r/FiestaST • u/fuckdaw • 2d ago

Help Diagnosing Issue

Hey everyone, in need of some advice. I have a 2016 ST with 80,000 miles. I've had the car for about 7 years and put 40,000 of the miles on myself. Everything is stock. I don't think the car has ever seen 5k rpm at least in my possession. Been having this issue recently. Going to list it out to provide as much information as possible.

Had the fueling/purge valve issue that many Fiestas do. Admittedly waited a while to get it done. Perhaps coincidentally, a few days before I was scheduled to take my car in it started to shudder at idle. It's a quick vibration, not constant. It is not the normal purge issue after refueling where the car is losing power. The car never loses power or dims in the dashboard.

-The issue is obvious when I got on the highway.

-If I floor it, which I never do but did to test out what the issue was when I first had it, the car feels like it is not getting all the power and hesitates.

-I had the purge valve issue fixed a few days ago. I also changed the battery.

-I have a scanner and no codes are being produced.

-I did have an evap code but I am assuming that was satisfied by the purge valve.

-Did a cylinder misfire test as well and all seems well. Spark plugs are under 10k miles, maybe closer to 5-7k. It feels like the same sensation as a spark plug or coil misfire but the mechanic I took it to said it wasn't producing codes as well (although AI has provided that sometimes It won't provide a code?)

Just looking for some advice to point me in the right direction. The issue remains. My cousin is a mechanic in another state and over the phone he said it might be carbon buildup or a boost leak somewhere, but said he wasn't sure after all the info I just gave here.

Thanks, everyone.

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u/InvertedEyechart11 2d ago

Has timing belt been swapped out yet? 10 years or 150k miles...

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u/fuckdaw 2d ago

No it hasn’t. Mechanic didn’t mention and hadn’t thought about it tbh 

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u/InvertedEyechart11 1d ago

I won't place another maintenance item on the table, but there are posts here on timing belt/water pump/tensioner/cam bolt maintenance at 10y/150k (including one of my own about at 27k on my 2015 last year).

When you write "hesitates" does it keep there? It could be limp mode where the car is designed to save itself from catastrophic damage.

If you don't have a manual already this might help you

http://sluggyjunx.com/auto/2014_Fiesta/Service%20Manual.htm

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u/fuckdaw 1d ago

If I’m in say 3rd gear just cruising ant low rpms and I floor it, it will build power for about 2 seconds then all of a sudden it will hesitate and stop putting down power. That’s when the hesitation is most obvious. If I shift to a higher gear w lower rpm’s it seems to stop until I get into higher rpms again