Long Term Fuel Trim Question
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Joined: Jul 2017
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2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
Long Term Fuel Trim Question
I have an M278 and had the CEL come on last fall. I have an iCarSoft scanner and it appears the B1 LTFT was high, anywhere from 12.5-19. A local mechanic diagnosed it as a faulty O2 sensor. I had both B1 O2 sensors replaced, and while the CEL went off it did not solve the high numbers on the B1 LTFT. I took it to another mechanic, but he didn't think it was an issue worth looking at. I have since noticed while the B1 LTFT is high at idle, it returns to normal while driving. Could this be a faulty injector(s) maybe sending to much fuel to a cylinder?
Out Of Control!!
Joined: Apr 2019
Posts: 12,272
Likes: 6,918
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
Half LEAN M278
I have an M278 and had the CEL come on last fall. I have an iCarSoft scanner and it appears the B1 LTFT was high, anywhere from 12.5-19. A local mechanic diagnosed it as a faulty O2 sensor. I had both B1 O2 sensors replaced, and while the CEL went off it did not solve the high numbers on the B1 LTFT. I took it to another mechanic, but he didn't think it was an issue worth looking at. I have since noticed while the B1 LTFT is high at idle, it returns to normal while driving. Could this be a faulty injector(s) maybe sending to much fuel to a cylinder?
Right now you have two 4-Cyl. banks, dragging each other mad out of balance. Take it easy with high loads. This freaks out ECU timings.
Your LTF1 is calling for a lot MORE fuel.
Your single fuel rail is pressurized by two HPFP both supplied by one smart tank pump. This is a return less fuel system -- Very likely ok here: unbalanced.
Nothing wrong with your ignition.
Use your scanner to read:
tank pump pressure
fuel rail high pressure
both LTFT 1+2 at warm idle.
Basically your O2 was/is correct !!
Your Bk1 is way too LEAN... ie. it has too much air or not enough fuel.
Any other misc engine issues or CAN faults that may be overlooked here ??
You're gonna need to figure this one out soon...
maybe GDI injectors are plugged up by heavy oil contamination ??
How many Miles: low/high ?
How much oil burns through: more than 1Qt per ??
How is this engine used ?
How about leaking wategake or cracked turbo manifold?
what were previous maintenance/repairs done?
The sooner you can zip to the finish line by discovering the obvious cause, the cheaper the fix will be.
Last edited by CaliBenzDriver; Jun 21, 2026 at 11:54 PM.
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Joined: Jul 2017
Posts: 525
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2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
Note both banks need to run the same upstream Lambda for a chance to balance out your engine.
Right now you have two 4-Cyl. banks, dragging each other mad out of balance. Take it easy with high loads. This freaks out ECU timings.
Your LTF1 is calling for a lot MORE fuel.
Your single fuel rail is pressurized by two HPFP both supplied by one smart tank pump. This is a return less fuel system -- Very likely ok here: unbalanced.
Nothing wrong with your ignition.
Use your scanner to read:
tank pump pressure
fuel rail high pressure
both LTFT 1+2 at warm idle.
Basically your O2 was/is correct !!
Your Bk1 is way too LEAN... ie. it has too much air or not enough fuel.
Any other misc engine issues or CAN faults that may be overlooked here ??
You're gonna need to figure this one out soon...
maybe GDI injectors are plugged up by heavy oil contamination ??
How many Miles: low/high ?
How much oil burns through: more than 1Qt per ??
How is this engine used ?
How about leaking wategake or cracked turbo manifold?
what were previous maintenance/repairs done?
The sooner you can zip to the finish line by discovering the obvious cause, the cheaper the fix will be.
Right now you have two 4-Cyl. banks, dragging each other mad out of balance. Take it easy with high loads. This freaks out ECU timings.
Your LTF1 is calling for a lot MORE fuel.
Your single fuel rail is pressurized by two HPFP both supplied by one smart tank pump. This is a return less fuel system -- Very likely ok here: unbalanced.
Nothing wrong with your ignition.
Use your scanner to read:
tank pump pressure
fuel rail high pressure
both LTFT 1+2 at warm idle.
Basically your O2 was/is correct !!
Your Bk1 is way too LEAN... ie. it has too much air or not enough fuel.
Any other misc engine issues or CAN faults that may be overlooked here ??
You're gonna need to figure this one out soon...
maybe GDI injectors are plugged up by heavy oil contamination ??
How many Miles: low/high ?
How much oil burns through: more than 1Qt per ??
How is this engine used ?
How about leaking wategake or cracked turbo manifold?
what were previous maintenance/repairs done?
The sooner you can zip to the finish line by discovering the obvious cause, the cheaper the fix will be.
Out Of Control!!
Joined: Apr 2019
Posts: 12,272
Likes: 6,918
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
There is 109K miles on the car. I had a few minutes today and was able to get a few readings. Warmed up and at idle the fuel rail pressure is about 1715 psi, which appears to be about 300 psi high. When I revved the engine up the pressure didn't really change, which I thought was strange. I am going to drive around with the scanner on tomorrow. The scanner measured the fuel pressure at 4.2 to 4.65 bar. If this is this the low pressure fuel pump then it appears to be ok. I am not sure how to measure the high pressure fuel pump.
No pressure gauge needed.
Last edited by CaliBenzDriver; Jun 22, 2026 at 11:57 PM.
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2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
I did read the fuel rail pressure. It was about 1715 psi at idle. From what I understand normal is 1420 psi. What would cause a high fuel rail pressure only at idle?
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2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
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Out Of Control!!
Joined: Apr 2019
Posts: 12,272
Likes: 6,918
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
The ECU has control over rail pressue through the pump valves. It's helping compensate for your lean trim a little.
> Your summary reads :
- lean engine Bank1 has rich LTFT1
- fuel pressures H/L: ok
- no ignition misfires: lean!
- no bad injector: no misfire!
- Lambda b1: new
I asked for maintenance record...
-- Did someone had a chance to messup intake seals recently??
-- How old is the PCV system : 110kMi?
-- Did MAF + MAP sensors get cleaned up a little?
At least this has a chance to be a cheap repair for now. Regardless your mileage is due for fuel delivery soon 3x fuel pumps + filter. After this Lean is fixed!
Last edited by CaliBenzDriver; Yesterday at 03:09 PM.
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Thread Starter
Super Member
Joined: Jul 2017
Posts: 525
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2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
Great! Both High & Low fuel pressures check out ok.
The ECU has control over rail pressue through the pump valves. It's helping compensate for your lean trim a little.
> Your summary reads :
I asked for maintenance record...
-- Did someone had a chance to messup intake seals recently??
-- How old is the PCV system : 110kMi?
-- Did MAF + MAP sensors get cleaned up a little?
At least this has a chance to be a cheap repair for now. Regardless your mileage is due for fuel delivery soon 3x fuel pumps + filter. After this Lean is fixed!
The ECU has control over rail pressue through the pump valves. It's helping compensate for your lean trim a little.
> Your summary reads :
- lean engine Bank1 has rich LTFT1
- fuel pressures H/L: ok
- no ignition misfires: lean!
- no bad injector: no misfire!
- Lambda b1: new
I asked for maintenance record...
-- Did someone had a chance to messup intake seals recently??
-- How old is the PCV system : 110kMi?
-- Did MAF + MAP sensors get cleaned up a little?
At least this has a chance to be a cheap repair for now. Regardless your mileage is due for fuel delivery soon 3x fuel pumps + filter. After this Lean is fixed!
Out Of Control!!
Joined: Apr 2019
Posts: 12,272
Likes: 6,918
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
The manifold has not been touched.
Other than oil and air filter changes,
the only engine work that has been done has been done after this started.
Since this started I have had :
the plugs,
coolant lines,
O2 sensors and
the cam sensors replaced.
I think I may change the vacuum pump valve this weekend.
Other than oil and air filter changes,
the only engine work that has been done has been done after this started.
Since this started I have had :
the plugs,
coolant lines,
O2 sensors and
the cam sensors replaced.
I think I may change the vacuum pump valve this weekend.
This helps the process of elimination.
Clean the MAF + MAP sensors then clear related faults.
Last edited by CaliBenzDriver; Yesterday at 07:01 PM.
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2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
