1971 911S with MFI hot start issue

I have a Targa 1971 911s with MFI.
Every time I start the engine in the morning I have no problem however when I stop for 30 min the car will not start until 30min after.
I think I know more or less the problem.....vapor lock.
What is my solution ?
Tony

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[Casey at Pelican Parts]

You need to check the residual fuel pressure. My bet is you have a residual fuel pressure loss.

- Casey

[jclotz]

I had this same problem with a freshly rebuilt 72 MFI engine. I think the problem was related to the thermo time switch. When the engine is too warm to activate the cold start circuit, but too cold to start without well without it, there is a problem. I ended up adding a intermittent switch to the dash that is connected to the solenoid on top of the fuel filter unit. In that situation, I hit the switch a couple of times which activates the solenoid and primes the MFI with fuel - and things start easily.

[regency]

I did the same thing jclotz did..........problem solved.

1973 911 T MFI Coupe, Aubergine

Steve

[dicklague]

Originally Posted by jclotz

I had this same problem with a freshly rebuilt 72 MFI engine. I think the problem was related to the thermo time switch. When the engine is too warm to activate the cold start circuit, but too cold to start without well without it, there is a problem. I ended up adding a intermittent switch to the dash that is connected to the solenoid on top of the fuel filter unit. In that situation, I hit the switch a couple of times which activates the solenoid and primes the MFI with fuel - and things start easily.

I did the same thing and took out the bi-metallic disk warm up regulator and put in and adjustment there. 6mm diam long bolt to exact and a fabbed cover.

I have a push button on the dash and a relay to prime the engine when cold. Works like a charm.

[Porboynz]

Originally Posted by Casey at Pelican Parts

You need to check the residual fuel pressure. My bet is you have a residual fuel pressure loss.

- Casey

Not on a MFI motor

[bkreigsr]

What is your right foot doing to the gas pedal while you are trying to hot-start?
Pressing or pumping during cranking will produce these results.
Your high idle lock lever should also be zeroed for hot starts.
Bill K

[RSTarga]

#1 what fuel are you using?
#2 try cracking your throttle about half way when activating starter. No pumping. Give it short burst of starter.
Most of these problems are due to the crappy fuel we get these days.
Double check your timing and dwell too.

[BURN-BROS]

Hi Tony,

Is this a Hot weather issue, cold weather issue, or both?

Vapor lock is becoming an issue an why people are playing with their cold start systems.

[Flieger]

It wasn't vapor lock for me, it was the thermal time switch. I use a push-button since I don't have the connection to the thermostat. Pump is fully leaned all the time, push button really helps starting, especially when hot.

[BURN-BROS]

Originally Posted by Flieger

It wasn't vapor lock for me, it was the thermal time switch. I use a push-button since I don't have the connection to the thermostat. Pump is fully leaned all the time, push button really helps starting, especially when hot.

Max! When vapor lock is present, little to no fuel is produced from the injectors. You are simply overcoming a lean condition due to fuel volatility when you bypass the cold start/ thermal time switch.

I discovered this issue a few years ago. I was tuning a car and when ambient temps reached 80 degrees, Idle mixture adjustment were no longer responding and I had very hard hot start issue.

I loosened one of the hard lines to the injector and notice nothing but fuel vapor was escaping. I removed the hard lines and replaced with plastic lines. this solved 90 percent of the hot run and start issues. The only part of the fuel system that was subjected to head temps was now the injector. The plastic lines also allowed me to see the bubbles that were forming when the hot engine sat for a period of time.


I believe fuel volatility is the real reason behind most of the hot start issues MFI cars have.

[Flieger]

I'll defer to the expert. I figured in my case it was just that the car had cooled down more than the thermal time switch "thought" so that it needed some enrichment to start. But I guess since it would keep running once started the vapor lock makes more sense.

[daepp]

Somewhere (either here or in the owner's manual) I learned to put my right foot well into it - at least half way down, turn key, let the pump run for 5-10 seconds, then crank. ALWAYS works for my MFI motor. IDK - maybe just lucky but it's worked for about a decade.

[Porboynz]

Originally Posted by daepp

Somewhere (either here or in the owner's manual) I learned to put my right foot well into it - at least half way down, turn key, let the pump run for 5-10 seconds, then crank. ALWAYS works for my MFI motor. IDK - maybe just lucky but it's worked for about a decade.

This works for me as well. Half throttle and crank when warm, hand throttle and crank when cold. Never bothered with waiting 10 seconds myself, no patience.