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Volvo XC70 D5244T4 2008: rough idle when warm and no load


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Hello, I have a manual Volvo XC70, D5244T4, 2008, 300000km with a rough idle when warm and no load. No trouble codes. After engine gets warm and i stop the car from moving, shut down the AC and lights, the engine speed varies from 670 to 730 instead of staying at 700RPM. If i switch on the AC then the idle stabilizes at 700 +/- 5RPM. If the engine is cold there is no problem with or without load. I have replaced EGR valve and actuator because even after cleaning the deposits, the valve required force to move it and the actuator was damaged and read EGR position was all over the place. Swirl flaps were exchanged and compartment cleaned of deposits. Swirl flaps were replaced because the diameter shrank from rubbing against deposits. Throttle body cleaned. MAF cleaned. Alternator swapped. Fuel pump regulator exchanged and variation in fuel pressure dropped from 3000kpa to 1000kpa. Injectors checked and are within tolerance. If i look with the diagnose tool at the graphs: the engine rpm, load_pct, maf readings fluctuate with no load, but as soon as i turn on the AC they all stabilize. 

The car runs good otherwise and i also do not notice this issue often because AC is mostly on. But I know the problem is there and it kills me. I do not want to waste more money in replacing or checking random things but i do want to fix the issue as I want to keep the car. 

Please help me Scotty,  and of course, the rest of the community. 



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1 Answer

Check for vacuum leaks. I would also check the air filter, oxygen sensors and PCV valve.

Checked for vacuum leaks with smoke and starter fluid spray. Also i suppose a vacuum leak would manifest when engine is cold or, when warm, when it has the AC on. Vacuum leak shouldn't mess the fuel air mixture? Also fuel air mixture was reported by a mechanic as being good. I could double check myself but i do not know exactly how to check the mixture.
Air filter was chand twice since troubleshooting this old problem.
O2 sensor reading seems steady on the graphs. What can I chek on a oxygen sensor?
Pcv valve only checked visually for leaks. How should that be checked?

Also, can you please explain how these correlate with the symptoms i described: rough idle when warm engine and no load? And how can i check it with the diagnostic tool?
I want to eliminate guesswork by checking physically everything, a lot of mechanics already did this, this is why I am appealing to this community. Smarter disgnostics.
Thank you.

PCV Valve and air filter: if both of these components are dirty or clogged, they will not function properly.
Oxygen sensor: use a multimeter to check that they are not failing.
Vacuum Leaks: If you hear a sound from the engine, then your engine will not run correctly.