Mis-fire and limp mode.

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Finn001
New
Posts: 2
Joined: Mon Jun 10, 2024 10:36 am
Drives: 2010 Moda
Location: Sout coast

Mis-fire and limp mode.

Post by Finn001 »

Hi everyone,

I picked up a 2010 Polo on the cheap with a short MOT and a few issues. I’ve fixed the obvious stuff like the rear shocks and sloppy hand brake etc. However, there is an EML issue I need to sort out.

I got a diagnostic thingy for the OBD port and reset the EML which was due to a mid-fire in cylinder 2. When first started the engine will obviously mis-fire and run rough at idle but after a minute or 2 of driving the idle has settled down and the car drives ok. I’ve replaced the plugs and cleaned the coil pack, checked the earth and visually inspected the wiring. All seems ok.

The generally runs well but the goes into limp more from time to time. It seems to do it (according to the diagnostic app and driving experience) at just under 2000 rpm, typically in 4th gear at round 40mph. This is actually quite consistent. Clutch in > key off > wait a few seconds > restart and limp mode disappears. The EML stays on sometimes. If I drive it at above 2000 rpm it’s fine.

At higher revs the engine will naturally smooth out but the performance seems ok for a 60 hp 3pot engine. If it was consistently running on 2 cylinders I think the performance drop would be pretty obvious.


So, I’m wondering if this is an ECU error?
2nd hand ECU’s are cheap enough on eBay (£20) but can I just plug it in and see or does need to be coded to the vehicle?
I’m also wondering if a tuner can fiddle the ECU to disable limp mode?

Any thoughts are most welcome.
2226
Silver Member
Posts: 355
Joined: Fri Apr 12, 2019 9:35 pm
Drives: Cross Polo 1.2TSI 2017
Location: Cape Town, South Africa

Re: Mis-fire and limp mode.

Post by 2226 »

Could be a bad ground maybe.

Misfires are not only from ignition. Fuel delivery can also cause it. Misfires are detected off engine speed at the TDC and BDC. If the ECU reckons it slowed for some reason then it logs a misfire. Perhaps you have a dirty injector?

I have close to 40,000 misfires logged per cylinder in my 1.2TSI. All plugs fine, fuel delivery fine. I ran CRC GDI engine cleaner through it and it stopped. In my case it might be related to carbon build-up possibly causing the intake valve to not close completely. It could have lost combustion pressure into the intake, slowing the piston speed enough to be detected as a misfire. That's the only theory I have for my situation and results.
Finn001
New
Posts: 2
Joined: Mon Jun 10, 2024 10:36 am
Drives: 2010 Moda
Location: Sout coast

Re: Mis-fire and limp mode.

Post by Finn001 »

Thanks for that.

I have checked the earth on the coil packs and they seem ok. I’ll look further down the loom for obvious issues.

I did find a compression test in the documentation which suggested lower compression on cylinder 2 which I thought was a red herring but now that you have explained how the misfire is determined by the ECU that could be a factor.

I have an injector cleaner in there ATM and I’m about to do a 100 mile run on the motorway so I’ll add more fuel system cleaner with the fuel.
CarbonChaos
Silver Member
Posts: 418
Joined: Mon Sep 03, 2018 9:43 pm
Drives: 2015 6C GTI DSG Oryx white 3 door.
Location: South west uk

Re: Mis-fire and limp mode.

Post by CarbonChaos »

If its a 1.2 3 cyl then the low compression will definitely be the major factor in your misfire and eml light . these engines are known for low compression and burnt valves or worn rings/bores
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