2017 F150 XLT 3.5L turbo with 103k miles has started making soft knocking sound coming from engine. At about the same time, this started happening my gas mileage significantly decreased from 17 MPG city to about 13.5 MPG driving under the same conditions on the same route. I took it in for an oil change yesterday and asked if they could identify exactly where that was coming from and they said it sounded like the lifters. Has anyone on here experienced anything similar to this?

 

This is the sound it’s making

 

Ouch... what are the oil change intervals you're doing? and what kind of oil are you using?
I got my 2020 3.5 ecoboost recently just put over 1k miles and did an oil change at Ford, got surprised to see that they use a semi-syntetic 5w30, I'll definetely start doing the oil changes myself and use a better quality oil Full Synthetic, specially with this engines being so finicky with phasers and turbo.
In addition, since you live in a cold place (like me in Canada) I'd plug it in overnight assuming that you probably have a block heater installed to keep the oil warm so it can reach to the top of the engine faster for cold starts.

 

Since I have had it over year now maintenance has been spot on, full synthetic 5w30 oil change every 3 to 4 k, new plugs, trans fluid and filter change, adaptive learning reset which fixed some shifting issues it was having. I will try that, hoping if it is the lifters I can buy some time before I have to replace.

 

There is nothing wrong with the Motorcraft Semi-Synthetic. We have one 2017 F-150 with the standard 3.3N.A. V6 that has over 300,000 miles on it that has always had the Motorcraft semi-synthetic oil. A couple of others are also north of 200,000 miles. Now I do use full synthetic in my vehicles for the same reasons you mentioned, and I do my own oil changes. I am only stating that the semi-synthetic is not that bad of an oil either. I do prefer the full synthetic.

Regarding the noise, is there a larger rattle noise upon start-up and then fades? This engine would be on the tail end of the 3.5's that have cam chains stretching. If related, the stretching is due more to long oil change intervals more than oil type or brand.

 

Yes, and the full synthetic is just a preference for me! No, the noise is only at idle or I can only hear it during idle. There’s no other noises and it runs extremely smooth, but I can’t figure out why the gas mileage would’ve dropped so much.

 

Overhead cam engines don't have lifters.
That "knock" sounds severe to me. I wish you luck but full synthwtic won't fix that problem.

 

You might want to do a sanity check. 3.5 ecoboost has lifters.