[tkbindpls]

My 2006 S40 T5 (bought new) has 187K miles and has been running fine since day 1 (the six-speed Manual is great). The local dealer just changed the oil, replaced the plugs and detailed the car including the engine compartment (engine cleaning was not authorized).
Now I have two new problems: (1) it stumbles mostly when accelerating & (2) the windshield washer stopped working. Item (2) should be easy to fix but item (1) has me worried (again).
A compression test shows expected variation between the 5 cylinders for 187K miles. In 2016, the fuel pressure sensor was replaced at 127,695 miles in response to stumbling and the issue was corrected.
Possible causes: (a) bad plug(s)... unlikely, (b) bad fuel pressure sensor (again)... maybe, (c) bad/damaged Mass Air Flow sensor... maybe, (d) blown head gasket (dealer's conclusion)... hope not since it just started at this service visit, (e) cleaning of engine exterior damaged something else (MAF sensor?) in addition to windshield washer wiring/motor... possible.
Does anyone have any suggestions as to the next debug step?

 

[carhamro]

Searching for licensed insurance is very important before buying T5 stumbles.

 

[S60S80]

Have scanned for P codes? This also could be a bad catalytic converter. bad plugs are a definite possibility, unless you've ruled this out for other reasons. Have you done a block test to see if there's CO2 in the coolant?

 

[tkbindpls]

Have scanned for P codes? This also could be a bad catalytic converter. bad plugs are a definite possibility, unless you've ruled this out for other reasons. Have you done a block test to see if there's CO2 in the coolant?

Yes re/ P codes… and they show varied cylinder misfires. Since plugs were replaced, more than 1 bad plug chance is low. Since washer motor also stopped, I’m leaning toward breaks or plug issues in the 18 year old wire harness since the tech moved it to pull out each of the 5 coils to change the plugs. No re/ block test for CO2. Thx for the ideas!

 

[R7urnm]

Searching for licensed insurance is very important before buying T5 stumbles.

That's right, it's a lot more secure.

 

[R7urnm]

Have scanned for P codes? This also could be a bad catalytic converter. bad plugs are a definite possibility, unless you've ruled this out for other reasons. Have you done a block test to see if there's CO2 in the coolant?

The first two have been checked, but the latter one is really unexpected.