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Hesitating at full throttle ?

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  • #16
    Hello,
    we ran a test at night. There is no spark from ht leads. But we noticed the cat became red. So I guess the problem is about ignition and fuel is burnt into the cat. HT leads seem to be ok. Is there a power supply module for ignition coils ? I don"t think 3 coils can fail at the same time.
    Last edited by marcjero; 19-08-17, 06:58 PM.

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    • #17
      Coils powered from ECU as far as I know.
      Are all the electrical earths OK? (coils especially).
      If not ignition related, then possibly massive overfuelling but hard to think of a cause.

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      • #18
        Yes earths are fine. I don't think it's overfueling else it should smell bad. Fuel trims are close to 0. Could be an ecu issue ?

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        • #19
          Hi,

          I changed the crankshatft postion sensor but the problem is still here. It takes more time to appear though. Maybe because the weather is cooler now. I also noticed that the intercooler fan is not working anymore. Does a failing intercooler fan could lead to misfire problems ?

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          • #20
            Originally posted by marcjero View Post
            Hi,

            I changed the crankshatft postion sensor but the problem is still here. It takes more time to appear though. Maybe because the weather is cooler now. I also noticed that the intercooler fan is not working anymore. Does a failing intercooler fan could lead to misfire problems ?

            Wouldn't have thought so though the ECU will pick up higher IAT and make some compensation. Try hot wiring the IC fan to eliminate as suspect. Anyway, at speed, I'd guess (cooling) airflow through the IC is mainly from the scoop.
            Time to pull the plugs and look for a specific cylinder being the cause?

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            • #21
              I agree. Do you mean a cylinder leakage test ? What is strange is this happening after a 15 minutes drive. The engine is running very well until the problem start.

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              • #22
                Please remove all plugs and post photos of their business end. Make sure to mark them so you know where they came from.

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                • #23
                  You have checked for leaks between the the turbo manifold and the exhaust haven’t you?

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                  • #24
                    Originally posted by marcjero View Post
                    I agree. Do you mean a cylinder leakage test ?



                    No, just read then for colour looking for signs of a rich or weak mix on one or more cylinders.


                    [QUOTE=marcjero;n879646What is strange is this happening after a 15 minutes drive. The engine is running very well until the problem start.[/QUOTE]

                    Implies a temp problem - over-fuelling, or some electrical/electronic component fading with temp..Would have thought though that an over-fuelling problem would manifest itself sooner than 15 mins - but then there's the glowing cat....

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                    • #25
                      I think I know. Fuel pump could be damaged and overheating : fuel pressure starts dropping, engine runs lean and just stalls when boost is coming. Running lean increases EGT. I will hook a pressure gauge to the injection rail.

                      Yes confirmed ! Fuel pressure drops to about 1 bar when misfire occurs. Fuel pump noise is different too when this happens. Blocked fuel filter (recently changed), power supply issue, or just a weak pump ? We will check the pump voltage during a run but I'm pretty sure the pump is dead. What brand of pump would you recommend ? An ethanol compatible pump would be great.
                      Last edited by marcjero; 23-10-17, 04:56 PM.

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                      • #26
                        So the fuel pump voltage is fine. And the fuel filter was changed few miles ago. So I think it's time to change the fuel pump. Is it possible to do this job without removing the fuel tank from the car ? I wasn't able to get the ring out from the boot until now. Thanks guys !
                        Last edited by marcjero; 25-10-17, 07:56 PM.

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                        • #27
                          Quick and easy on a Roadster as fuel sender unit is accessible from the top. Two days job on a 450. Arm yourself with a fuel sender unit nut removal tool and a kettle to make piping hot water. Pour piping hot water over the large nut to ease its removal. I prefer the three legged removal tools. Mine is a Laser 4768.

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                          • #28
                            Originally posted by marcjero View Post
                            Fuel pressure drops to about 1 bar when misfire occurs.
                            Fuel pressure regulator?

                            Originally posted by marcjero View Post
                            Fuel pump noise is different too when this happens.
                            Faster? Slower?

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                            • #29
                              Another possibility is one or more injectors remaining open.

                              If fuel pressure should be around 4 bar above manifold pressure, then a drop to 1 bar is barely enough to overcome manifold pressure when the turbo is boosting and reducing fuelling to such an extent would make the mixture so lean as to not ignite at all - flame out.

                              If however the pressure drop is due to injectors open so long that the pump cannot keep up, then over fuelling would be the result (and an overheated cat).

                              If you can shut down the motor ASAP after a misfire event and pull the plugs, then dry or soaking/black plug or plug(s) should tell you which. Or, if the fuel pump slows (faulty pump) or quickens (open injectors, reduced load on pump).

                              A persistently open injector could be caused by a short in the wiring (they open on being grounded) and if batch fired, a single short would affect all three injectors. The engine moving on its mounts due to torque reaction during full throttle running may be bringing a part of the wiring harness into contact with a potential earth path (as occurred with early 964 Porsches when their ignition system shorted to the ABS and caused both a misfire and the ABS warning light to illuminate).

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                              • #30
                                I used to get a hesitation that went away after I fitted a spring clip to the vacuum hose that went to the fuel regulator, it may be worth checking that hose.

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