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Thread: MFI troubleshooting - high idle, pops and bucks at part throttle

  1. #71
    Serial old car rescuer Arne's Avatar
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    Quote Originally Posted by edmayo View Post
    Just simply remove the feed line from the discharge of the pump which goes up thru the frame section right where it enters the inlet fitting of the fuel filter console. Install a pressure gauge there and see if you're getting 30 psi or so of pressure, then run that hose into a small coffee size can and the pump should fill that in less than a minute. If it does all that then jack up front of car and repeat. If it fails that then fill a five gallon can with gas and with several feet of extra fuel hose run the car at an angle from the five gallon can. This will prove whether it is a pump problem or a fuel feed problem involving the car's gas tank and lines. And by the way make sure that the fuel hose from the bottom of the filter is going to the passenger side of the MFI pump, and the driver side of the MFI pump is going to the inlet fitting of the console. ( the fitting facing rear of car)
    Well, I'm back, and no less frustrated than before.

    I started going through some of the tests here with a borrowed pressure gauge. Pressure at the inlet of the filter console would jump to ~40 PSI when the pump was switched on, then the pump's relief valve would open and it dropped to a steady 28 PSI and stayed there. Did not matter the attitude of the car, same result with the nose in the air.

    Verified that the lines to and from the MFI pump are correct, bottom of filter to the right side of the pump via large banjo fitting.

    Started to look into volume testing by running the pump into a bucket, but noticed that the fuel pump was starting to leak again. (I had replaced the large o-ring that seals the motor case to the pump body before I got into this process.)

    As a stopgap to deal with the leak, I bought a 2-port pump conversion kit from Zims to get the leaking pump back off the car. (Will decide later if I want to try to rebuild my original or buy one of the new Chinese pumps from Porsche Classic.) This new pump eliminates the bypass valve and return hose from the pump, and uses the check valve in the filter console to regulate pressure. (My check valve is new and factory Porsche part.) So I can no longer check pressure at the inlet side of the filter console. I will need to check it between the bottom of the filter and the MFI pump inlet. But I had to return the borrowed gauge, so I have not tested pressure since the pump replacement.

    But I'm going to guess that with this new pump in place I shouldn't have a shortage of pressure. And...

    No change. I'm not really certain at this point if I am really losing pressure when I start up a grade, or if it is just that going uphill just puts more load on and it starves out. But the result is that trying to pull up a modest grade at normal RPMs for town driving runs it out of fuel. Sputters, pops, and will even sometimes die.

    I feel I must be missing something very basic. I will get the gauge back tomorrow for a bit more testing with this new pump. I will (assuming the pressure tests out OK) go back to the volume test into a can, and probably also try to run the engine out of a gas can to eliminate all existing tank and line possibilities.

    But I'm really starting to wonder if there is something else afoot. Could the MFI pump itself have issues that would manifest themselves in this way?
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  2. #72
    I'm curious to see what you get when running out of a gas can vs. the tank. Even though that in tank filter looked perfect I don't think you ever took it out and inspected it did you?
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  3. #73
    Serial old car rescuer Arne's Avatar
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    No Ed, I never took it out and inspected it myself. It looks new when seen through the sender hole, and when looking at the connections coming out the rear of the tank I saw obvious new copper washers. But it could still be a problem.
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  4. #74
    Serial old car rescuer Arne's Avatar
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    After playing with gas cans, gallon paint buckets, wheel ramps, and a couple lengths of fuel hose, I've ruled out fuel delivery as my problem. Doesn't matter what or how I get fuel to the pump, there was no change.

    In fact, today when it started sputtering going uphill, I found that if I could get the revs up above 4000 it would pull again. Between 2000-3500 when the engine is warm and under load it just flat doesn't want to run. So it must be getting fuel, it just isn't metering it properly at some speeds and loads.

    Pulled the plugs for a look today, the side electrodes are now showing gray-tan color, not black and sooty as before.

    The baffling thing is that it runs much better when cold, pulling up hills nicely. Then after some driving (5 minutes?) and it gets warm, it changes to really ugly. The change is rather sudden, maybe over 30 seconds or so.

    This would seem to be related to the thermostat on the injection pump. My thinking has been the mixture is pretty good when cold, then when the 'stat warms and leans the mixture it gets really bad.

    So I took off the 'stat to check it out.

    Quote Originally Posted by 2.5MFI View Post
    Thermostat: On the end of the thermostat rod, after the 25 pair of thermo disc's, are various sized shims (from .10mm to 2.0mm) that make the adjustment for how rich the fuel will be at cold starts. One .30mm shim can make a 10 to 15% difference in fuel delivery. When below 32 degrees this will move the main rack for 35% to 45% more fuel for the cold start. Adding shims will lean out the cold start fuel, removing shims will richen up the cold start fuel. Once the thermostats 25 disc's warm up and expand to the max, about 105 to 110 degrees F, it has reached the warmed up condition and stays there till you shut off the engine and it cools way down.
    Mine does still have all 25 pairs of discs, and a single 2.0mm shim. As best I can determine using my heat gun, this gives me about 3mm of movement at the pushrod.

    Since it seems to run better when cold (pushrod 3mm shorter than warm), I tried shimming the thermostat out 3mm in hopes that once warm it would run like it often does cold. That failed miserably. Ran so badly cold that I aborted the test drive and limped home. Oddly, pulling the thermostat out 3mm made it run cold just about the same as how it runs normally when warm. That seems very wrong to me, as either direction the pushrod goes seems to make it run much the same (badly).

    Totally frustrated now, and confused. I'm going to take a couple days off and head out of town with my wife. Feel free to offer more suggestions and advice while I'm away.
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  5. #75
    That is new information, that it will run somewhat better if you can get it above 4,000 RPM. That could mean that the pump main adjustment is at least part of the problem. Since you don't have an AFR meter you'll just have to make adjustments and record what they are. Normally everyone just wants to jump in and screw with the rack adjustments, and now you've seen there are many other checks to make first, such as that bad check valve. By the way, reducing shims richens the thermostat. We're going to do this seat of the pants method. Richen the main rack adjustment 4 clks counterclockwise ( I proof read this time) record the change, and repeat your test. Four clks is a big step. If we're going in the right direction it ought to be an improvement. If so richen again 4 clks. By this time it will be obvious this is either working or it isn't.
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  6. #76
    Serial old car rescuer Arne's Avatar
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    Got back home yesterday, this morning I did the 4 clicks richer and took it for a drive. Once warmed up, the results were horrible. Had to struggle to get it back up the hill to the house.

    So I back off that change, and then went 4 clicks leaner instead. Still undrivable. Very little difference. An 8 click swing, no real difference. Which is consistent with how this has gone through the whole process. Even significant changes don't seem to make much difference.

    At this point I've thrown in the towel. I called Mark at MFI Werks, and will ship him the pump and all 6 injectors for inspection, cleaning and calibration.

    It is definitely possible that a proper calibration of the pump may not fix my issue. There may yet be something else at play that I haven't found. But the MFI is so far out whack now that I need to get it to a baseline for further testing.

    I (and possibly Mark) will update this thread as news develops. Probably nothing before late next week at best.
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  7. #77
    Serial old car rescuer Arne's Avatar
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    Also noticed during initial disassembly that the coldstart squirter (nozzle) for #3 is broken and was bypassed. Placed a Wanted ad for it -

    http://www.early911sregistry.org/for...with-T-fitting

    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  8. #78
    I'll be curious to see if a stuck rack was your problem.
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  9. #79
    Senior Member 2.5MFI's Avatar
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    Curious myself to see what is really going on. The term "stuck rack" is used a lot, but what is really happening is one or more plungers are getting stuck or frozen in they're cylinders. When a plunger is stuck/frozen it prevents the rack from moving. If a plunger is frozen, that cylinder will not fire and the rack will not respond to any space cam changes. The problem Arne is having does not seem to be a frozen plunger from all the symptoms he has described. But I have been surprised before.......
    Last edited by 2.5MFI; 04-29-2019 at 06:03 AM. Reason: added more info
    Mark Jung
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  10. #80
    Senior Member tcsracing1's Avatar
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    i am still struggling with my car..... going to bring it back to my Indy and try some things.
    You can see why some people swap to carbs!

    But im not giving up!
    LOOKING FOR 1967S TRANSMISSION #103586
    Looking For 1969T Engine #6195922
    Looking For 1969T Transmission #7194313

    Looking for 1969T Transmission #7195495
    www.tcspeed.com

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