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Thread: MFI warm start issue

  1. #41
    '72 911T 3,0 liter MFI Albert Blue street/DE toy Jeff Higgins's Avatar
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    In my case, the final solution was one that was mentioned earlier in this thread - simply bypass the thermal time switch and wire the bypass solenoid to a push button on the dash. I understand that this may not be acceptable to those with entirely stock cars who would like to keep them that way. The push button is, however, very discrete, at least where I mounted it. There was an unused hole in my dash, directly to the right of the steering column, that was covered by the dash facia panel. I simply cut a hole in that panel with a razor knife. The hole was the perfect size to accept a momentary contact push button from Radio Shack. Anyway, since performing this modification, I have never been stuck with a vapor locked system. I can manually introduce fuel, and that will get it running long enough for the MFI pump to push fresh, cool fuel to the injectors.
    "God invented whisky so the Irish wouldn't rule the world."

  2. #42
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    I'm chasing this too on my one-year-only 1969 S MFI dual solenoid with the cold-start rack enrichment solenoid above the cut-out solenoid on the MFI pump.

    I'm lucky enough to have TWO Thermo time switches- 'cold' and 'frigid' @ $350 a pop.
    They are called on the wiring diagram: "71" Temperature switch (start) for temperate weather and "72" Temperature switch (start assist) for the raw gas primer sprayer for arctic driving.

    Both go off when your ice skates are in the trunk.

    And a '69 one-year-only 'slow reacting' relay interrupts both after a few seconds once the start position of the key switch is released.
    Hopefully the engine kicks over-
    Repeated cranking without the engine starting may deliver quite a lot of fuel- everywhere.

    I eliminated the raw-gas prime (Start assist) solenoid since the '69 has plastic velocity stacks and the hokey spray nozzles above them built into the air filter housing and I KNOW thats a fire hazard.

    The 'Frigid' raw-gas enrichment solenoid at the top of the fuel console is for really, really cold weather and my car sleeps when there is brine on the road.
    A Mercedes transmission plug filled the spot where the 'Frigid' enrichment solenoid lived. (Don't ask the price of that felon- mine works and is now in my "For Next Owner" box)

    I've tested the '69's one-year-only 'slow reacting' relay and it seems to be breaking the continuity to the relay that energizes the Thermo-time switch that when cold enough, energizes a solenoid that bumps the the MFI mixture rack full rich for a few seconds to start a 'cold' engine. (Like the old Mouse-Trap game!)

    The issue for many is the '69 Thermo-time switch is calibrated too cold. A later model switch works better.

    Although I'm looking to install a newer model thermal time switch- if I cannot get this sorted-out bone-stock I'm going to tap into the handbrake warning light and use that as my 'cold-start' button.

    Handbrake on for first start of the day and foot brake thereafter-

    But the stock system is best. That (without the raw gas sprinkler) is my goal.

    I know I'll get it sorted and the 'slow-reacting' relay will quit-

    So I bought a spare...

  3. #43
    Quote Originally Posted by G69 View Post
    I'm chasing this too on my one-year-only 1969 S MFI dual solenoid with the cold-start rack enrichment solenoid above the cut-out solenoid on the MFI pump.

    I'm lucky enough to have TWO Thermo time switches- 'cold' and 'frigid' @ $350 a pop.
    They are called on the wiring diagram: "71" Temperature switch (start) for temperate weather and "72" Temperature switch (start assist) for the raw gas primer sprayer for arctic driving.

    Both go off when your ice skates are in the trunk.

    And a '69 one-year-only 'slow reacting' relay interrupts both after a few seconds once the start position of the key switch is released.
    Hopefully the engine kicks over-
    Repeated cranking without the engine starting may deliver quite a lot of fuel- everywhere.

    I eliminated the raw-gas prime (Start assist) solenoid since the '69 has plastic velocity stacks and the hokey spray nozzles above them built into the air filter housing and I KNOW thats a fire hazard.

    The 'Frigid' raw-gas enrichment solenoid at the top of the fuel console is for really, really cold weather and my car sleeps when there is brine on the road.
    A Mercedes transmission plug filled the spot where the 'Frigid' enrichment solenoid lived. (Don't ask the price of that felon- mine works and is now in my "For Next Owner" box)

    I've tested the '69's one-year-only 'slow reacting' relay and it seems to be breaking the continuity to the relay that energizes the Thermo-time switch that when cold enough, energizes a solenoid that bumps the the MFI mixture rack full rich for a few seconds to start a 'cold' engine. (Like the old Mouse-Trap game!)

    The issue for many is the '69 Thermo-time switch is calibrated too cold. A later model switch works better.

    Although I'm looking to install a newer model thermal time switch- if I cannot get this sorted-out bone-stock I'm going to tap into the handbrake warning light and use that as my 'cold-start' button.

    Handbrake on for first start of the day and foot brake thereafter-

    But the stock system is best. That (without the raw gas sprinkler) is my goal.

    I know I'll get it sorted and the 'slow-reacting' relay will quit-

    So I bought a spare...
    keep us posted on how it goes - I put in a cold start button years ago and never looked back
    73 911S Targa

  4. #44
    Very good thread
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  5. #45
    Very informative guys. Just ran into this after a warm day run and then parking in a parking lot in the sun for an hour. It finally cranked but man did it take a while. Short term solution? Find some nice shade at least!

  6. #46
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    I have a 72E with recently fully Gus overhauled MFI. (Which was still taking 20 seconds to catch when the hot motor is restarted after about 30 minutes....)

    I've been experimenting with a $5 adjustable timed relay to bypass my working thermo-time switch. This can be adjusted from about 0.5 to 10 seconds.
    I currently have it configured to squirt for one second on cranking. It is powered by the yellow wire that was previously connected to the thermo-time switch. Mine currently squirts once, each time you crank but usually fires first crank, even when warm. It could, however, be combined with a thermal thermal switch set at say, 75C, to lock it out if the motor is too warm. (To prevent flooding.)

    I need to crack open the throttle slightly before cranking.

    The circuit is tiny and is easily hidden away near the motor. Existing wiring has not been modified.

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    Similar to this (no commercial interest....)-

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  7. #47
    Senior Member NorthernThrux's Avatar
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    A lot of hacks to get around what is a pretty elegant electromechanical beast (911.617.117.00). Nowadays some programmer put in several hundred lines of code in an ECU to do the same thing. There are a number of visual and electrical checks that should be made on these switches to ensure they are correct and can work as intended.

    The bi-metallic switch in the brass housing responds to temperature and disables the cold start enrichment if above a certain temperature. That temperature is stamped on the thermo-time switch and should be 45 C (113 F if you don’t speak metric) at least for the 70- MFI cars. There is also a heating element that is driven by the wire to the starter. The longer you crank, the more you heat the bi-metal switch. When the bi-metal switch opens (at 45 C !) it prevents further cold start enrichment by interrupting the circuit from the relay to ground. The colder the motor, the longer it takes for the circuit to get interrupted and the more fuel is dumped into your intakes. Also the colder the motor is, the sooner the thermo-time switch cools off and again allows cold start enrichment. Somewhere in the neighbourhood of 10s in the temps most of you drive. So you have to wait between tries of the starter if it doesn’t catch the first time if you want the benefit of enrichment.

    Even here there is a catch. Original switches are marked 3W, 8W or 40W. Depends on year of manufacture and MFI, CIS etc. The 40W one heats up faster and hence stops enrichment faster. I think it is the one that is correct for MFI cars. It’s also what the new ones are marked as. I think the faster heating ones were used to prevent oil dilution with gasoline, but may compromise starting by limiting the cold squirt duration. I haven’t timed how long it squirts in my car, but the engine catches in 1-2 s, so it is adequate.

    Now here’s the ole timey mechanic checks. Often thermo-time switches have never been removed from the motor, even during rebuilding. The switch has a brass housing, an aluminum or copper sealing washer and is screwed into the magnesium breather cover. All are subject to contact corrosion from the dissimilar metals or oil crud in the valley of leaks in that area. The switch should be removed and all the surfaces cleaned to ensure continuity to ground. And speaking of ground, the transmission braided ground should be checked as this is part of the path to ground for the switch and that braid and its attachment points are on the bottom of the car and really get disgusting. I changed mine and it made a difference to cold starting and smoothness of the idle. Also check that the fuel console is grounded to the body stud just behind it or your cold start solenoid won’t open.

    Now when the car is warm, the recommended procedure is to depress the accelerator fully and crank. The extra fuel from WOT should be enough to start the car with a warm motor. But if you have vapor lock, you may not get fuel to the injectors. This is where using the cold start enrichment injectors helps as they aren’t in direct contact with the hot engine and aren’t so prone to fuel vaporization. The delay relay above can be used to fire the cold start solenoid for a second every time the starter is turned and that may help starting if your fuel system is not tuned perfectly. Probably no point in adding a thermostatic switch to that as all you are doing is effectively recreating the thermo-time switch but with a limited squirt duration and no waiting between subsequent starting attempts. Plus without any temp control you have a chance to defeat vapour lock on hot starts.

    If everything is working properly as when the car was new, the original system is all that is needed and works perfectly. I haven’t had issues with hot starting in 34C weather after 15 minutes or 2 hours on a fully warmed up engine. Of all the hacks, the delay relay is probably the one I’d chose as it requires no switches. You just pick the initial squirt duration that works for you and forget it. If you are a fair weather driver and don’t need to start in cold temps, the fixed duration squirt will be fine and will probably help your hot starts to the point that you don’t need to depress the accelerator pedal either. You could wire the delay relay in parallel with the thermo-time switch to get enrichment on hot starts but preserve the proportional enrichment duration with lower temperature that the stock switch offers in colder weather.

    If you really wanted to be fancy, replace the trim pot that adjusts the duration on that relay with a thermistor with the right characteristics and then squirt duration would be a function of temperature. But then all you’ve really done is build your own thermo-time switch! Some kind of plug-in replacement for the relay in the engine console that deals with all this in a logical manner would be perfect.
    Last edited by NorthernThrux; 07-19-2020 at 05:26 AM.
    Early 911S Registry # 2395
    1973 Porsche 911S in ivory white 5sp MT
    2015 Porsche Macan S in agate grey 7sp PDK

  8. #48
    Quote Originally Posted by NorthernThrux View Post
    A lot of hacks to get around what is a pretty elegant electromechanical beast (911.617.117.00). Nowadays some programmer put in several hundred lines of code in an ECU to do the same thing. There are a number of visual and electrical checks that should be made on these switches to ensure they are correct and can work as intended.

    The bi-metallic switch in the brass housing responds to temperature and disables the cold start enrichment if above a certain temperature. That temperature is stamped on the thermo-time switch and should be 45 C (113 F if you don’t speak metric) at least for the 70- MFI cars. There is also a heating element that is driven by the wire to the starter. The longer you crank, the more you heat the bi-metal switch. When the bi-metal switch opens (at 45 C !) it prevents further cold start enrichment by interrupting the circuit from the relay to ground. The colder the motor, the longer it takes for the circuit to get interrupted and the more fuel is dumped into your intakes. Also the colder the motor is, the sooner the thermo-time switch cools off and again allows cold start enrichment. Somewhere in the neighbourhood of 10s in the temps most of you drive. So you have to wait between tries of the starter if it doesn’t catch the first time if you want the benefit of enrichment.

    Even here there is a catch. Original switches are marked 3W, 8W or 40W. Depends on year of manufacture and MFI, CIS etc. The 40W one heats up faster and hence stops enrichment faster. I think it is the one that is correct for MFI cars. It’s also what the new ones are marked as. I think the faster heating ones were used to prevent oil dilution with gasoline, but may compromise starting by limiting the cold squirt duration. I haven’t timed how long it squirts in my car, but the engine catches in 1-2 s, so it is adequate.

    Now here’s the ole timey mechanic checks. Often thermo-time switches have never been removed from the motor, even during rebuilding. The switch has a brass housing, an aluminum or copper sealing washer and is screwed into the magnesium breather cover. All are subject to contact corrosion from the dissimilar metals or oil crud in the valley of leaks in that area. The switch should be removed and all the surfaces cleaned to ensure continuity to ground. And speaking of ground, the transmission braided ground should be checked as this is part of the path to ground for the switch and that braid and its attachment points are on the bottom of the car and really get disgusting. I changed mine and it made a difference to cold starting and smoothness of the idle. Also check that the fuel console is grounded to the body stud just behind it or your cold start solenoid won’t open.

    Now when the car is warm, the recommended procedure is to depress the accelerator fully and crank. The extra fuel from WOT should be enough to start the car with a warm motor. But if you have vapor lock, you may not get fuel to the injectors. This is where using the cold start enrichment injectors helps as they aren’t in direct contact with the hot engine and aren’t so prone to fuel vaporization. The delay relay above can be used to fire the cold start solenoid for a second every time the starter is turned and that may help starting if your fuel system is not tuned perfectly. Probably no point in adding a thermostatic switch to that as all you are doing is effectively recreating the thermo-time switch but with a limited squirt duration and no waiting between subsequent starting attempts. Plus without any temp control you have a chance to defeat vapour lock on hot starts.

    If everything is working properly as when the car was new, the original system is all that is needed and works perfectly. I haven’t had issues with hot starting in 34C weather after 15 minutes or 2 hours on a fully warmed up engine. Of all the hacks, the delay relay is probably the one I’d chose as it requires no switches. You just pick the initial squirt duration that works for you and forget it. If you are a fair weather driver and don’t need to start in cold temps, the fixed duration squirt will be fine and will probably help your hot starts to the point that you don’t need to depress the accelerator pedal either. You could wire the delay relay in parallel with the thermo-time switch to get enrichment on hot starts but preserve the proportional enrichment duration with lower temperature that the stock switch offers in colder weather.

    If you really wanted to be fancy, replace the trim pot that adjusts the duration on that relay with a thermistor with the right characteristics and then squirt duration would be a function of temperature. But then all you’ve really done is build your own thermo-time switch! Some kind of plug-in replacement for the relay in the engine console that deals with all this in a logical manner would be perfect.
    great post, you really nailed it!

    I'm still convinced that some hot start issues are caused by a faulty overflow valve...
    73 911S Targa

  9. #49
    Senior Member NorthernThrux's Avatar
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    One last point. If your hand throttle isn't working or you don't depress the accelerator, the car isn't ever going to start. The cold start enrichment is on the intake side stack of the throttle plates so all you will do is pool gasoline above the throttle plates, some of which may drip through depending on how worn your throttle plates and bores are. Porsche's cold start procedure is aimed at ensuring that the throttle plates are open so the cold start squirt results in gas going to the combustion chamber, not pooling in the ITBs.

    I've lately wondered whether having the cold start nozzles on the other side of the throttle plate could have dispensed with the hand throttle and other awkward starting procedures. That would require the air screws to provide enough combustion air when starting, which I think they do. After all, you wouldn't idle otherwise. Although, thinking further, even modern cars have an automatic fast idle and the normal idle bypass airflow may not be enough when cold. Modern cars have a bypass solenoid across the throttle plate(s) to allow for the high idle and a way of metering that extra air to get the AFR right. MFI doesn't allow for that. CIS did. We'd need a solenoid to pull open the throttles, mimicking the hand throttle basically.

    Which raises one last point (honest). The cold start nozzles do have a mesh in the tip and they do atomize the fuel a bit. You aren't just dumping a stream into the intake air stream. Those meshes can degrade or get clogged and you lose the spray pattern. Nobody every checks this, but it does happen and it will make a difference to starting. I replaced one bad plastic one last year and of course a few weeks ago, I replaced the entire circuit with the brass ones that Sierre Madre and others sell. Noticable improvement in starting from 3-4 seconds of cranking to 1-2 s.
    Last edited by NorthernThrux; 07-19-2020 at 11:43 AM.
    Early 911S Registry # 2395
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    2015 Porsche Macan S in agate grey 7sp PDK

  10. #50
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    Sorry, I should have been clearer above. My Cold start circuit, fuel console and MFI have been recently completely rebuilt and work perfectly for cold starts and 90% of the time. My thermo-time switch circuit operates perfectly as intended.

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