Hello all,
Four years ago we purchased a used xc90 2.5t with 150k miles and after a few months began noticing a stumble/misfire sensation when accelerating from hwy speeds. Some times it is almost unnoticeable but other times quite alarming. When cruising at continuous speeds it is fine but when accelerating it is right there.
Since the problem began I started replacing the obvious/cheap and easy stuff first such as spark plugs, timing belt, oil vapor system, oil pump, coil packs and more recently have replaced the fuel injectors, fuel rail pressure sensor and fuel system ecu , fuel filter and finally yesterday replaced the fuel pump.
Despite this continuous issue we have put almost 40k miles on it (now approaching 198k) and have had no faults in the ecu, no check engine light.
I have a nice scan tool that provides real-time monitoring of all engine functions and I never see anything out of range including misfire detection or fuel pressure deviation.
Id really appreciate any ideas on what I might be missing here because I don’t know what to do next.

 

Some spitball thoughts: when you did the spark plugs, did you go with Genuine Volvo to ensure the correct fit/gap? Have you tested for intake air leaks (ie smoke test) etc? tested that the airway boots are secure and no cracks? Have you tested compression? How does the temp gauge act and does this happen with both warm and cold engine or just once warmed up? I'd also look at the wiring harness to the coils to make sure there's no cracked connectors, broken insulation etc. What fuel octane do you run? Regarding a CEL, misfires are accumulating codes so a random burp won't set the code but if you force the car to stumble a few times on a drive, you can probably force one to set (I had a coil issue once and did this to set a code to find the culprit).

 

Are you sure it's a misfire you are trying to fix? Could it be the common symptom that a worn cv joint shows? That is a rumble/vibration when accelerating and goes away when not under load or decelerating. Granted you say it's been the same for 40k miles - a cv joint might have gotten worse during that period.

 

I don’t recall the specific spark plug but they were Bosch from AutoZone, however I did not gap them. That is a thought. I have also never done a compression test. I think my next course of action will be to replace the plugs with genuine Volvo and do a compression test while I’m in there. I’ve had the intake and airway boots off a few times and have never seen cracks or leaks and have sprayed carb clean around those and vacuum ports, but will take another look. I do not believe it is related to CV joints axles or drivetrain .

 

one other thought is to clean the contacts to the ECU and connectors for various sensors. Idea here is if there's some corrosion (say on the connector for the ECT) you could get a slightly off reading back to the ECU which will alter fuel trim w/o setting a code.