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2017, Mk3 Focus RS. Stock, for now!
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Discussion Starter · #1 ·
Bit stumped here. I’ve taken car to Ford and they’ve reported nothing wrong. I’ve no engine management light either.

Problem: car revs surge when down at idle. Say the car idles normally at approx 700/800rpm. All of a sudden however, the revs will drop down to 500 and the car shudders, revs jump back up to the normal idle, then drop back down to the 500, then jump back upto normal idle.

It seems to happen randomly, but a sure way to induce it would be to have the A/C turned on when approaching a stop. Once you stop it’ll do the double surge thing then go back to normal idle.

Anyone any ideas? Is it a load issue? Should I try replacing the battery?
 

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Its the fuel rail sensor, it not RS related but Ford related
Only at idle

I posted about the issue and its resolution.

I tried replacing the front o2 first, cuase I got a stuck lean condition and figure it was a bad O2.

The fuel rail sensor is way easier, should have done that right away.
$60 for tasca IIRC and 10 min of work time.
Look for my post on the issue including the part no
 

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2017, Mk3 Focus RS. Stock, for now!
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Discussion Starter · #5 ·
Its the fuel rail sensor, it not RS related but Ford related
Only at idle

I posted about the issue and its resolution.

I tried replacing the front o2 first, cuase I got a stuck lean condition and figure it was a bad O2.

The fuel rail sensor is way easier, should have done that right away.
$60 for tasca IIRC and 10 min of work time.
Look for my post on the issue including the part no
Thanks for this. What would I need to diagnose the fuel sensor issue before I start taking it apart? If there a data logger or something I can purchase?
 

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Its so cheap and easy to do...

Watch your LTFT and STFT at idle you see them climb after a reset...if they climb to offscale it will throw a code...
have you throw a code yet?
 

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2017, Mk3 Focus RS. Stock, for now!
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Discussion Starter · #7 ·
Its so cheap and easy to do...

Watch your LTFT and STFT at idle you see them climb after a reset...if they climb to offscale it will throw a code...
have you throw a code yet?
No codes yet no. No Engine Managment Light or anything. Ford said there was previously a code in the Power Control Module, but it didn't throw the EML on. It has been resolved too after a PCM update, but my rev surging issue remains.

What data logger do I need to see the LTFT and STFT values? Any Bluetooth OBD readers that can do it?

My problem with going straight at a fix is I don't have any space to do it at home (live on main road/no front yard) so would have to go to a buddies nearer town.
 

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No codes yet no. No Engine Managment Light or anything. Ford said there was previously a code in the Power Control Module, but it didn't throw the EML on. It has been resolved too after a PCM update, but my rev surging issue remains.

What data logger do I need to see the LTFT and STFT values? Any Bluetooth OBD readers that can do it?

My problem with going straight at a fix is I don't have any space to do it at home (live on main road/no front yard) so would have to go to a buddies nearer town.
Accessport will tell you what's going on.
 

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Its so easy, do the fix....in the the road its that easy....

An AP you can watch but you still will need the repair...and its $500
And your a UK driver so the AP will have to be for a UK car, we cant help....
 

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You can buy a cheap OBD-II bluetooth reader and use an Android phone with Torque installed.

The part is about $70, and by the time you wait for the OBD-II reader and figure out Torque you could have the part swapped...
 

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Does it happen like every time you fill up the gas tank? Mine does exactly that, more if I top off the tank. Probably some evap thing.
Same here. Just filled up earlier today, and it'll do this when I first start it up after filling. Usually goes away after I drive it a couple blocks.
 
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2017 Focus RS (Canada) / 2022 Hyundai Elantra N
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I put together some instructions for replacing the Fuel Rail Pressure sensor here:


There appear to be 3 common causes for hunting/surging idle on our cars:
  1. faulty Fuel Rail Pressure (FRP) sensor
  2. faulty front oxygen sensor
  3. faulty Evaporative Purge Valve
 
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Mine had the same issue a couple weeks back. Was a faulty oil pressure sensor valve down by the oil filter. Mine actually burst and started
Leaking oil all over my driveway. I was
Lucky it wasn’t on my way home in the fast lane at 100mph


I think 16 Ford Focus RS Grey
Eibach Sways /Catless Dp /Adamtuned Auxfuel Kit/ Cobb AdamTuned/ Dsc Sport Controller/ Radium PcV CC./K+N air filter /Tema 4x4 30mm spacer/e85/massive sway endlinks//

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I put together some instructions for replacing the Fuel Rail Pressure sensor here:


There appear to be 3 common causes for hunting/surging idle on our cars:
  1. faulty Fuel Rail Pressure (FRP) sensor
  2. faulty front oxygen sensor
  3. faulty Evaporative Purge Valve
The problem he had with rough idle initially was intermittent and not related to filling up. He changed the fuel rail pressure sensor which fixed that problem. Now it's when filling up. It's crazy how some cars can have these problems while others don't.

I'll let him know about the other 2.

Thanks

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The problem he had with rough idle initially was intermittent and not related to filling up. He changed the fuel rail pressure sensor which fixed that problem. Now it's when filling up. It's crazy how some cars can have these problems while others don't.

I'll let him know about the other 2.

Thanks

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i think its a faulty purge valve when you get the idle dip after filling up.
 

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Discussion Starter · #18 ·
New FRP sensor coming on Friday. Will report back if it fixes!



Annoyingly, I tried to have ford diagnose it today when I was ordering the part, but the car decided it would idle fine and not do anything wrong. Data logger showed everything (pressure, LTFT, and STFT) as perfect ☹
 

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yeah its intermittent...and only at idle...no worries Ford over here is useless.....
 
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