SL55/63/65/R230 AMG: Possible causes of a lean AFR
Thread Starter
Senior Member
Joined: Feb 2009
Posts: 396
Likes: 21
SL63, GTR
Possible causes of a lean AFR
So I've been trying to troubleshoot a lean AFR that I have on my SL65. I got it back on the dyno today after shutting off the rear o2s completely and the SAI. I also switched my intake piping from 4" to 3" for better air flow with less turbulence as I had to severely oval the bottom half of the 4" piping to fit between the coil pack and coolant overflow tank.
I flashed the stock ECU file back and made two dyno pulls. The AFR is still lean even on the stock file. I now need to start troubleshooting why there is a lean condition. I'm picking up a fuel pressure gauge tomorrow and will hook it up to the high pressure fuel test port to monitor the pressure. I also have a new fuel filter on the way.
If one or both of the primary o2 sensors were bad, would they cause a lean AFR? Is there one or two (low pressure and high pressure) fuel pumps on the SL65? Is it in the same location as the SL55 above the rear axle?
Are there any other causes of a lean operating condition to check? The AFR dips down to around 12 from 3,700-4,200 RPM but flatlines at 13 below and above that.
1. Failing fuel pump(s)
2. Clogged fuel filter
3. Dirty injectors
4. Primary o2 sensors
I flashed the stock ECU file back and made two dyno pulls. The AFR is still lean even on the stock file. I now need to start troubleshooting why there is a lean condition. I'm picking up a fuel pressure gauge tomorrow and will hook it up to the high pressure fuel test port to monitor the pressure. I also have a new fuel filter on the way.
If one or both of the primary o2 sensors were bad, would they cause a lean AFR? Is there one or two (low pressure and high pressure) fuel pumps on the SL65? Is it in the same location as the SL55 above the rear axle?
Are there any other causes of a lean operating condition to check? The AFR dips down to around 12 from 3,700-4,200 RPM but flatlines at 13 below and above that.
1. Failing fuel pump(s)
2. Clogged fuel filter
3. Dirty injectors
4. Primary o2 sensors
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Joined: Dec 2015
Posts: 146
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2005 SL65
Member
Joined: Jun 2013
Posts: 148
Likes: 21
2005 SL65 AMG, 04 S600
If your fuel pressure is fine, maybe have your injectors flow tested and cleaned. When I built my intakes, my AFR stayed pretty much the same.
On 100% stock setup and tune, my stock AFR was 11.9-12.0 after 4k rpm.
As I kept rasing the boost, afr still stayed around 12.0, even after I removed the factory airboxes and built new intakes.
On 100% stock setup and tune, my stock AFR was 11.9-12.0 after 4k rpm.
As I kept rasing the boost, afr still stayed around 12.0, even after I removed the factory airboxes and built new intakes.
Thread Starter
Senior Member
Joined: Feb 2009
Posts: 396
Likes: 21
SL63, GTR
Thanks boostd and DUDMD. With an engine this expensive, I obviously want to me make sure that it's operating safely and at it's peak performance.
So far I've hooked up a fuel gauge to the fuel test port and drove around while monitoring the fuel pressure. During WOT, the fuel pressure averages around 55 PSI which according to my tuner is good. Therefore, it seems as if the fuel pump is operating correctly. If it was a volume issue and not a pressure issue, it would be leaning out on the top end which it is not.
Just yesterday I replaced both primary o2 sensors and the fuel filter. Unfortunately my dyno day isn't until Monday of next week so I won't know if either of these things have fixed the issue until then.
On another note, it appears that the IAT magic number is 158 degrees when the ECU starts to cut boost/timing. Obviously the colder the intake charge the better but as long as the IAT stays below 158 degrees during WOT pulls, the engine should be making maximum power, correct?
So far I've hooked up a fuel gauge to the fuel test port and drove around while monitoring the fuel pressure. During WOT, the fuel pressure averages around 55 PSI which according to my tuner is good. Therefore, it seems as if the fuel pump is operating correctly. If it was a volume issue and not a pressure issue, it would be leaning out on the top end which it is not.
Just yesterday I replaced both primary o2 sensors and the fuel filter. Unfortunately my dyno day isn't until Monday of next week so I won't know if either of these things have fixed the issue until then.
On another note, it appears that the IAT magic number is 158 degrees when the ECU starts to cut boost/timing. Obviously the colder the intake charge the better but as long as the IAT stays below 158 degrees during WOT pulls, the engine should be making maximum power, correct?
