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Help, 740 engine dies every few miles 700 1990

1990 740GL, engine just finished overhaul. Engine stops running almost every 3 miles and can't start(it cranks and the spark is fine) until after 10 minutes. The problem is more frequent in the day time when the air is hot.

The mechanics changed the main fuel pump under the chassis and check the fuel pump relay fine but the problem is still in existance. They have been working in the problem for two days and still can't find the clue. Does anyone have any idea?

Regards,


Mark








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    Help, 740 engine dies every few miles 700 1990

    Check the radio suppression relay and the rpm sensor, both classic sources of "no hot start" problems.








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      Help, 740 engine dies every few miles 700 1990

      My 740 uses an K-JETRONIC mechanical fuel injection system so I think it has no rpm sensor in it. And where is the radio suppression relay located? Thanks.

      Mark








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        Help, 740 engine dies every few miles 700 1990

        The main suspects are the fuel pump relay or the Hall Effect sensor in the distributor.(your car`s version of the RPM sensor)

        You don`t have a radio supression relay on a K Jet 740 (I assume your 740 is not a US model as they never had K Jet on 740s)

        The way to narrow down the search for the culprit is:-

        The next time it stops, immediately check for a spark at the plug leads and/or note whether the tachometer needle twitches when you try to restart.

        No spark or tacho movement indicates impies an ignition problem (probably the Hall sensor)

        If you have a spark it`s likely to be the pump relay.

        Note that on these cars these cars the fuel pump relay will not switch on the fuel pump if it does not sense ignition pulses.

        Colin.

        1990 740SE B200E/M47, remote C/Locking.








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          Help, 740 engine dies every few miles 700 1990

          The car is not a US model. The Hall sensor and fuel pump relay are all fine.

          The new finding today is that the in-tank pre-pump fails.

          The mechanics said the pre-pump failure has led to the hot start problem. Here is their logic: the pre-pump failure --> resistance to feed the main pump increases --> the main pump overloaded --> main pump overheated --> pressure to the fuel distributor dropped below working condition --> engine stalls

          Any comments on their logic? They will change the pre-pump tomorrow.

          Regards,

          Mark








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            Help, 740 engine dies every few miles 700 1990

            My thoughts are that they are chasing the problem by replacing parts rather than by diagnosis.

            When (if) they find the problem don`t pay for the parts that didn`t cure it.

            If the main pump was weak you could expect problems at high power/revs or cold start.

            The engine needs very little fuel to start when hot.

            Let us know the outcome!!

            Colin.

            1990 740SE B200E/M47, remote C/Locking.








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              Help, 740 engine dies every few miles 700 1990

              Today the mechanics changed the in-tank pump. Although it doesn't uproot the hot start problem, it does alleviates it. The car ran some 50Km today and died only once and resumed working in 2 minutes.

              The car is running in an air temperature of 95 Degree Fahrenheit.

              The main fuel pump is not a BOTCH but an OEM PIESBURG. There is a foam sleeve covering the main pump as soundproof. I am now concerning if the hot start problem has something to do with the OEM main pump, which is overheated in hot air.

              I will have the mechanics removed the main pump foam sleeve tomorrow and see if the problem can be solved.

              Any suggestion is appreciated.

              Mark








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                Help, 740 engine dies every few miles 700 1990

                If a foam sleeve makes the pump go too hot, the pump is failing. The pump is nominally cooled pretty good by the fuel that passes through it. It should run fine in ambient temperatures of 120F (did for me). Unless your fuel pressure regulator is bad, or the fuel return line restricted, the pump should be running cool enough -- almost at the temperature of the fuel itself.

                That's what I make of it, at least. A failing/failed pre-pump may lead to premature main pump failure, so the mechanics might be right for once :)

                Cheers, Kuba








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                  Help, 740 engine dies every few miles 700 1990

                  The mechanics changed the pre-pump, the fuel filter, and took off the foam sleeve covering the main pump. Now the car seems running fine, but the ambient temperature is not as high as before. I have to waited if it is working all right in hot weather.

                  The fuel filter may be the key culprit, which had not been changed in three years. The failed pre-pump was also the blame. I read from the FAQ that the pre-pump should be able to maintain idle even when the main pump is not working, is that true?

                  My 740 does have any ECU as it uses a K-Jetronic engine, so the number of sensor is limited.

                  Thanks very much for all your suggestions.

                  Mark







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