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  1. #1

    Default V6 2.8L/C4 auto - loss of power when accelerating, low power, stalling

    Hello everyone, I have trouble getting my '79 Mustang V6 2.8L/C4 auto to run properly. Here's the symptoms :
    It starts up fine, idles fine, goes in all gears perfectly, doesn't smoke, but when I hit the road and start accelerating, it loses power, sometimes to the point of stalling, most of the times it will just be very slow and the maximum speed will be around 60mph...and consume a lot of fuel.
    Again, all gears change smoothly, so smooth I can barely feel them.
    Also, it tends to run out of fuel when there is still plenty in the tank.

    I've done a LOT of work to the engine to attempt to solve the issues. The car ran perfectly before I pulled the engine and transmission to paint the engine bay. Here's what I've done :
    - pulled engine and transmission in one piece, followed by a cleaning with a pressure washer (it was covered in a mix of undercoating, dirt and oil)
    - nothing was removed from the engine, I didn't touch the kick down arm or carb or ignition, nothing moved.
    - put it back a few months later
    - replaced all the fluids
    - replaced transmission filter, oil filter, fuel filter
    - flushed the transmission fluid as much as I could and refilled with new Type F fluid
    - replaced water pump (it was leaking at the shaft) and water hoses
    - replaced fuel pump and hoses
    - new spark plugs, wires, cap and rotor, coil
    - rebuilt the carburetor (the symptoms were already there, didn't help much)
    - checked the interior of the gas tank and sending unit, both in perfect condition
    - checked the fuel line, perfect, not plugged

    I do not know what to verify anymore. I was thinking maybe the vacuum modulator at the transmission could be bad, or the vacuum advance diaphragm as well.
    What are your ideas ?

    Thanks

  2. #2

    Default

    Is the gas tank vent clear/open? What's the carburetor float level at?

    A bad modulator (diaphragm leaking (or vacuum leak)) would shift hard, and would be drawing transmission fluid into the intake... lotsa smoke, and transmission fluid level dropping. A leaking vacuum advance diaphragm would just be a vacuum leak, and a lack of idle/cruise ignition advance... wouldn't affect max power/speed or use lots of gas or cause stalling like running out of gas when there's gas in the tank... really sounds like maybe the gas tank isn't being allowed to vent, countering the suction of a mechanical fuel pump running for a time... try running it with the gas cap off, see if it's fine then, and report back...
    Last edited by Walking-Tall; 04-19-2018 at 02:39 AM.
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  3. #3

    Default

    Thanks, I'll try that

  4. #4

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    Allright so I've replaced the tank cap, no improvement. But then I remembered I had changed the soft hoses with slightly oversized ones, and when I opened one, it was all dry. I removed them and while they felt tight enough to stay in place, they were more "squeezed" than making good contact against the line. I have no hose of this size so I had to scavenge some from my parts car, it's bad but it's the right size.
    Well after a good drive , it seems the carburetion problem is gone.


    But the car is still slow, it took around 30 seconds to go to 60mph. I would expect at least half that...

  5. #5

    Default

    If it’s not missing or popping then that sounds like timing. Either the vacuum advance is stuck at zero or the temp controlled vacuum valve is staying shut or the diaphragm is broken or the base timing is retarded.

  6. #6

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    It's not popping, I've checked the timing...again, set it at 9 again (at idle, vacuum removed and pluged), not better.
    I plugged the diaphragm directly to the carburetor ported vacuum, no improvement.
    When I plug everything back and check the timing, it will go up to around 12 at idle, when I accelerate, it goes down to 10

  7. #7

    Default

    Quote Originally Posted by SuperDuty455 View Post
    Allright so I've replaced the tank cap, no improvement. But then I remembered I had changed the soft hoses with slightly oversized ones, and when I opened one, it was all dry. I removed them and while they felt tight enough to stay in place, they were more "squeezed" than making good contact against the line. I have no hose of this size so I had to scavenge some from my parts car, it's bad but it's the right size.
    Well after a good drive , it seems the carburetion problem is gone.


    But the car is still slow, it took around 30 seconds to go to 60mph. I would expect at least half that...
    Replaced the tank cap? My suggestion above was to try running and driving the car some without the tank cap on, which would basically point to the tank vent being blocked if it ran and drove well...
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  8. #8

    Default

    Quote Originally Posted by SuperDuty455 View Post
    It's not popping, I've checked the timing...again, set it at 9 again (at idle, vacuum removed and pluged), not better.
    I plugged the diaphragm directly to the carburetor ported vacuum, no improvement.
    When I plug everything back and check the timing, it will go up to around 12 at idle, when I accelerate, it goes down to 10
    Now that sounds very much like the mechanical/centrifugal ignition advance (the weights and springs under the distributor "breaker" plate) is stuck not advancing or not working as it should in general. Check your timing with your light again, vacuum advance disconnected and hose plugged again, but this time rev the engine up to say up past 3000rpm while watching the timing mark, to see if the mechanical/centrifugal ignition advance is advancing the timing or not with increased rpm... it should be... if not, that would explain the lack of power and likely some of the excessive fuel consumption too...
    Last edited by Walking-Tall; 04-20-2018 at 11:41 PM.
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  9. #9

    Default

    That was exactly that ! THANKS !

    I took the distributor out and carefully took it apart. This is was I found under the plates...



    The weights didn't move properly, so the diaphragm couldn't do much. After a lot of WD40 and thinner, and a light sanding to remove the surface rust, I put it all back together with a proper amount of grease on all moving parts. Adjuted the timing, checked with the hose plugged on the diaphragm, and the timing went all the way out of the numbers

    A quick test drive, the 0-60mph is 14.5s, less than half of what it did before.

  10. #10

    Default

    Very nice. Now, going forward, everybody take note, take care of details like that before they bite you in the ass. Everything works real good when dialed in.
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  11. #11

    Default

    Glad it’s sorted!

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