Misfire/Shunting
Discussion
Looking for Advice (apologises in advance for the lengthy write up).
Bought a 2004 DB9 few months ago privately and the car has been regularly serviced by a mix of independents and dealerships. Most recently it has been serviced by a dealership and the previous owner bought the car with a warranty from a dealership, the warranty expired few month before I bought it.
The car developed a lumpy idle and I could feel a small shunt (bump) initially but as the weeks progressed it got worse. I took it to the local dealership who had serviced and maintained the car previously. They had a quick listen and said there was definitely a misfire and that I should book it in for a diagnosis. A few weeks passed and before I booked it in for a diagnosis the MIL light and emission warning came up.
I bought a Foxwell NT520 and that showed a misfire code P0303. Which I interpreted as a misfire on cylinder 3 on bank 1. I reset the code.
I took the car to the dealership for diagnosis. Their diagnoses was that there was a misfire on cylinder 3 on bank 2 and recommended replacement of all the coils on bank 2. All the coils on bank 1 had been replaced c.15 months a go together with 12 new spark plugs by the same dealership.
I had the repair done. Collected the car and drove about 5 miles before the shunting started again, spoke to the technician at the dealership and took the car back.
He put it back on the computer and having waited at least 45 minutes they gave up and I had to get back to work. He said he couldn’t run a full check as the kept on crashing and the car wasn’t definitely misfiring and could I book it in for further investigation. I booked it in for following week.
I got about 2 miles down the road, the MIL light and emissions check message came on. Drove it back to the dealership and left it there for them to investigate further.
They reported back saying that the one of the new coils they fitted was faulty. It was cylinder 3 bank 2. (the same coil that was causing the initial error code).
They replaced it and I collected the car. On the way home I noticed the shunt again and reported it back to the dealership. Bought the car back a few weeks later. They hooked it back on the computer and run the diagnosis, they even let me into the service bay as the test was being performed. The computer showed no errors or mis-fires. I showed the technician the output from the NT220 (which now showed a misfire in cylinder 4 of 74) and he said he could not take that into consideration and it wouldn’t be considered as a misfire unless the count reached at least 100.
On the way out I noticed the shunt (very slight). So I got the technician to seat in the car. He explained that the slight shunt was because of the torque converter. There was nothing wrong, the car was in drive with the brakes on, the torque was building up. The engine management is expecting the car to move forward but when it didn’t it was making adjustments to the firing which was causing the slight shunt.
The following were also confirmed by the technician.
No problems with torque converter.
No problem with the gear box.
No problems with the cats as both pre and post sensors were not showing any issues.
No issues with breathing system.
The shunting is very noticeable now when the engine is warm and in drive and idling and, less noticeable when in neutral or park. The NT220 is showing misfire on cylinder 4 in the region on 70-140 (I’ve checked it a few times now).
I’ve attached a video to show the shunting and you might be able to pick up some sound as well.
https://youtu.be/y6Oj4CNeNKM
I need advice on the following:
1. What the problem may be and how to test for it/remedy it.
2. How to proceed with the dealership.
Thanks in advance.
Abs
Bought a 2004 DB9 few months ago privately and the car has been regularly serviced by a mix of independents and dealerships. Most recently it has been serviced by a dealership and the previous owner bought the car with a warranty from a dealership, the warranty expired few month before I bought it.
The car developed a lumpy idle and I could feel a small shunt (bump) initially but as the weeks progressed it got worse. I took it to the local dealership who had serviced and maintained the car previously. They had a quick listen and said there was definitely a misfire and that I should book it in for a diagnosis. A few weeks passed and before I booked it in for a diagnosis the MIL light and emission warning came up.
I bought a Foxwell NT520 and that showed a misfire code P0303. Which I interpreted as a misfire on cylinder 3 on bank 1. I reset the code.
I took the car to the dealership for diagnosis. Their diagnoses was that there was a misfire on cylinder 3 on bank 2 and recommended replacement of all the coils on bank 2. All the coils on bank 1 had been replaced c.15 months a go together with 12 new spark plugs by the same dealership.
I had the repair done. Collected the car and drove about 5 miles before the shunting started again, spoke to the technician at the dealership and took the car back.
He put it back on the computer and having waited at least 45 minutes they gave up and I had to get back to work. He said he couldn’t run a full check as the kept on crashing and the car wasn’t definitely misfiring and could I book it in for further investigation. I booked it in for following week.
I got about 2 miles down the road, the MIL light and emissions check message came on. Drove it back to the dealership and left it there for them to investigate further.
They reported back saying that the one of the new coils they fitted was faulty. It was cylinder 3 bank 2. (the same coil that was causing the initial error code).
They replaced it and I collected the car. On the way home I noticed the shunt again and reported it back to the dealership. Bought the car back a few weeks later. They hooked it back on the computer and run the diagnosis, they even let me into the service bay as the test was being performed. The computer showed no errors or mis-fires. I showed the technician the output from the NT220 (which now showed a misfire in cylinder 4 of 74) and he said he could not take that into consideration and it wouldn’t be considered as a misfire unless the count reached at least 100.
On the way out I noticed the shunt (very slight). So I got the technician to seat in the car. He explained that the slight shunt was because of the torque converter. There was nothing wrong, the car was in drive with the brakes on, the torque was building up. The engine management is expecting the car to move forward but when it didn’t it was making adjustments to the firing which was causing the slight shunt.
The following were also confirmed by the technician.
No problems with torque converter.
No problem with the gear box.
No problems with the cats as both pre and post sensors were not showing any issues.
No issues with breathing system.
The shunting is very noticeable now when the engine is warm and in drive and idling and, less noticeable when in neutral or park. The NT220 is showing misfire on cylinder 4 in the region on 70-140 (I’ve checked it a few times now).
I’ve attached a video to show the shunting and you might be able to pick up some sound as well.
https://youtu.be/y6Oj4CNeNKM
I need advice on the following:
1. What the problem may be and how to test for it/remedy it.
2. How to proceed with the dealership.
Thanks in advance.
Abs
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