Intermittent Misfire After Extended Driving - Heat Soak Related?
I've got an intermittent misfire issue that could have several causes. Hoping someone here recognises the exact symptoms and can point me in the right direction before I start throwing money at parts.
Background
Picked up the car in November 2025 and drove it 335 km from the dealer. Parked it for the winter and did the following engine-side mods:
How It Happens
I had been cruising at 120 km/h on the highway for about two hours. Downshifted to 4th and went full throttle. It immediately started to misfire, so I backed off. I have a custom display showing misfires in real time, and it was occurring on cylinder 3.
On the Dyno
I already had a Syvecs ECU on order, so a few days later it went on the dyno. During the tuning session (done by Romain at RaceCal), everything ran fine for a few hours until the misfire came back. Romain checked the logs and found that the ECU was commanding maximum fuelling, so his first guess is the solenoid in the high-pressure fuel pump. We turned the car off, opened the hood, and let it cool for about 20 minutes. Started it back up and the misfires were gone. That's an expensive part to swap just to test a theory (around 2500€ in Sweden), which is why I'm posting here first.
The Pattern
Since then the misfires have appeared a few more times. It always happens after the car has been driven for an extended period, and specifically after holding the same speed for a long time. My theory is that something is heat-soaking over time and stops functioning properly.
Any input from someone who has experienced the same issue or has solid knowledge of these engines would be greatly appreciated.
I've got an intermittent misfire issue that could have several causes. Hoping someone here recognises the exact symptoms and can point me in the right direction before I start throwing money at parts.
Background
Picked up the car in November 2025 and drove it 335 km from the dealer. Parked it for the winter and did the following engine-side mods:
- Eventuri intake
- Hand-built stainless exhaust without OPF
- Airtec Stage 1 intercooler
- Oil cooler sandwich plate with AN outputs
How It Happens
I had been cruising at 120 km/h on the highway for about two hours. Downshifted to 4th and went full throttle. It immediately started to misfire, so I backed off. I have a custom display showing misfires in real time, and it was occurring on cylinder 3.
On the Dyno
I already had a Syvecs ECU on order, so a few days later it went on the dyno. During the tuning session (done by Romain at RaceCal), everything ran fine for a few hours until the misfire came back. Romain checked the logs and found that the ECU was commanding maximum fuelling, so his first guess is the solenoid in the high-pressure fuel pump. We turned the car off, opened the hood, and let it cool for about 20 minutes. Started it back up and the misfires were gone. That's an expensive part to swap just to test a theory (around 2500€ in Sweden), which is why I'm posting here first.
The Pattern
Since then the misfires have appeared a few more times. It always happens after the car has been driven for an extended period, and specifically after holding the same speed for a long time. My theory is that something is heat-soaking over time and stops functioning properly.
Any input from someone who has experienced the same issue or has solid knowledge of these engines would be greatly appreciated.
