Rough idle on a Seat Leon Cupra 300 4drive

[The Goethe]

Hello everyone!

2 days ago I purchased a Seat Leon Cupra 300 4drive with the DSG6 gearbox. Since I'm waiting on some paperwork, I was unable to test drive the car (I drove it before with the previous owner's plate before purchasing). Before continuing to describe the problem, do note that I've never owned an automatic car nor DSG, and this is my first petrol-powered car. I have a manual 2.0 pre-facelift 110kw diesel, which runs extremely smoothly on idle even when it's cold.

The car runs rather rough when starting up, it starts quickly, no problems with that. But it starts vibrating for brief moments, the revs drop, then increase (between 600-1000 rpm) and the exhaust sounds different when these rpm-drops occur. Until the oil temperature reached 80 degrees, the car felt very shaky and jerky (was sitting inside the whole time monitoring the situation), after the oil reached 80 degrees celsius, the shaking stopped and the revs stabilised around 680-710. However, according to my OBD device, the coolant water went above 95 degrees celsius (the dashboard reading remained steady at 90), and this is what concerns me the most. I revved the engine once to 3000 rpm, but after that the rpm's remained steady during idling. No DTC's and according to my device, the last DTC was cleared 65500 km ago (car has 170k).

The car has a digital service book, oil changes every 10-20k kms, DSG, and Haldex oil replaced 40k km ago, so it's still between the regular intervals. Coolant level (when cold) was well over the minimum mark, but below the maximum by an inch or two. The oil level was also between the markings, slightly lower. The oil filler cap is completely clean, and so is the coolant expansion tank cap. I mean, the reason I bought the car because it was maintained at the very same workshop since its first kilometres, and the owner was the original owner who bought it in 2017. No stage 1 or any modifications to the engine, however the previous owner was running a Hurricane V2 exhaust, but the car was never tuned (i believe him). Gearbox isn't mapped, in normal mode, it does shift up around 1900-2000 rpm.

The car was in PARK for the whole process. Please help me ASAP, because I am really doubting my purchase right now.

 

[nd-photo.nl]

Can you scan for misfires? Also check fuel trims & maybe also inspect the spark plugs. When was the last time they were swapped out with new spark plugs?

I have an ST280 with the DSG6 (aka DQ250), which is more or less the same car. On cold start it will idle a bit higher to heat up the cat as soon as possible, before dropping down in rpms a bit. I think this takes (over) a minute, before dropping down in revs for normal idling.

Coolant temps will always fluctuate, but I havent done the comparison between OBD and what is on the screen. Normally the dashboard will keep pointing at 90 degrees while the coolant temp is fluctuating as the thermostat opens and the cooling system does it job. This cycle is repeated off course.

I think the jerkyness has to do with ignition or some airleak. Thats why I am asking about misfires and fuel trims

 

[mty12345]

As said, the temp gauge will stay on 90 even though the actual temp is moving around in the normal operating range (85C-98C) It won't start to move unless temps are getting above normal.

 

[The Goethe]

So quick update for all replies: I did do a live data monitoring while idling in P until the engine reached operating temperature. The problem is still present, and clearly visible on the ignition timing, the ECU significantly delays the ignition, which leads to the vibration and rpm drop. The delay is as high as 33* degrees, show on the picture below.

The other thing that I’ve noticed is that the intake air temperature is way too high, 68 degrees Celsius.

Conclusions: the delay is done by the ECU on purpose to prevent overheating in the combustion chamber.
The intake air temperature is too high. Coolant temperature remained at a steady 102* degrees Celsius, it didn’t fluctuate in this timeframe. So my guesses are: either the thermostat is faulty which would be my best guess, or the cooler’s flaps are damaged and they simply cannot cool down the air any better when stationary (the previous owner used this car on the Autobahn with high speeds) which would be the reason why the flaps are damaged.

Although, it’s important to note that the jerking, shaking disappears after the oil reaches 80 degrees, and intake temps and coolant temperature drops slightly to a normal level. Also worth noting that the car was sitting in the sun, outside temps were above 38 or even 40 degrees.

The thing is why I am really concerned about the issue is that my other car (2.0 TDI 110kw/manual) doesn’t do this. Did the same on that car, same ambient air temperature, and the temperature simply refused to go above 92 degrees Celsius on the coolant. Obviously it’s a diesel and I’ve never had a petrol engine with 300hp

As soon as I get a license plate on the car, I’ll do a test drive with live data to see if the temperatures and ignition timing are working well. Also a workshop visit to change all the fluids, and do the G12 evo conversion will be booked. Thank you for your help, I’ll keep all updated so if anyone experiences a similar issue, might refer to this thread.

 

[nd-photo.nl]

Can you feel the hoses going in/out of the radiator? That will tell you if the thermostat works. Also there should be a trickle stream in your coolant expansion tank in the return line, this will tell you that the waterpump is working. It can be a bit deceiving, as the car also has 1 or 2 AUX coolant pumps. 1 is located behind the engine and 1 above the gearbox. But I believe those only work when the car is switched off

At least we know the system works by design because your IATs are too high.

 

[The Goethe]

Can you feel the hoses going in/out of the radiator? That will tell you if the thermostat works. Also there should be a trickle stream in your coolant expansion tank in the return line, this will tell you that the waterpump is working. It can be a bit deceiving, as the car also has 1 or 2 AUX coolant pumps. 1 is located behind the engine and 1 above the gearbox. But I believe those only work when the car is switched off

At least we know the system works by design because your IATs are too high.

I did some more testing, now with colder ambient temperatures (22C). IAT’s didn’t go over 38C, but the coolant still went up to 102C. This time, I let the engine run for a lot longer, about 15-20 minutes, mostly in normal mode with ~650-700rpm. The vibration, and the ignition delay disappears whenever I put it in Sport/Cupra mode, idling at ~1100rpm feels a lot smoother. Also IAT’s seemed to slightly decrease when I was idling at ~1100, it was 34-36C.

I’m a bit confused now, coolant was definitely circulating so the water pump did work. My guess is still the thermostat, water temp shouldn’t be over 100C imo.

will keep this thread updated as soon as I get the paperwork sorted and the license plate on first route will be a workshop still for sure.