Adjusting Idle on 5.9 Cummins

[jjrowan]

I have a 1998 Gulf Stream Sun Voyager Diesel pusher, 5.9 liter 12 valve Cummins with Allison 4 speed trans. When I start the engine (starts right up) it idles around 700 rpm. When she warms up she stays between 1000 and 1200 rpm making shifting from neutral into drive or reverse abrupt. Why does the idle go fro 700 up to 1000 - 1200 when at temperature and can I safely / easily adjust it down to 700 without affecting startup idle and oil pressure?

[jjrowan]

Drove RV today, when arrived at destination RPMs were again at 1200, tried revving quickly to 2000 and off, no change, tried a few times up to 2000 quickly but idle comes back to 1200. I have to check with Cummins on the possibility of the return spring issue.

[sjholt]

That looks like an 98 1/2 Motor with a VP44 Pump?
In the first picture, below the throttle cable, there is a standoff or a peg there that should have a return spring on it? It should go from the peg to the bracket that the throttle and cruise cables mount.
If you have the engine all warmed up and idling fast, pull back on that peg and see if the motor slows down. If it does then I would say that a return spring is missing.
I hope this helps as it is a WAG.

[jjrowan]

I don't know if this is the throttle linkage. It didn't look gummed up. Is this what you're suggesting I spray a cleaner on? The problem happens within 5 minutes. I drive from here to a parking lot 2.5 miles away and the idle is already a problem.

[Mote]

I have the same engine in my pickup truck. When cold it will idle around 700rpm, when warm around 900. Not sure why yours goes all the way up to 1200.
I had to turn the idle up a little as it was around 500rpm when cold and I'd have to keep my foot on the pedal for the first minute so the engine wouldn't die.
I would agree with the idea of getting a can of carb cleaner and cleaning off and dirt and grime around the throttle linkage. Just make to do it with the engine off and cold.

[sjholt]

You might check your return springs on the throttle in the engine compartment and see if there are two of them. If they are there- get some lithium spray lube and lubricate the linkage.

[Dog_Trainer]

mpfireman wrote:
Cummins engines have no glow plugs, but a heat grid that pre heats the air prior to ignition. This heat grid will cause the engine to idle faster, ( sometimes ) as it is re charging the battery. Thus as stated, you may have to rev up your engine for a few seconds, thus allowing the RPM to drop into the safe zone.

If you have a 12 valve then you do not have a grid heater. There are also no glo plugs. I am not sure what is advancing the rpms maybe some additional information such as what happens if you rev the motor and then where are the rpms? or how long after the start does it take to get these higher rpms?

[mpfireman]

Cummins engines have no glow plugs, but a heat grid that pre heats the air prior to ignition. This heat grid will cause the engine to idle faster, ( sometimes ) as it is re charging the battery. Thus as stated, you may have to rev up your engine for a few seconds, thus allowing the RPM to drop into the safe zone.

[powderman426]

On the 5.9s it idles at 700 but will ramp up if idling too long to protect the engine. If you Rev it for a second or so it should drop back to normal idle.

I hope I understood the question.