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Thread: 73/3 MFI preheater flapper contraption

  1. #1

    73/3 MFI preheater flapper contraption

    I had kind of a notchy, jerky accelerator pedal. And the pedal some times did not return to full stop. So I cleaned and lubed all the ball and sockets, checked the travel in the tunnel etc. Basically an end to end service. I snapped everything back together and still had the problem. Sometimes the carpet moves up against the accelerator pedal but that wasn't it. I have the MFI pre heater so checked that. Turned out the interface of the rod with roller and the control cam lever was bone dry and gooped up with oil and dirt and crud. Once cleaned and lubed, bingo problem solved. So just a heads up for one more place to service on linkage issues.
    Last edited by steve shea; 06-30-2020 at 02:39 PM.
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  2. #2
    Senior Member NorthernThrux's Avatar
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    I bought my flapper box on Ebay and did a full service on it before installing. Shuts with a crisp snap when the accelerator is let off. That movement is quite important to get right as it can also interfere with the MFI microswitch actuation if the set screw doesn't quite touch the switch.

    Ravi
    Early 911S Registry # 2395
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  3. #3
    Serial old car rescuer Arne's Avatar
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    Good info for the future. My car's flapper unit is in storage right now, as the car doesn't have the correct heat exchanger for the left side. Has one for '69-71 MFI, so there is the hookup for the thermostat, but not the secondary exchanger for the warm air flapper. Someday I'll replace the HEs with the correct style and reinstall the flapper. Will need to make certain it works smoothly when I put it back.
    - Arne
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    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  4. #4
    I see a lot of MFI cars with the warm air regulator removed. An experienced MFI mechanic, who helped me get my car to run well, advised me to remove it. He said debris can come up the hose from the heat exchanger and get into the MFI pump temperature sensor. I ran my car for a while without it. Definitely louder! I didn't like the way it looked without it (looking into the air filter), so I made a plate out of sheet aluminum that locks the flapper in the position which closes off the flow from the hose. Seems to be working well. I don't use my car when the ambient is below 40 deg F.
    73 911S Targa

  5. #5
    Senior Member NorthernThrux's Avatar
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    The temperature discs are easy to clean from time to time, but your MFI isn't set right if it doesn't use the warm air hose or has the flapper blocked in the cold air intake position. It means the MFI control rack isn't ever in the right place at idle. The mixture should be rich if those disks don't expand. Yes you can play around to try compensate (the part load screw I think), and that's what your guy probably did, but it's not as God and Porsche intended.

    If you want to block it off, then the thermostatic disks should be removed and a screw into a plate used to position the enrichment rod to where it would be when things are properly warmed up. The contraption works by the bimetallic thermostatic discs retracting the rod when at ambient temp and extending it as it heats up until the linkage in the MFI pump hits a stop. The screw you put in a block off plate would push it in to the fully extended position. If you back off on that screw, it's essentially a richening screw. At that point the thermostat is hot, the rod the discs are on is fully extended and no longer affects the mixture. CMA specs that at 113F to 127F depending on pump. Further heating of the thermostatic discs just causes compression of the spring in the thermostat but no further leaning out of the mixture. How much all this matters probably just depends on how cool the weather gets where you are and how much tolerance you have for a poor idle during warmup.

    Note the warmup hose action is really only relevant at idle. The flap in the flapper box also has a mechanical linkage to the throttle and when you depress the throttle, the flap moves to admit mostly cold air. Even when you are still warming up.

    Ravi
    Last edited by NorthernThrux; 07-10-2020 at 12:36 PM.
    Early 911S Registry # 2395
    1973 Porsche 911S in ivory white 5sp MT
    2015 Porsche Macan S in agate grey 7sp PDK

  6. #6
    Quote Originally Posted by NorthernThrux View Post
    The temperature discs are easy to clean from time to time, but your MFI isn't set right if it doesn't use the warm air hose or has the flapper blocked in the cold air intake position. It means the MFI control rack isn't ever in the right place at idle. The mixture should be rich if those disks don't expand. Yes you can play around to try compensate (the part load screw I think), and that's what your guy probably did, but it's not as God and Porsche intended.

    If you want to block it off, then the thermostatic disks should be removed and a screw into a plate used to position the enrichment rod to where it would be when things are properly warmed up. The contraption works by the bimetallic thermostatic discs retracting the rod when at ambient temp and extending it as it heats up until the linkage in the MFI pump hits a stop. The screw you put in a block off plate would push it in to the fully extended position. If you back off on that screw, it's essentially a richening screw. At that point the thermostat is hot, the rod the discs are on is fully extended and no longer affects the mixture. CMA specs that at 113F to 127F depending on pump. Further heating of the thermostatic discs just causes compression of the spring in the thermostat but no further leaning out of the mixture. How much all this matters probably just depends on how cool the weather gets where you are and how much tolerance you have for a poor idle during warmup.

    Note the warmup hose action is really only relevant at idle. The flap in the flapper box also has a mechanical linkage to the throttle and when you depress the throttle, the flap moves to admit mostly cold air. Even when you are still warming up.

    Ravi
    Hi Ravi - thanks for the response! I think you may have misunderstood me - I have a functioning hose that comes from the heat exchanger to the MFI pump. This hose is really 2 hoses, I believe the inner hose is foil, and I always thought the "double wall construction" was for thermal insulation. The hose I have blocked off is the second, larger diameter hose that also comes from the heat exchanger and goes directly to the warm air regulator, not the MFI pump. Does what I said make sense and change your post, or I am a completely clueless, which is very possible?
    73 911S Targa

  7. #7
    Senior Member NorthernThrux's Avatar
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    Indeed I did misunderstand. You were talking about the MFI pump thermostat getting dirty, so that’s what I was going on about. Yes, the hose you are talking about heats a coil in the flapper valve box. Initially that allows warm air from the heat exchangers into the airbox and engine, but slowly starts to temper it with cool air as the warm air from the HEs heats the coil. Eventually you get only cool air into the air box because the flap in the air regulator blocks off the warm air from the HEs. However, even when the flap is allowing warm air from the HEs to come into the airbox, stepping on the throttle causes the flap to close and primarily cool air to come in. I guess the assumption is when you are accelerating you want the coolest air into the engine, no matter what.

    So the air regulator really just feeds warm air at idle to the airbox. Pressing the accelerator changes the bias to cold air. I imagine for a lot of fair weather owner-drivers, they won’t see much difference, which is why they remove it. Probably makes a difference in cold start emissions at idle, where emissions testing used to be, so Porsche essentially disabled that as you accelerate.
    Last edited by NorthernThrux; 07-10-2020 at 05:45 PM.
    Early 911S Registry # 2395
    1973 Porsche 911S in ivory white 5sp MT
    2015 Porsche Macan S in agate grey 7sp PDK

  8. #8
    I had mine removed for years, put it back on to be correct for concour judging, and I swear it ran ever so slightly better during warm-up phase, so I've left it on.
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  9. #9
    Senior Member NorthernThrux's Avatar
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    Quote Originally Posted by edmayo View Post
    I had mine removed for years, put it back on to be correct for concour judging, and I swear it ran ever so slightly better during warm-up phase, so I've left it on.
    It can't do anything for cold starting because the HEs are cold for the first minute anyway, but it does seem to prevent stumbling at idle when the engine is warming up and allows the hand throttle to pushed down faster than if it isn't connected.
    Early 911S Registry # 2395
    1973 Porsche 911S in ivory white 5sp MT
    2015 Porsche Macan S in agate grey 7sp PDK

  10. #10
    Senior Member frederik's Avatar
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    I’m missing mine and am looking for one. I did wonder about the noise difference but Boulder Bob just answered that question for me. Like my mechanic said: everyone removed those in the past but somehow they’re very difficult to find nowadays.
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