition shows its self. I discussed the problem with Guss and he suggested using the gas pedal to lessen the problem. I would like to know if any one here has any thoughts on the subject. Any help would be greatly appreciated.

I do have the cold start system disabled.

[matt demaria]

Put a fuel pressure gauge on the inlet line to the pump to see if it loses pressure while sitting. The check valve is in the filter housing.

[356RS]

I don't think it's your injectors leaking. If they were leaking/dripping fuel when the engine is off, you would be having a big idle problem and running problem for that matter. The MFI metal fuel lines & injectors are not designed to hold fuel pressure with the engine not running, un-like the CIS injection system which does hold pressure. As long as there's fuel in the metal lines the first second of cranking the engine/MFI pump (not the electric fuel pump) you will have 240 psi in the lines.
When your starting problem occurs, is there a lack of fuel? to much fuel?
You mentioned the cold start device was disconnected?

[dtw]

Are you opening the throttles on hot restart per the owner's manual?

[Terry Worick]

Ill try to answer a few of the questions......

When its hot, within the first 0-30 minuets it will start instantly with no effort or throttle.

Its if you wait longer the problem occurs. It seems like the non firing cylinders are not getting fuel.

I have tried a few starting methods,

1.) turn on key, wait 15 seconds, lift hand throttle full, crank 5 seconds, repeat.
If its completely cold this will make it start well on the second cranking.


2.) depress accelerator pedal 3/4 and crank. ( makes it start faster when problem happens).

It really seems like some cylinders are slow to get fuel when problem happens.

356RS, I sent you that beautiful 72 oil tank have you put it to good use?. Thanks for your help.

[jclotz]

I had this same problem with my newly-built 2.7 MFI. It would cold start fine, and start fine after a short off period when hot. But would be hard to start if it sat for about an hour or so - when not hot or cold. I fixed the problem by adding a momentary switch (actually a button) at the dash that would trigger the cold start solenoid and inject some fuel. Now when in that condition I turn the ignition on to start the fuel pump, open the throttle half way, give the button a couple of pushes and it starts great.

[Terry Worick]

jclotz- I seriously thought about doing that.........

[BURN-BROS]

I have seen fuel percolate @ the injector and exhibit the same hot start issue.

You can see it if you run plastic injection lines.

The problem is that it's not leaking anywhere....Gasses are forming within the lines preventing the injector from firing for a few cycles.

jclotz solution allows for a bit of extra fuel to help get thru the issue.


Edit; The condition can be worse with steel lines as it allows head temps to run up the lines further than with the plastic lines.

[regency]

I too had the same prob....I did the exact same thing that jclotz did and end of problem.

1973 911 T MFI Coupe, Aubergine

Steve

[kwikt 911]

Same problem here and only happens in very hot conditions. I know it will always start though but annoying.....sort of like a nagigng wife.

[GeorgeRud]

Fuel percolation certainly sounds like a reasonable cause. The vapor pressure of today's ethanol laced fuel is much different than it was in the 70's, and boiling fuel is a real problem.

[cmcfaul]

I would put my money on a cold start valve malfunction. If the cold start valve in on it will flud the engine making it hard to start when hot. When the engine is cold it sounds like it is working properly and priming the engine for start until fuel reaches the injectors.

It's relatively easy to reach back there and pull the wire off off the cold start switch. Or, even easier to pull it right off the top of the fuel filter. I bet it solves the problem.

Chris

73 911 E