In the process of changing the spark plugs , can you check if there is something you might have disturbed? Could this just be coincidence?

There are only so many places for exhaust to leak and one of them is where the exhaust manifold is bolted to the head of the engine.
If there is a leak there, you could have a problem seeing and finding it especially if it is underneath and cannot be seen easily.

One other consideration could be the heating/ventilation system is normally pulling air in through the cowl. If that is restricted by debris the air could be getting sucked in from an opening in the rear of the vehicle where the exhaust normally goes out through the tailpipe.
Don't forget once that exhaust odor gets into the vehicle it can stay on the material and stink for quite a while.

Time to start looking elsewhere if it can't be found in the usual areas. What do you think?

 

Hello all,
Glad I found this thread as I have the identical problem and can find nothing anywhere else on the internet. I do not have a Ford but a 2015 BMW 328i but please hear me out. Here's my story:

In March 2021, i had my spark plugs changed at a non BMW garage. When i got the car back, i had exhaust coming into cabin through the ventilation, much worse when stopped at a red light. At first i thought it was exhaust coming from car in front of me but seemed quite strong for it to be that. Then i realized it was also happening when there was no one else around. It was so bad, i would be dizzy after 5-10 mins of driving. I brought the car back to the garage and they said nothing was wrong. Kept driving like that for 3-4 days and i couldn't stand it so brought the car back again and he took out all the spark plugs and pulled out coils as well and noticed one coil was all oxidized or tarnished kind of brownish color. They said there must've been a leak and replaced that one, car seemed to be running but perhaps slightly less power and engine had a slightly different sound when revving. I drove off and the heavy exhaust smell was gone but there remained a remnant smell coming through the ventilation. I believe whatever was leaking from spark plug, was getting sucked straight into ventilation, as the ventilation intake is directly beside the spark plug area. Also when the leak was happening, from outside the car, i could put my nose near the right side of the edge of the hood near the lower windshield (right near the spark plug region) and could smell this horrible semi combusted oil/gas smell which was the same as what was coming in through ventilation. When they fixed the spark plug i could no longer smell it near the hood and windshield but some remnant odor was still coming in through the vents.

I strongly believe that while the spark plug was leaking semi-combusted fumes and coming in the vents for 3-4 days of driving, the ventilation ducting got covered in soot and it remains stuck on the walls of the ducting to this day. I can change the air cabin filter every month which the vent's air runs through, and it always has the same smell of exhaust/semi-combusted oil. Every time i start the car after its been sitting a few hours, i get a huge blast of exhaust smelling air in my face, worse when its hot outside. And when opening car after being parked a while, you can smell it inside the car before even turning it on. I think the smell is always emanating from the vents whether air is flowing or not.

Unfortunately i have no solution for this other than taking apart dash and replacing all the ventilation ducting which seems like a huge expensive undertaking.

Do you think this is what's happening on your side as well?

 

Oh man, that's not good. Thanks for sharing your story.

My situation hasn't resolved but differs from your a little in that the HVAC doesn't blast me with smells unless I'm idling at a red light, where as yours seems to have been tainted and hits you with a stored blast after it's been sitting.

As long as I'm moving the car smells normal, with HVAC on or off. But stop moving and it comes through the vents within about 5 seconds.

I can smell it under the hood as well but can't pinpoint from where, as the spark plugs are more or less directly above the exhaust manifolds.

​​​​Based on your story I'm curious to maybe pull the front 3 coils and see if anything is sooty, but those back ones are a nightmare to get to.

 

I see what you're saying, probably as soon as you start moving the wind dilutes it to a point that you cant notice it anymore. I would definitely have your spark plugs checked if they were torques properly or whatever else could be causing a non tight seal.