R56 NGK 97968 spark pugs gap adjustment
NGK 97968 spark pugs gap adjustment
I have 12 R58 Mini coupe JCW with 50k miles and car runs great but has misfire when it's cold. I replaced Bosch ignition coils few months ago and today I replaced plugs with NGK 97968 but I feel like it's more misfiring in cold start but once car gets warmed up, I don't feel any misfire at idling and driving. I asked chatgpt about this situation and they recommend to reduce the plugs' gap. I purchased plugs from fcp euro and checked the gap size is .03" (.762mm) but chatgpt suggested me to reduce the gap into 0.022". Is that correct?
6th Gear
Joined: Jul 2023
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Any fault codes?
Google Gemini AI has a different response:
Google Gemini AI has a different response:
It’s a bit of a classic Mini "charm," isn't it? You’ve already done the right first steps with the coils and the 97968s (the correct High Power plugs for the N14/N18 engines), but since the issue is strictly cold-start related and disappears once the engine expands and warms up, we need to look at the "usual suspects" for the Prince engine.
Here is a breakdown of what is likely happening with your JCW:
1. Carbon Buildup (The Most Likely Culprit)
Since you’ve already done the "Stage 1" ignition refresh:
Here is a breakdown of what is likely happening with your JCW:
1. Carbon Buildup (The Most Likely Culprit)
Since your car is a 2012, it has the N18 engine, which is direct-injected. Fuel doesn't spray over the intake valves to clean them, leading to carbon deposits (coking).
The Symptom: On a cold start, the carbon acts like a sponge, soaking up the initial fuel spray and disrupting airflow. Once the engine is hot, the "wet" carbon matters less, and the car runs fine.
The Fix: A walnut blasting service. At 50k miles, if it’s never been done, your intake valves likely look like a BBQ pit.
2. High-Pressure Fuel Pump (HPFP)The Fix: A walnut blasting service. At 50k miles, if it’s never been done, your intake valves likely look like a BBQ pit.
Mini HPFPs are notorious for failing, often starting with "stutters" or misfires only during the cold-start enrichment phase.
The Symptom: Rough idle or stalling for the first 30–60 seconds, which clears up as the pump stabilizes or the car enters "closed loop" operation.
The Check: Look for a Shadow Code (a code that doesn't always trigger a Check Engine Light) for "Fuel pressure volume flow control" or similar.
3. Intake Manifold / PCV LeaksThe Check: Look for a Shadow Code (a code that doesn't always trigger a Check Engine Light) for "Fuel pressure volume flow control" or similar.
Plastic components and rubber gaskets shrink when cold and expand when hot.
The Symptom: A tiny vacuum leak at the intake manifold or a cracked PCV hose can lean out the mixture just enough to cause a cold misfire. Once the engine heat causes the material to expand, the leak seals itself.
The Check: Perform a smoke test while the engine is stone cold.
4. Vanos SolenoidsThe Check: Perform a smoke test while the engine is stone cold.
The N18 engine uses Vanos (variable valve timing) on both intake and exhaust.
The Symptom: If the oil is thick or the solenoids are slightly gummed up, they may struggle to advance the timing correctly until the oil reaches operating temperature.
My RecommendationSince you’ve already done the "Stage 1" ignition refresh:
1. Check for Shadow Codes: Even without a Check Engine Light, a BMW-specific scanner (like INPA, ISTA, or an enthusiast tool like Bimmergeeks ProTool/Carly) can see if the fuel pressure is dipping during start-up.
2. Inspect the Intake: If you haven't had a walnut blasting service in the last 30k miles, that is almost certainly your issue. It's a rite of passage for R58 owners.
2. Inspect the Intake: If you haven't had a walnut blasting service in the last 30k miles, that is almost certainly your issue. It's a rite of passage for R58 owners.
Last edited by Maybe, maybe not; Mar 4, 2026 at 06:53 PM.
No fault codes. I recently replaced vanos solenoid as well. I believe it might be carbon buildup or intake manifold / pcv leaks.
MINI Alliance Ambassador
Joined: Sep 2005
Posts: 144
Likes: 33
I have 12 R58 Mini coupe JCW with 50k miles and car runs great but has misfire when it's cold. I replaced Bosch ignition coils few months ago and today I replaced plugs with NGK 97968 but I feel like it's more misfiring in cold start but once car gets warmed up, I don't feel any misfire at idling and driving. I asked chatgpt about this situation and they recommend to reduce the plugs' gap. I purchased plugs from fcp euro and checked the gap size is .03" (.762mm) but chatgpt suggested me to reduce the gap into 0.022". Is that correct?
If the car has not had a walnut blasting then get it done. I believe I may have said that when you were replacing the turbo. Or keep driving it and soon it will be time for a new engine.
Here is a good article on spark plugs. It is worth the read.
https://www.motoringalliance.com/art...ark-plugs.108/
Last edited by MCS02; Mar 13, 2026 at 03:57 PM.
MINI Alliance Ambassador
Joined: Sep 2005
Posts: 144
Likes: 33
Also if you don't have one of these code readers I would get one they are great and have saved me plenty of money. No connection to this but I saw it on the market place.
https://www.northamericanmotoring.co.../market/372712
https://www.northamericanmotoring.co.../market/372712
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