Dear All,

Quick questions for you, on the dirty diesel, what controls the fueling at idle. My understanding is:

MAF is only used when an EGR is in place

MAP sensor determines the air density which in turn determines the fueling - is this at idle for just normal engine running?

Idle Control Valve - the diesel doesn't have one of these

Air temp sensor - as the name states

The reason I ask is that every 60 seconds or so my revs dip on idle and recovers. This doesnt happens when AC is off ( new ac pump has been fitted so not that). I thought it could be a sensor or something. I have a replacement (second hand) 4th injector installed which I hope rules that out as being an issue. Could it be the Crank Position Sensor?

Many thanks

Tom

Does it dip in time with the AC compressor clutch kicking in? On a petrol engine the HEVAC tells the engine ECU that the compressor is about to engage so it can adjust the Idle Air valve to maintain the idle speed. Presumably the diesel has something similar but no idea what. One of our resident diesel experts should be able to tell you.

Yes, it dips when the AC clutch engages which I understand is normal. But then it continues to do so every 30-60secs.

Perhaps I am being stupid but once the clutch has engaged, does it remain engaged until the ac or engine is turned off?

Many thanks

Tom

I'm pretty sure it'll disengage/engage as the temp reaches set point and drops.
Mine does the same, more of a cough only when ac is on, never really timed it but at a guess every 20 secs or so. No issues when driving just idling. I've only had the bus 11 months and working ac for about a month.

tking88 wrote:

Yes, it dips when the AC clutch engages which I understand is normal. But then it continues to do so every 30-60secs.

Perhaps I am being stupid but once the clutch has engaged, does it remain engaged until the ac or engine is turned off?

No it doesn't, it engages every so often to raise the pressure in the system then disengages until the pressure drops to keep it in the correct pressure window. I understand the AC compressor on a diesel is a bit buried but on a V8 where it sits on top of the engine so in easy view, you can watch it continuously engaging and disengaging. As the compressor has no built in regulation, if it was to run all the time the pressure would get far too high.

The diesel one is quite hard to see the clutch face particularly so. I've thought a blob of paint on the face would be a way here to see it engaging/disengaging.?

Ahhh right. That might explain the dips in the revs.

Does the diesel not increase the fuel/revs when the AC engages? If it does, what controls this feature?