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I doubt a rich sensor readings only could be enough for the cat to be overdue . Have you ever noticed smell or smokes , cracking when you step on the gas pedal while it's idling ?
2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
Originally Posted by BenzV12
I doubt a rich sensor readings only could be enough for the cat to be overdue . Have you ever noticed smell or smokes , cracking when you step on the gas pedal while it's idling ?
I have noticed no smells, but have had two unusual episodes over the past 2 months. Twice when I floored the car it went into limp mode with no CEL. When I turned the car off and back on, everything was normal. It was a random issue that I could not duplicate.
I was able to get a quick scan this morning at warm up.
Here is what I found.
Tell me what I am looking at.
.... you are looking at money saving not replacing good parts. LTFT #1 vs. #2
both numbers should be nearly equal They're over 10% apart.
Worry about combustion before exhaust scrubbers
2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
I have owned the car for 8 years and have kept up on maintenance. The car has 97K miles and I just had the plugs replaced a couple of months ago. Is the long term fuel trim the downstream O2 sensor?
I have owned the car for 8 years and have kept up on maintenance.
The car has 97K miles and I just had the plugs replaced a couple of months ago.
Is the long term fuel trim the downstream O2 sensor?
... Injection LTFT deals with UPSTREAM sensor measuring the cylinders combustions.
> Your Bk1 is running too lean, getting extra fuel
> Your Bk2 is running evenly well
13% is not the end of the world...
some ppl get over 15% imbalanced.
Engine runs with reduced output.
It would be good to get back closer to 5% The ECU can not run well with unbalanced timings.
You are near 100kMi with all new sparkplugs.
thus we can say you're not dealing with bad spark
It's not bad fuel pressure pressure proven by Bk2.
Get Bk1 CYLINDERS inspected + compressions checked.
This will show you root cause: drafty cylinders?
What's the service history: annual oil change ??
Try 5kMi service with cleaner oil.
I don't believe injectors are your problem nor the solution.
Last edited by CaliBenzDriver; Sep 5, 2025 at 02:27 PM.
you're right this is misleading... OUCH: "LTFT downstream" + "not affecting fuel mixture" : WRONG!
LTFT + STFT combined directly affect the injected fuel mixture.
These numbers are not measured but computed mostly based on UPSTREAM combustion sensor.
The key here is exhaust sensor only help analyze engine combustion.
Your issue is not sensors nor cats. These only witness cylinders combustion results.
Deal with Bank1 cylinders, not the exhaust lines.
Last edited by CaliBenzDriver; Sep 5, 2025 at 03:10 PM.
2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
Originally Posted by CaliBenzDriver
Just like I noted prior... range significance
LEAN engine tips...
> LOOSY CYL.
They forgot to add the drafty cylinders with higher blow-by losses.
Measure cyl. compressions
Relative crank RPM
(Bore scope optional)
> CYL. HEAT ....
Your scanner showed an interesting number:
cooler weak output
The number shows the lean cyl. contribution being cooler.
The combustion losses endup venting through the crankcase and back through intake valves.
That was on start up, after about 5 minutes. After driving for 15-20 minutes, the temps were 475 and 455. While Bank 1 was stiller cooler, those look like acceptable numbers. It's interesting the article said the CEL would not go on until a sensor was at 25%, but yet mine has come on at 12.5%. If the LTFT is downstream it looks it could be a faulty O2 sensor, leaky injector, fuel pressure regulator, coolant temp sensor, or an exhaust leak upstream of the LTFT sensor. One thing to add, it appears the LTFT Bk1 12.5% is staying constant. In my research, this appears to indicate the cat is functioning as intended. If the LTFT fluctuates like the STFT, then it indicates the cat is not longer cleaning the exhaust effectively. Doesn't look like anything I can do in the parking lot of my condo.
Last edited by Davery; Sep 5, 2025 at 05:40 PM.
Reason: clarity
That was on start up, after about 5 minutes. After driving for 15-20 minutes, the temps were 475 and 455. While Bank 1 was stiller cooler, those look like acceptable numbers. It's interesting the article said the CEL would not go on until a sensor was at 25%, but yet mine has come on at 12.5%. If the LTFT is downstream it looks it could be a faulty O2 sensor, leaky injector, fuel pressure regulator, coolant temp sensor, or an exhaust leak upstream of the LTFT sensor. One thing to add, it appears the LTFT Bk1 12.5% is staying constant. In my research, this appears to indicate the cat is functioning as intended.
If the LTFT fluctuates like the STFT, then it indicates the cat is not longer cleaning the exhaust effectively.
Doesn't look like anything I can do in the parking lot of my condo.
.... Fuel trims are ECU computed values, not sensor values.
The exhaust temp is a Lambda measure.
2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
Lambda Equivalence ratio is 0.953 and 0.989, which means bank 1 is running a bit richer than bank 2? I have not noticed any change in the exhaust. I also need to mention my engine has been tuned, so that may explain the Lambda number.
Last edited by Davery; Sep 5, 2025 at 06:13 PM.
Reason: clarity
for a cat to work, the car will not be at lambda 1.. it will actually fluctuate between lambda .95 and 1.05 and bounce in between. It has to do this "bouncing" to keep the cat working correctly. Back in the narrow sensor days, the "o2" gauges looked like a epileptic christmas tree,
Is there anyway to do check on the o2 sensor to ambient see if the sensor is ok?
Replace ignition coils. You had plugs replaced recently…but still on original coils? Buy all new Bosch or Beru coils. You described typical multiple misfire event, disappears after restart. You could also have some minor compression issue…but solving that is not money well spent at this point. My opinion, but based on some experience .
2012 E550 Cabriolet 2011 VW Eos 1985 Pininfarina Spider
Originally Posted by S-Prihadi
Why no one mentioned this crazy 99% ?
Is the scanner healthy ? or some other issue ?
Short term fuel trim (S2) at 25% would already trigger some sort of DTC, how can we be seeing 99% ?
I was under the impression the Short Term Fuel Trim S2 measured the percent of fuel that is burned. It also my understanding that since this number is normal, my cat is good (I hope). https://support.bluedriver.com/suppo...m-n-a-or-99-2-
I was under the impression the Short Term Fuel Trim S2 measured the percent of fuel that is burned. It also my understanding that since this number is normal, my cat is good (I hope). https://support.bluedriver.com/suppo...m-n-a-or-99-2-
No , fuel trim means this :
Let say your engine need 100 cc of fuel per minute, at that given load. When load change, fuel burn quantity change too.
The front wide band oxygen sensor ( lambda sensor, AFR sensor and other names ) is the one reading the decision for ECM to inject the fuel above and below the target 100cc per minute.
Let say you accelerate hard, and the Lambda sensor is detecting value that said lean ( not enough fuel ), ECM will know and add more fuel, let say 10cc more, now total is 110cc.
The short term fuel trim then will register +10% and no more 0%.
Below is for wide band Lambda sensor only and not for narrow band oxygen sensor.
The long term fuel trim is when ECM slowly log the fuel trim requirement and shift short term fuel trim to become a long term fuel trim.
Example, 2 hours of driving of which 100cc per minute deemed not enough by ECM seeing the Lamda value and ECM then decided to set
fuel at 110cc as long term. And then you climb a mountain, need more power and the short term fuel trim will be showing more fuel, say extra 5cc.
Now you have 115cc of fuel per minute, of which standard 100cc , plus 10% long term and 5% short term. Thus total fuel trim is 15%.
Fuel trim is to be read as Long Term + Short Term, to see actual trim or correction the ECM is doing, based on the input of the engine load and the lambda sensors.
The Lamda sensor ( wide band oxygen sensor no 1, front or upstream ) is the one telling ECM how good the fuel mix is based on the fuel used. If gasoline, it needs 14.7 air and 1 of fuel.
This value is 1.0 as stoich or optimum fuel to air ratio.
The narrow band oyxgen sensor at the CAT is more for emmission, as the CAT needs rich and lean swing to get rid of the baddies.
This sensor is low cost and simple and the operation graph is below : 0.1 volt to 0.9 volt is the range.
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