Odd High-Idle Behavior

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Kevin Means

Kevin Means

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Hereford, Arizona
In preparation for an upcoming trip, I degreased and freshwater-rinsed the radiator, then started the engine to blow any remaining degreaser and water out of the radiator. After the engine started, it immediately went to high-idle on its own (about 1050 RPM.) It usually idles at 800 RPM.

As a test, I turned on the high-idle feature and the RPM increased to about 1250 RPM, so I disengaged high-idle, and the RPM dropped to 1050 again - still about 250 RPM higher than normal. I shut the engine off for a moment, and then restarted it. All was normal - it idled at 800 RPM and high-idle was about 1050 RPM.

There were no check-engine lights, and I don't think there are any problems. It's just unusual behavior for our engine, which is a Cummins 8.9. Anyone ever experience that before, or know why it might happen? Is there a water-sensitive component behind the radiator that controls the high-idle feature that I may have inadvertently soaked?

Kev
 
Sounds like a thermo sensor or electrical plug for the fan (if it is electric driven) got soaked and made a temp connection. Does your side radiator have an electric thermo switch, or is it all gear driven? 
 
John From Detroit said:
Often a cold engine idles faster.
Click to expand...

I don't believe that's the case with electronically controlled diesel engines, at least not mine
 
Does your chassis have a variable speed fan?  I think most late model side radiator diesels do, but not ure about rear rads.  It's a key factor because with it, the engine computer will increase fan speed if needed for more cooling or a/c operation, but without it the computer has to increase engine rpms to get more fan speed.  This doesn't really affect anything until the engine reaches operating temperature, though.

The engine computer responds to a/c compressor engaging, both because of the increase in engine load and the greater need for air flow through the  a/c condensor.  Try it with the dash a/c both on & off to see if that changes the rpms.
 
Marty, it's a rear radiator, and it does have a temperature controlled auxiliary cooling fan that comes on when a certain temp is exceeded, and I was wondering if I might have inadvertently soaked the sensor that triggers it when I was rinsing out the radiator. That fan definitely draws additional horse power when it's running, because it's pulling a lot of air through it. That might explain it.

John, thanks but when our engine is first started, it (normally) idles at 800 RPM, no matter if it's near freezing or 100 degrees outside, or if the engine is warm or cold. It wasn't necessary to warm it up to get the glitch to go away. It started working normally as soon as I shut the engine down and restarted it.

Gary, since the engine was cold when the glitch occurred, I agree that it wasn't a temperature-induced glitch... although if I soaked a sensor that turns the aux cooling fan on, it may have "thought" it was hot. (Just guessing at that) I have to back the RV out of the garage each month to exercise the generator, and when I do, I always run the dash air to keep the seals lubricated (per the owners manual.) The tach has never indicated any change in RPM when the dash air is on or off.

Thanks for everyone's input. It seems to be working fine, so unless it happens again, I think I'm just going to write it off to something getting wet when I was rinsing off the radiator - unless someone's got a better suggestion.

Kev
 
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