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Thread: MFI .. thermostat question

  1. #1

    MFI .. thermostat question

    I’m trying to get to the bottom of an issue with my
    2.4S , its all is set up really well and drives great at normal road temps ( 80deg engine temp) and the afr’s are good too with hard acceleration at 7000rpm in 3rd afr is about 13.5

    What I’ve found though is after some real hard driving , and the temp gauge reading 90deg the mixture is leaning out right at the top end and showing about 14.2-14.4 afr at 7000+rpm

    I gradually went 6 clicks rich on the main rack and Ive brought those top end afrs back into the 13’s but when back at normal road engine temp I feel the car is just a bit too rich which o know for experience can lead to oil dilution.

    On the back of the above, when the engine was ‘road temp’ I pulled off the triangular top cover on the mfi pump for the the thermostat lever and and found I could move it still maybe 3-4mm until it bottoms out

    The thermostat rod, discs and spring are all clean and correct. I’m assuming at normal road temps they are only closing the cold enrichment off so far and when at 90deg + the rod is extending that little bit further and Leaning it out. In this case The mfi has been tuned based on normal engine temp

    Anyone know if the enrichment lever within the mfi pump should be pushed as far as it can go by the thermo rod ? Or should there be a little bit of movement before it bottoms out ?
    Last edited by james; 09-19-2021 at 06:59 AM.
    1973 2.4S
    1993 964 C2
    987 Boxster Spyder

    'hot66'
    www.ddk-online.com

  2. #2
    Think the answer is there shouldn’t be any more movement which implies the thermostat rod isn’t extending far enough when hot. Need to now work out why as it appears to be getting uninterrupted hot air from the heat exchanger
    1973 2.4S
    1993 964 C2
    987 Boxster Spyder

    'hot66'
    www.ddk-online.com

  3. #3
    Senior Member 2.5MFI's Avatar
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    Quote Originally Posted by james View Post
    Think the answer is there shouldn’t be any more movement
    That's correct James. After the engine has warmed up to normal temps, 190 F plus, the thermostat disc's have expanded and the rod ( mfi pump "f" setting ) has pushed the lever ( under the triangleular cap ) all the way over and compressed the spring on the MFI pump's "f" settings adjustable pad.
    Hot air from heat exchanger should make the thermostat very warm/hot to touch after engine is fully warmed up.
    Mark Jung
    MFI Werks
    Early 911S Registry #972
    Carrera T w/LWB, MT, RWS, PCCB
    72 T 66 x 100 MFI Twin Plug Coupe
    R Gruppe #686

  4. #4
    Serial old car rescuer Arne's Avatar
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    I'd also verify that the thermostat has the full complement of bi-metal discs. I seem to recall that there should 25 pairs of discs, as well as adjustment shims. The number and thickness of the shims can vary as they are used to regulate how much enrichment is provided when cold. As I recall, my thermostat had a total shim thickness of around 1.5mm.
    - Arne

    Mostly done
    - 1972 911T coupe, Silver Metallic, Fuchs, Appearance Group, F&R sway bars, factory installed Frankfurt, upgraded factory A/C.
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  5. #5
    Quote Originally Posted by 2.5MFI View Post
    That's correct James. After the engine has warmed up to normal temps, 190 F plus, the thermostat disc's have expanded and the rod ( mfi pump "f" setting ) has pushed the lever ( under the triangleular cap ) all the way over and compressed the spring on the MFI pump's "f" settings adjustable pad.
    Hot air from heat exchanger should make the thermostat very warm/hot to touch after engine is fully warmed up.
    Quote Originally Posted by 2.5MFI View Post
    That's correct James. After the engine has warmed up to normal temps, 190 F plus, the thermostat disc's have expanded and the rod ( mfi pump "f" setting ) has pushed the lever ( under the triangleular cap ) all the way over and compressed the spring on the MFI pump's "f" settings adjustable pad.
    Hot air from heat exchanger should make the thermostat very warm/hot to touch after engine is fully warmed up.
    Hi Mark

    Thanks for the reply .. you actually rebuilt this pump back in 2010 ?? or similar so I'm going to assume all is ok internally. My suspicion the thermo has never fully extened but we have tuned around it .. . its only recenlty when I've been road tuning it ( I have a twin daytona sensors afr box ) and I've leaned it out a bit that the top end lean mixture at high temps has become apparent .

    190F ( 87.7 Deg ) .. in the UK summer , according to my temp gauge in normal quick road driving is approx 175F and to get it up to 190F I need to be really thrashing the car hard incl 120mph long pulls ( of course my guage might not be accurate ) . I'm going to remove and check all the flexi hoses running from the heat exchanger to the therm discs to make sure non are blocked ( unlikely ) .

    Arne ... The number of discs is correct ( 25 + spacer either end & 2 washers ) . It feels like its getting pretty hot.

    I've got a few solutions if heat is getting to it .... add a spacer, or remove & fit a fixed rsr style system ... or tune around it and tune for road use
    1973 2.4S
    1993 964 C2
    987 Boxster Spyder

    'hot66'
    www.ddk-online.com

  6. #6
    Senior Member frederik's Avatar
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    I had the exact same problem in the beginning. My mechanic was able to adjust the thermostat so it fully extended at normal operating temperature: no more rich running. I don't know exactly how he did it however, but Mark surely does!
    1972 911 2.4T Targa Aubergine (MFI)
    2000 996 Carrera Ocean Blue
    Member #3833

  7. #7
    wonder if he just fitted an extra spacer ?

    Just been reading in CMA and it reckons the thermostat enrichment stops at 53 deg c ( 130 F ) .

    In the short term, to confirm its the thermostat that the problem, I'm going to replace it with a cold start delete plate and adjust it so enrichment is fully off all the time. If nothing elese this should confirm the AFR's stay stable with temp increase

    https://porshop.co.uk/911-r-st-rs-rs...ing-screw.html
    Last edited by james; 09-21-2021 at 10:12 PM.
    1973 2.4S
    1993 964 C2
    987 Boxster Spyder

    'hot66'
    www.ddk-online.com

  8. #8
    Senior Member 2.5MFI's Avatar
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    James, my typo, should be 130 F.
    Mark Jung
    MFI Werks
    Early 911S Registry #972
    Carrera T w/LWB, MT, RWS, PCCB
    72 T 66 x 100 MFI Twin Plug Coupe
    R Gruppe #686

  9. #9
    So… I replaced the thermostat with a fixed rsr style plate and screw so I could eliminate the thermo.

    Did some more power runs and measuring the afr’s the same happens still. Afr is good .. say working up to 13.6 through to about 7000 rpm … but just in that last little bit of the final 200rpm to 7200 rpm it leans out to 14.4 … almost as if it drops off the fuel ‘map’ .

    These power runs were WOT in 3rd gear from 3500 rpm all through to the redline

    As an experiment I tried to see wether I could get that last bit of fuelling to about 13.8. I did manage it but by going 12 clicks rich !!! Obviously at that it’s too rich in normal Rev range .

    I’m going to set the pump back to what it was and re install the thermostat.

    Any ideas why the fuelling might ‘drop off the map ‘ in those last 200rpms ?
    Last edited by james; 09-26-2021 at 12:41 PM.
    1973 2.4S
    1993 964 C2
    987 Boxster Spyder

    'hot66'
    www.ddk-online.com

  10. #10
    Senior Member 2.5MFI's Avatar
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    James, your WOT runs are very close to normal AFR's except for the last 200 rpm as you said. So, re-adjust the main rack back to it's original setting first. You need to tighten up the Hi Speed screws on the main spring capsule to fix this problem. This requires removing the idle adj plate off the pump cover, lots of oil will pour out so you will have to deal with that. After oil has stoped pouring out, identify the two Hi Speed screws. They have a lighter color head than the Mid Speed screws which are very dark. So with a small head screwdriver turn each Hi Speed screw in CW 2 clicks. It's a small adjustment but it should put a little more tension on the counter weights and keep the space cam in a richer area @ 7200 rpm. This adjustment will only address your WOT and will not change your Mid Speed or Idle settings.
    Mark Jung
    MFI Werks
    Early 911S Registry #972
    Carrera T w/LWB, MT, RWS, PCCB
    72 T 66 x 100 MFI Twin Plug Coupe
    R Gruppe #686

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