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5.7 Volvo Penta running on/dieseling


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I will give you full description of the boat, 1998 252 Sunesta with a 5.7 GS Volvo Penta. Plugs are a year old with about 40 hours on them, I use pump gas 87 octane unleaded and I have adjusted the idle down as low as I dare. I have bought a return spring to insure the throttle plates are closed yet haven't installed it yet. Any suggestions to stop the occasional dieseling/running on.

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You touched on the idle being set too high so that's covered. Piston dome carbon build up will raise the compression ratio also causing the problem, but unless you troll a lot that's not likely. If you do spend hours at idle speed, then get some General Motors TEC (top engine cleaner) and follow the directions. It will clean a piston dome cleaner than I can do with a wire brush on a drill. Amazing stuff. Too much base ignition timing, plugs that are too high of a heat range, engine operating temperature too high, or really poor quality fuel will also cause your issue. If she is raw water cooled, then debris in the cylinder head water jackets, or even a weak raw water pump will caused localized combustion chamber excess heating, even though the operating temp looks normal. Also, after a hard run it is important to allow the engine to idle unloaded for say 15 seconds to allow the temperature to normalize throughout the block, as all engines heat soak and parts of the engine, especially the cylinder heads will spike in temperature even after the heavy load is removed.This time of year the refiners are switching over to summer blend gasoline which has less butane, and is also less susceptible to pre-ignition. If it were me I'd wait until May first when winter blend will be gone for the summer, and put a tank of 89 octane in her, as it too is less susceptible to run-on. Check your plug heat range too, as automotive plugs for the same engine are typically one or two heat ranges higher. It's not the the plugs fire hotter, it's that they don't dissipate heat as quickly. Important in a car engine, but death to a boat engine. Lastly,if she is running on the high end of the specified operating temperature range, flush the water jackets, replace the thermostat, and do a good PM on the raw water make-up pump.    W

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Another possibility would be a vacuum leak. Even a minor one will cause "run on". Spray a little carb cleaner around the carb base area / any intake manifold fittings while engine is running and see if idle speed changes. If it changes, there is a leak and needs to be looked at.

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I Would have each sparkplug removed. One at a time. See if any of them have a SUPER WHITE color. Check everyone of the plugs.  If any have the metal ends rounded ? Tiny blisters on the white insulator ?  Check the sparkplug number on ALL OF  the plugs with what the engine should have.

A too slow of a idle can cause after running due to incomplete combustion of the gasoline mixture.  Then the unburned carbon starts cooking onto the insides of the cylinder parts. It forms a VERY HARD BLACK shiny layer on cylinder parts.  If you find that ?  Use Wingnut General Motor TEC. 

Set the idle speed to the correct rpms.........use a tachometer. A accurate one.

 

 

I did that trying to save a buck.

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