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  • #16
    So one thing to try is to place the coolant temp sensor (probably have to get a second one) in ambient air and ground the pipe thread end with an alligator clip. Basically trying to lie to the ECM about a lower cooling water temp.

    Without an O2 oxygen sensor and catalytic converters, the classic meaning of closed loop doesn't fit.

    So the ECM strategy is most likely full open-loop (which is just a "lookup table" injector pulsewidth based on speed and load...more or less the crank signal, the TPS and the MAP sensor) but is modified by the coolant temp sensor for a fuel-rich start for reliable starting, which ramps up to a more chemically-correct ratio for economical running, but always on the side of slightly-rich to avoid stumbles and sags when you always want power.

    It would seem like the very-rich start is working well and the getting-less-rich warmup period is OK, but the stable running phase is where you get cyclical idle.

    Going to guess here at possibiities...but it hints at the fuel pressure seems wrong = too low if there are no vacuum leaks. Since there's no O2 sensor to "correct" an air-fuel ratio that's too lean (integrated over time), the open loop condition of slightly lean causes the idle to stumble and lose RPM which drives an IAC response and the MAP sensing that increases the RPM. A side possibility is the MAP sensor is causing the issue by reading incorectly and it might be easier to work on that first. There is some mechanical fuel pressure regulation somewhere in the fuel rail that keeps the fuel pressure at a constant pressure relative to the intake (meaning its cyclic up and down as the throttle is opened and closed) and usually there is some spring somewhere that influences the baseline pressure.

    Not sure if I confused you more here or am getting towards clarity about what makes the system run.

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    • #17
      It seems that the thermostat is either stuck or partially open. The temperature swing does not look right. Is this engine fresh water cooled? Has the sea water pump been serviced lately? Has it ever lost some of it’s blades? If so were they all accounted for?
      Kenner, Louisiana 1980 Donzi 18'. 2+3 6.2 MPI Alpha 1

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      • #18
        Saline Marina, I'm following you 👍
        So you say disconnect the lead to the coolant temp sensor, reconnect it to a new sensor ( all while leaving the orig one in place essentially plugging the coolant port) and ground the threads of the new one with a alligator clip lead, correct? This should trick computer to reading a cooler ambient temp, and therefore cause a smooth idle.
        As for the map sensor, I'm less familiar with it, how it exactly works and how to test.
        There is a fuel pressure regulator at the fuel rail. I've removed it for inspection and its clean. Speaking with the VP shop yesterday, they said the fuel pressure for both pumps was within VP specs & there were no signs of vacuum leaks.

        Kenny Nunez, the temp swing is quick when cooling down, but takes it a bit of time to rise back upward to 165-170°. In fact it often takes a bit of throttle advancement to warm it up enough to get the thermostat to open up again. It appears to me the the thermostat is operating correctly. I'm operating on the hose and it's very cool well water, so I could see how it would cool very fast and keep the operating temps down during idle.

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        • #19
          Good, sorry I was getting tired last night and felt like I was not being fully cohesive in my thoughts.

          Yes that was my thought. I think the metal end has to be grounded for it to work right, many sensors state "no teflon tape" as it could insulate the path to ground. But in ambient air its going to read "cold". It is a possibility it could set an ECM code by doing this for not being in a certain range after a certain time, but I think you'd get insight into diagnosing the idle problem.

          In a truck engine you'd have the long term fuel trim from the O2 sensor feedback to diagnose and correct rich or lean running, I think this has up to around a +/-10% authority to modify injector pulsewidth. Fuel trims are a very slow stable signal that's averaged/integrated over a long period of time. Also in a truck engine you'd replace every vacuum hose on it with a new one just to (shotgun) avoid & correct the possibility of vacuum leaks.

          The MAP sensor is manifold absolute pressure or a vacuum gage. It directionally aligns with the TPS but also consider at idle the TPS is pegged to 0% (more or less) and the MAP is used as one feedback mechanism for IAC....as well as the engine speed from distributor or crank pulse. As I recall they are tested by plugging into a handheld vacuum pump like MityVac and stabilized at a certain level and a certain voltage should result. It might be easier to throw a new part at it, but I'm not sure of the cost. If its a $20 sensor I'd do it, but but if its a $100 sensor then I'd be inclined to do some more testing!

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          • #20
            Saline Marina, you were very clear on your prior message and I thank you for your time👍
            I will try the coolant temp sensor and see what happens. I know I've already thrown a code yesterday when I disconnected the IAC connector.
            I rented a scanner but have since sent it back. If I'm throwing codes for doing these different tests, will that affect my future tests/ diagnosing without clearing the old codes that have been stored 1st?

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            • #21
              Does this have a crankshaft sensor like the newer mpi/gxi ? This could cause such an issue and be intermittent too.

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              • #22
                Does this have a crankshaft sensor like the newer mpi/gxi ? This could cause such an issue and be intermittent too.

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                • #23
                  hope you figure it out...thinking about it gives me a headache...glad I still have these....lol.....
                  1988 Four Winns 200 Horizon
                  4.3 OMC Cobra

                  98 Jeep Grand Cherokee 4.0 Selectrac
                  07 Jeep Grand Cherokee Limited 5.7 Quadradrive II

                  "While air doesn't freeze....rust never sleeps"

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                  • #24
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                    QBhoy, this does have a crank sensor (it's a 2008 motor) and was just replaced by my VP mechanic. A brand new VP sensor was installed and no fix with my idle issue.

                    I haven't had the opportunity to test the coolant temp sensor like Saline Marina has suggested.
                    Outside of this sensor test, I'm completely at a loss. It definitely appears to be temp related.

                    I'm afraid my VP machanic shop is at the point that they are slowly and quietly "backing out" as their last communication was that the fuel burn was incorrect because they, for one reason or another, think my new motor was a "New" aka remanufactured motor with different internals than a factory motor and therefore the computer is trying to adjust for a different piston & bore. This meeting of the minds idea came, supposedly from a conversation my VP shop said they had with VOLVO PENTA themselves.

                    I've been assured by the company I purchased the motor from that my new motor is a BRAND NEW FACTORY GM VP motor and the exact same specs as the factory installed motor.

                    This is driving me insane. It has to be something simple but for the life of me ( and my VP shop) the issue just hasn't raised its ugly head to the point that anyone can put their finger on it. I hate to incur additional costs but maybe I need to find another VP shop near me in the Tampa Bay area to take a look, to get a second opinion? I'm just very very hesitant because I know the extreme hourly rates of these shops and I've already spent a Significant Amount of $$$ money $$$ already!
                    Last edited by Regal1973; September 5th, 2019, 10:31 AM.

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