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Thread: High Fuel Flow

  1. #1

    Default High Fuel Flow

    I have two 8.1 MPI crusaders. I have Murphy Power View engine gauges and one of the things I monitor is the fuel flow. Recently I noticed higher than normal fuel flow on the stbd. engine at normal cruise power. Port was normal at 15 gph., stbd. showed 18 gph. Engine sounded just fine and other items were normal. What could cause this ?

  2. #2
    Join Date
    Dec 2011
    Location
    San Jose del Cabo, MX, Osprey, FL
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    311

    Default Re: High Fuel Flow

    If you find that you now have to advance one throttle ahead of the other to maintain equal rpm, then you have a cylinder not firing (either spark or burnt valve). You will burn 8/7 x 15 gph in that engine (allowing for rounding pretty close to your observed 18 gph) to maintain equal power output.

    If the throttles are equal as they were before at cruise, then I would look for an injector stuck on. You can pull the plugs and look for the one running super rich and send it out to be cleaned or replaced. Or do them all. It's likely time.

  3. #3

    Default Re: High Fuel Flow

    You said >> You can pull the plugs and look for the one running super rich and send it out to be cleaned or replaced. << How do I tell if a spark plug is running rich?

    Anyhow, plugs are new, have about 30 hrs. on them. Could old fuel (one year old) have anything to do with it?

  4. #4
    Join Date
    Dec 2011
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    San Jose del Cabo, MX, Osprey, FL
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    311

    Default Re: High Fuel Flow

    Too rich is usually indicated by lots of carbon on the plug.

    In answer to your question old fuel can certainly lead to problems. Not sure if more fuel usage would be among them, however. If you had crud/snot in the old fuel it could clog an injector. In that case you would need more throttle to keep the same rpm as the other engine. Because you would be pumping more air (throttle open more) your ECM would tell your injectors to stay on longer (causing more fuel usage) but because one of the eight injectors was clogged it would use less (7/8). End result should be about the same fuel usage (which is not what you are seeing).

    Check your relative throttle positions. That is an important diagnostic point.

  5. #5
    Join Date
    Sep 2008
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    Maryland - USA
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    6,824

    Default Re: High Fuel Flow

    a MAP or ECT sensor going bad will change the operating point on the fuel delivery curve.....as would a connection that has gone bad at either of those sensors....

    Old fuel, especially if it wasn't treated correctly, won't have as much energy potential so it will take more to produce a given amount of torque....

  6. #6
    Join Date
    Dec 2011
    Location
    San Jose del Cabo, MX, Osprey, FL
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    311

    Default Re: High Fuel Flow

    The professional approach is always to scan the engine and compare to specs. It is also useful in cases such as this to carefully compare the data to that from the other engine. To do this you will need a scanner and the Diagnostic manual for the ECM (computer) on your engine. The ECM will be an MEFI (2 or 3 or 4, etc.) depending upon the year of manufacture. The Diagnostic manual is also very useful in understanding the operation of the engine and trouble shooting, even if you don't have access to a scanner.

    The scanner is useful for most trouble shooting and occasionally is the only way to find and correct problems (such as with the infamous "cam retard" adjustment).

    You did not mention scan data so I assumed you do not have a scanner. My suggestions were based upon experiences I had with a similar MPI engine. And in earlier engines (mine are 2003), injector performance and individual cylinder spark performance is not monitored by the ECM (engine computer).

    You can check out Mark's suggestions of the manifold air pressure sensor or the engine coolant temperature sensor with a scanner, or you can simply swap them between engines and see if the problem moves. They are a quick and easy swap.

    What year are your engines? Have you had occasion, for any reason, to loosen the distributor clamp (this would change the cam retard)?

  7. #7

    Default Re: High Fuel Flow

    They are 2007, no distributer.

  8. #8
    Join Date
    Dec 2011
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    San Jose del Cabo, MX, Osprey, FL
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    Default Re: High Fuel Flow

    OK, I would still take a look and plugs and compression to see if there is a hinky cylinder. Also note throttle positions at cruise. If that fails to point you in a direction, a scan of both engines may reveal what is different. You might also question if the Murphy is giving correct information. I don't know what drives it. A scan will give you the exact injector on times at cruise.

  9. #9
    Join Date
    Sep 2008
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    Default Re: High Fuel Flow

    The 2007's probably have the CAN-Bus based ECM-07 setup, not the MEFI-4 ECU....

    you could do the part swap approach with the 'other engine' but the fastest way would be to get a scan tool...the RINDA stuff is standard in the industry. They aren't cheap but, if you understand engines, you'll likely recover its costs in a couple of repairs....

  10. #10

    Default Re: High Fuel Flow

    Something else about the same engine that I didn't mention but I will. This engine has been loosing a very small amount of coolant (dex-cool). It didn't start until just after I had the elbows, risers and heat exchanger replaced, along with all new spark plugs. One other thing in reference to the high fuel flow. The fuel quantity has remained about the same in both tanks. Could it possibly be an indicating problem ? I'll see if I can find a mechanic with a scan tool.

  11. #11

    Default Re: High Fuel Flow

    Problem solved. Took on 200 gals. of fresh fuel and added 25 ozs. of Sta-Bil 360. Fuel flow back to where it should be.

  12. #12
    Join Date
    Sep 2008
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    Default Re: High Fuel Flow

    so, are you saying it was due to "stale fuel"??

  13. #13

    Default Re: High Fuel Flow

    Yup, absolutely.

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