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Vacuum problem

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  • Vacuum problem

    My car starts easy, runs great cold and for a little while once warmed up (I mean it runs perfect!), then it starts bucking, jumping, stalling. When it does this, the tablet shows the vacuum going as low as -1.6 then jumping back to normal (depending on the throttle position), naturally when this happens the AFR is all over the map also. If I drive the car with very light and steady throttle, I can limp it back home.
    I have no vacuum leaks, I just reinstalled the intake manifold thinking maybe I had a problem there.
    When the car is idling good, I can block off the throttle body and it dies.
    I have tested the exhaust system, no leaks there.
    Could it be that I have a bad MAP sensor?

  • #2
    Before replacing parts:

    Make sure the MAP sensors connectors are clean, and connecting.

    If there ok. Then do a reset of the system.

    If the sensor still reads funny. Then replace it......back fires can make it go out.
    At 43,861 MPH Elon Musk now owns the fastest car in this Galaxy


    • #3
      My initial gut reaction is that you probably don't have either a vacuum or MAP sensor problem but it wouldn't be a bad idea to check the MAP sensor connector for problems there. You can easily test the sensor with a Mity-Vac hand pump while monitoring the vacuum channel. I'm guessing that once you get out of the extra warm up enrichment fuel that your base fueling is too lean which is making the engine run badly thus it doesn't pull much vacuum which makes it run in a higher load section of the fuel map which could have fuel pulse values that are too small which exacerbates the problem. Let's cover the basics.

      What engine? What displacement? What cam profile? Which kit part number? What calibration have you loaded? This is most likely a simple calibration/tuning problem and not a mechanical system problem. Once warmed up, have you maybe tried trimming in some extra fuel?


      • #4
        The engine is a Ford 302 with a 331 stroker kit, the cam is a Comp Cams Magnum Series roller cam with 281 duration intake and exhaust, the Edelbrock kit is 3241, the base calibration is loaded.
        I should reiterate that the car starts and runs perfect, under all conditions, idle, acceleration, light throttle, full throttle, deceleration, for 15 minutes to as much as a half hour, the car is thoroughly warmed up and running great under all conditions, then it suddenly either has a massive vacuum leak or the fuel ratio is going crazy. The vacuum does not drop off slowly, one minute it appears to be correct for all throttle positions and the car is driving perfect, then while driving along at steady throttle (not stabbing the throttle or suddenly lifting), it suddenly drops to -1.6 then jumps back to correct then jumps to -20 something then back to -1.6, and just keeps doing it until I shut the car off and let everything cool completely.
        The MAP sensor connectors are clean and show no problems.


        • #5
          When the vacuum reading jumps up to -1.6 from -20, does the engine run very badly or does it just go a rich? If MAP sensor was bad and was reading near atmospheric pressure (same as at WOT) then I'd expect that the engine would just run rich but probably not so rich that it'd be jumpy or jerky or bucking or stalling. Have you confirmed the actual manifold pressure by using a mechanical vacuum gauge? Does the problem only occur when driving in low load? Does it continue to act bad after it starts happening and you just let the engine idle? Does the problem persist if you downshift a gear and apply more throttle?

          Is this a fresh install? If not, how long has the system been installed and has it always acted this way or did this behavior recently pop up? While I'm not a big fan of throwing parts at problems, changing out the MAP sensor is a $100 test you can try. Edelbrock MAP sensor is pn 36019.


          • #6
            When the vacuum reading jumps the engine runs very badly and stalls. I put a vacuum gauge on the car this afternoon, the vacuum is dropping off, but to around -5 when the tablet shows -1.6. When it happened this afternoon in the driveway, I noticed a very loud sucking sound from the throttle body hole for the IAC, the 1/2" hole on the top of the throttle body. The IAC on the tablet was showing 54% at the time. The problem occurs during normal light load driving, I have not tested it under WOT. Once the problem starts it will not go away until the car is off and cooled completely.
            I have around 4,000 miles on this engine with the Pro Flo 3 on it the entire time, this behavior is recent, it would have been very hard to get 4k miles on it with it acting like this.
            I have been talking to Edelbrock tech also. I sure wish they had a tech center that I could bring the car to.


            • #7
              I'm sure that with the engine running badly and dropping below the target idle rpm, the IAC is opening up to try and increase the engine speed. It's good to know that everything was fine for 4000 miles and that this is a recent issue. The fact that you removed and reinstalled the intake manifold and the problem improved but didn't completely go away does still kind of point towards a manifold sealing problem. Perhaps you still have a manifold sealing problem that results in a major air leak once the engine has warmed up. A simple test would be to spray something like WD-40 or brake cleaner around the intake manifold while the engine is running poorly to see if the engine speeds up.

              Unfortunately Edelbrock doesn't work on customer vehicles. If there are any doubts on your install then I highly recommend taking your car to a qualified shop or technician or tuner for assistance. Again, if you really think the MAP sensor is the problem, replace it. If the problem is fixed, great. If not, well then it's obviously something else and you'll need to keep troubleshooting.

              Also FWIW, there's no reason to "double tap" tech by both phone and here on the forum. You're getting the same information and answers from the same people. I'd ask that you go ahead an continue to work with tech primarily over the phone for the quickest response.


              • #8
                I checked the intake manifold when warmed up, couldn't find a leak. Edelbrock tech has my ECU now.


                • #9
                  Vacuum problem solved!
                  I tested the MAP sensor cold with a Mityvac, the tablet read the vacuum accurately, then I started the car, let it warm up and run for quite a while until it stumbled and died. I immediately checked the MAP sensor with the Mityvac and the tablet, the MAP sensor was completely unresponsive, note that the MAP sensor was only warm to the touch, not hot.
                  I installed a new MAP sensor and have now driven the car for several hours without a stumble at all. The car has always had a stumble since I first installed the Pro Flo 3, sometimes running great for an hour or so then one quick stumble, just enough to shake my confidence.


                  • #10
                    Thanks for the update and sharing your solution! Glad to hear you solved the issue and the car is running well.