[jas10]

I've been reading about hot idle issues - especially with 6-speed trannys - but I haven't seen anything that explains root cause yet, so I thought I'd start a thread to get current thinking.

I recently bought this truck. It has 102k miles on it. It runs very strong and smooth except for idle when it gets hot. It does have a bit of a long crank problem too - takes 2-4 seconds to crank when it's hot. On cool mornings it fires faster.

I put my AE on it and ran several of the tests. The only code I got was exhaust back pressure range. Cylinder contribution passes and cylinder deceleration looks good with the biggest difference being 1.2% on #2. I also monitored ICP and IPR duty cycle while cranking and the numbers were fine.

My idle problem is noticeable when the engine is fully up to operating temp. When cold it runs smoothly. As it gets hot, though, I get a noticeable shudder/shake/bump in the idle. It's enough to rock the truck a little bit.

Does anyone know what causes this? I just changed oil (Delo) and replaced the fuel and air filters. I haven't checked fuel pressure yet. Could slightly low fuel pressure cause a rough idle?

I'm looking for ideas.

Thanks!
-john

 

[klhansen]

Check your ICP and IPR duty cycle at cold idle and again when hot. If you're seeing a larger duty cycle when hot, it means you've got a leak in the HP oil system that gets worse when the oil gets hotter and thinner.

You should be seeing somewhere around 11-12% duty cycle at around 475 psi ICP.

 

[jas10]

I'll do that tomorrow. Thanks.

 

[Organdoner]

Check your ICP and IPR duty cycle at cold idle and again when hot. If you're seeing a larger duty cycle when hot, it means you've got a leak in the HP oil system that gets worse when the oil gets hotter and thinner.

You should be seeing somewhere around 11-12% duty cycle at around 475 psi ICP.

Sorry to but in on this, but I have pretty well the same problem, it hasnt done it in a week or so, but once i drive the truck and it comes up to temp, say i shut it off come back and drive it, it will do the "miss" when im sitting at a light or something idling.

I was trying to find a guide for all these abreviations actually, any such thing?

Trucks a early 99'' f250, 125k

 

[klhansen]

I was trying to find a guide for all these abreviations actually, any such thing?

Here you go
Grab a cup of coffee and start reading.
http://www.thedieselstop.com/forums/f46/sticky-common-acronyms-43966/

 

[jas10]

AE says my ICP and Duty cycle are on the money when the engine is good and hot... 475-480 and 11.33%.

I checked perdel again and cylinders 1 and 2 are the only ones that show anything significant: 1.2 and .9 respectively.

So what do you think? Should I replace o-rings on my IPR to see if it makes a difference or check fuel pressure first?