Engine Idle

[DMcV]

This is my first diesel truck and I am greatful to this forum for it's learned members helping me with my questions. Like this one :
When my engine is cold it will start and run at normal idle then after a few minutes it will increase in rpm. I have no problem with this happening, it goes back to normal the instant the brake pedal is touched. The PO said it was common with these engines but my question is why does it do it? Is it so it doesn't idle too long at one rpm? or what?
Thank you
Dennis

 

[Agnem]

That is by design. It is because the exhaust back presure valve has closed (creates a lot of back presure on the exhaust - which in turns heats the engine up faster). The high idle forces the warm up to occur. The idle goes down and the valve opens when you hit the brake like you said.

 

[h2odrx]

I've never tapped the brakes but mine does it then goes off real quick.

 

[69oiler]

even if you defeat the exhaust backpressure valve like i have (mine was sticking partially closed), the high idle still kicks in in colder weather. i've noticed that the colder it is outside, the higher the idle kicks up. i've seen it up to 1100-1200 rpm when it gets down around 0 degrees F

it does aid in warmup, and in battery charging, since the alternator doesn't put out max output at low idle.

 

[DMcV]

Thanks to all the replied.
It helps me understand what makes my truck tic...clatter...& rattle