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2017 Black RS
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Sounds good! I had been fighting an issue for a while, but the issue kept changing as I was throwing parts at it. The final issue was resolved after replacing my upstream o2, even though I had replaced it only about 500 miles before. I was going crazy, because I thought it couldn't have been the o2 since it was new, but my tuner told me to replace it again anyways. I guess it was DOA, or I fouled it somehow.
 

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2017, Mk3 Focus RS. Stock, for now!
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64 Posts
Discussion Starter · #62 ·
Diagnostic reveals...P0327

Diagnostic points to both NoX sensors having faults. Booked in for replacements now.

quick Google says nox sensors can apparently cause crummy idle..... So hopefully this is the source of my issue at last. Will soon see!

Update to this. It was the knock sensors not NOx sensors.

I'm not confident this will resolve my original issue, rather I think this is a separate issue 馃槀 oh the joy!

Ford want to replace both knock sensors, total cost around 拢740. Thankfully, covered under my extended warranty with motoreasy.

(Shout out to motoreasy/if anyone is considering going with them, let me know so I can refer you, think you get a discount. My first time having to use it, but it was a good experience. Emailed motoreasy 4pm 23rd December and had an accepted claim decision back at 8:30am on Christmas eve)
 

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2017, Mk3 Focus RS. Stock, for now!
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Discussion Starter · #63 ·
Update to this. It was the knock sensors not NOx sensors.

I'm not confident this will resolve my original issue, rather I think this is a separate issue 馃槀 oh the joy!

Ford want to replace both knock sensors, total cost around 拢740. Thankfully, covered under my extended warranty with motoreasy.

(Shout out to motoreasy/if anyone is considering going with them, let me know so I can refer you, think you get a discount. My first time having to use it, but it was a good experience. Emailed motoreasy 4pm 23rd December and had an accepted claim decision back at 8:30am on Christmas eve)

And sadly, did not fix my issue.

Interestingly, I thought it did initially as my issue disappeared. But after about 10-15 miles of driving, it came back. I suspect Ford reset my KAM, so the car must have needed to learn things again. And I wonder now if the source of my problem is a dodgy sensor somewhere, that is feeding the ECU bad info. Because it seems strange the car is fine (after a suspected KAM reset) but then once it beds in, the issue returns


Question is, how the hell do I figure out which one. If all the sensors are reporting things are fine, but one of them is not reporting real values to the ECU, how the heck do I find that lol.


Just to recap my symptoms:
1)after light driving, I come to a stop and the idle will do from 700, to 500, to 700 back to 500 then steady out at 700/750
2) when I ease into the throttle, I get two light jerks forward, then smooth power delivery
3) when I release the throttle after being on it lightly, I get again a couple light jerks then it spools down normally
4) if I sit in first gear at 5mph, clutch fully released and no accelerator, most of the time the car just lurches back and forward increasingly violently, with the revs jumping from between 900 and 1300, up down up down and the car lurching/jerking back and forward. But if I then get on the gas and get it upto 8-9 mph, it smooths out, again though a bit of jerking when I initially get in the gas


Totally stumped/no DTCs generated by this

Open to any suggestions on what to try to check etc
 

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2017 Black RS
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That sucks that the issue is still there, but it sounds like you might be on to something at least! Are you able to reset your KAM to see if you can reproduce the issue going away? If that's consistent, then I'd agree with your suspicion of a failing sensor, and it would only be a sensor that's used during closed loop.
 

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2017, Mk3 Focus RS. Stock, for now!
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Discussion Starter · #65 ·
That sucks that the issue is still there, but it sounds like you might be on to something at least! Are you able to reset your KAM to see if you can reproduce the issue going away? If that's consistent, then I'd agree with your suspicion of a failing sensor, and it would only be a sensor that's used during closed loop.
Hmm. Not sure how to reset the KAM to be honest. Do I need FORscan?

Regarding sensors used during closed loop - by any chance would you be able to list off what those are? I have a Carly OBD reader and night try to see if I can use that to look at life values 1 by 1 on particular sensors
 

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2017 Black RS
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971 Posts
Do you have an Cobb Access Port? If not, I'm sure there's a way to reset it... I'm just not aware of it.

For open loop sensors, I know the upstream o2 sensor, IAT, MAP, and the whole throttle body (since I don't think it's serviceable, and no separate TPS) are the main ones. I know there's some more, but I can't think of them off the top of my head.
 

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2017, Mk3 Focus RS. Stock, for now!
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Discussion Starter · #67 ·
So update on this. Still stumped. No codes and still have the issue.

New symptom I notice though. If I drive the car hard with the A/C turned off, then pull over and just idle, the revs sit at 1500 consistently. But if I then flick the A/C on, they drop down to 500 and nearly stall, jump back to 750, then down to 500 again before then levelling off at 750 and idling normally.

Anyone any ideas of sensors that affect idle but also could be getting exposed to heat when I drive hard/giving out dodgy readings to the ECU? Long shot I know.... I was thinking have a punt at replacing the intake air temp sensor, but it'd be a complete punt and the part is like 拢130
 

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The IAT should only be like $15-20, unless you're talking about the MAP-T sensor.

With your OBD tool, are you able to monitor the sensors... like what they temp sensor is saying during those times?
 

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Discussion Starter · #70 ·
Nah, not changed the IAT yet. Pricey part


I'm wondering though, does anyone think my symptoms fit that of a vacuum leak? Problems definitely seen to be worse with a warmer ambient temperature too
 
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