Mixture formation - Revision history

Pokseva: /* Resetting of mixture adaptation */

2022-07-24T20:21:19Z

Resetting of mixture adaptation

← Older revision		Revision as of 20:21, 24 July 2022
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	[https://www.autoserviceprofessional.com/articles/6237-fuel-trim-how-it-works-and-how-to-make-it-work-for-you]		[https://www.autoserviceprofessional.com/articles/6237-fuel-trim-how-it-works-and-how-to-make-it-work-for-you]

			== Resetting of mixture adaptation ==
			After replacing or fixing components related to mixture formation, the mixture adaptation can be reset using [[DAS]] menu:

			"ME-SFI 2.8 - Control unit adaptations - Learning processes - Resetting of mixture adaptation"

Pokseva at 21:13, 16 November 2021

2021-11-16T21:13:08Z

← Older revision		Revision as of 21:13, 16 November 2021
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	If the adaptation number is below this, the computer has seen a rich condition from the oxygen sensor, so is reducing fuel.		If the adaptation number is below this, the computer has seen a rich condition from the oxygen sensor, so is reducing fuel.
	If the number is above 1.00, the mixture was lean and the PCM is adding fuel.		If the number is above 1.00, the mixture was lean and the PCM is adding fuel.

			The Additive and Multiplicative fuel trim are both long-term fuel trims.

	The mixture self-adaptation can be monitored under the [[DAS]] menu "ME-SFI 2.8 - Actual values - Check self-adaptation".		The mixture self-adaptation can be monitored under the [[DAS]] menu "ME-SFI 2.8 - Actual values - Check self-adaptation".
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	\|}		\|}

	The overall Lambda reading can be seen under the [[DAS]] menu "ME-SFI 2.8 - Actual values - Test engine at idle speed. - Lambda control, upstream TWC".		The short-term fuel trim can be seen in oxygen sensor feedback. You can use this to view the short-term correction in manufacturer specific fuel control when Additive and Multiplicative fuel trim are used.

			The overall oxygen sensor feedback reading (Lambda reading) can be seen under the [[DAS]] menu "ME-SFI 2.8 - Actual values - Test engine at idle speed. - Lambda control, upstream TWC".
	[[File:W220_DAS_lambda_control_upstream_TWC.png\|thumb\|none\|As you can see, each bank is close to 1.00.]]		[[File:W220_DAS_lambda_control_upstream_TWC.png\|thumb\|none\|As you can see, each bank is close to 1.00.]]

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	Also, you may have a fuel delivery problem. Check fuel volume as well as fuel pressure, particularly with return-less systems.		Also, you may have a fuel delivery problem. Check fuel volume as well as fuel pressure, particularly with return-less systems.
	All of these fuel trim numbers are based on O2 sensors that are working well and promptly.		All of these fuel trim numbers are based on O2 sensors that are working well and promptly.
	They should be able to switch rich-lean-rich at least once a second.		They should be able to switch rich-lean-rich at least once a second, as during normal closed-loop operation, the PCM uses short-term fuel trim (STFT) calculations to constantly command the air/fuel ratio slightly rich and then slightly lean. This is necessary for the catalytic converter to do its job correctly, and the average of the rich/lean swings will be in or near the middle of the oxygen sensor's signal range.

	The O2 sensor readings can be plotted under the [[DAS]] menu "ME-SFI 2.8 - Actual values - Lambda control".		The O2 sensor readings can be plotted under the [[DAS]] menu "ME-SFI 2.8 - Actual values - Lambda control".
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	Of course, there are many causes of fuel trim codes, but using your [[DAS]] and fuel trim numbers should help reduce your diagnostic time.		Of course, there are many causes of fuel trim codes, but using your [[DAS]] and fuel trim numbers should help reduce your diagnostic time.

			[https://www.autoserviceprofessional.com/articles/6237-fuel-trim-how-it-works-and-how-to-make-it-work-for-you]

Pokseva: updated with pictures

2021-11-16T20:46:03Z

updated with pictures

← Older revision		Revision as of 20:46, 16 November 2021
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	Of course, attend to the basics first, such as measuring fuel pressure and doing a visual check of electrical connections, wire integrity, hoses, etc.		Of course, attend to the basics first, such as measuring fuel pressure and doing a visual check of electrical connections, wire integrity, hoses, etc.
	Then there are the many sensors and solenoids that determine how much fuel is actually injected.		Then there are the many sensors and solenoids that determine how much fuel is actually injected.
	If you have an ~~SDS~~, you will have access to the most comprehensive data available.		If you have an [[DAS]], you will have access to the most comprehensive data available.
	For example, look at coolant temperature as the engine warms up to verify that the thermostat is keeping the coolant within the proper range.		For example, look at coolant temperature as the engine warms up to verify that the thermostat is keeping the coolant within the proper range.
	Typically, if the thermostat is opening too soon the PCM will flag a Code P0128.		Typically, if the thermostat is opening too soon the PCM will flag a Code P0128.
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	Believe it or not, the oxygen sensor arrived on the automotive scene over three decades ago.		Believe it or not, the oxygen sensor arrived on the automotive scene over three decades ago.
	The PCM looks at the signal voltage it generates to tell if the mixture is rich or lean.		The PCM looks at the signal voltage it generates to tell if the mixture is rich or lean.
	If the signal indicates lean, the PCM will add fuel by increasing pulse width, and ~~vicesa~~ versa.		If the signal indicates lean, the PCM will add fuel by increasing pulse width, and vice versa.
	You can see the fuel trim indication in the data stream. Mercedes-Benz breaks down these fuel trim numbers into two readings.		You can see the fuel trim indication in the data stream. Mercedes-Benz breaks down these fuel trim numbers into two readings:
	The first is ~~“Additive”~~ fuel trim. This reading tells you what the PCM is doing to the mixture at idle and just off idle.
	In the ~~SDS~~, it is ~~also~~ referred to as the ~~“Idle”~~ trim reading, ~~and, on older systems, "lower part load~~."		(1) The first is "Additive" fuel trim. This reading tells you what the PCM is doing to the mixture at idle and just off idle.
			In the [[DAS]], it is referred to as the "Idle" trim reading.
			The [[DAS]] typically displays it as a millisecond adaptation (ms). The ms scale is simple.
			This is how much time in milliseconds the injector is being held open or closed.
			A negative ms number means fuel is reduced. If it is a positive number, then fuel is being added.
	If you see this number deviating from the center point, you should perform testing while the engine is at idle.		If you see this number deviating from the center point, you should perform testing while the engine is at idle.
	~~If the problem is above idle and under vehicle load, you should look at the "upper part fuel trim," or "Multiplicative" number.~~
	~~This represents fuel trims throughout the rpm range.~~

	~~'''A Rose By Any Other Name~~.~~'''~~		(2) The second is "Multiplicative" or part-load fuel trim. This represents fuel trims throughout the rpm range.
	~~These numbers can be represented on several~~ different ~~scales~~.		The part-load readings are split into two different rpm ranges, lower part load and upper part load.
	The ~~SDS~~ typically displays ~~them as either a millisecond adaptation (ms), or~~ in Lambda.		The [[DAS]] typically displays it in Lambda.
	~~If you’ll recall,~~ Lambda is a scale based on stoichiometry, the ideal ratio of 14.7 parts air to 1 part fuel by weight.		Lambda is a scale based on stoichiometry, the ideal ratio of 14.7 parts air to 1 part fuel by weight. This is represented by the number 1.00.
	This is represented by the number 1.00. If the adaptation number is below this, the computer has seen a rich condition from the oxygen sensor, so is reducing fuel.		If the adaptation number is below this, the computer has seen a rich condition from the oxygen sensor, so is reducing fuel.
	If the number is above 1.00, the mixture was lean and the PCM is adding fuel.		If the number is above 1.00, the mixture was lean and the PCM is adding fuel.
	The ~~ms scale is simple~~. ~~This is how much time in milliseconds~~ the ~~injector is being held open or closed~~.
	~~A negative ms number means fuel is reduced~~. ~~If it's~~ a ~~positive number~~, ~~then fuel~~ is ~~being added~~.		The mixture self-adaptation can be monitored under the [[DAS]] menu "ME-SFI 2.8 - Actual values - Check self-adaptation".

			{\|
			\|[[File:W220_DAS_self-adaptation.png\|thumb\|none\|In this case, the injectors of the left bank are pulsed .14 less due to a rich condition at idle]]
			\|[[File:W220_DAS_self-adaptation_lean.png\|thumb\|none\|In this case, the injectors are pulsed more due to a lean condition at load]]
			\|}

			The overall Lambda reading can be seen under the [[DAS]] menu "ME-SFI 2.8 - Actual values - Test engine at idle speed. - Lambda control, upstream TWC".
			[[File:W220_DAS_lambda_control_upstream_TWC.png\|thumb\|none\|As you can see, each bank is close to 1.00.]]

	You should compare Bank 1 and Bank 2 readings where applicable, but you can also tell if the engine has a problem at idle or at part throttle.		You should compare Bank 1 and Bank 2 readings where applicable, but you can also tell if the engine has a problem at idle or at part throttle.
	Use this information to isolate the problem.		Use this information to isolate the problem.

	~~'''One Bank, Or Two.'''~~
	When dealing with an engine with two banks, compare the fuel trims from one to the other.		When dealing with an engine with two banks, compare the fuel trims from one to the other.
	If you have a problem with only one bank, focus on that side's O2 sensor.		If you have a problem with only one bank, focus on that side's O2 sensor.
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	'''Problems Over The Road.'''		'''Problems Over The Road.'''
	Small problems like a vacuum leak usually do not affect the fuel trim much above idle.		Small problems like a vacuum leak usually do not affect the fuel trim much above idle.
	An injector with a weak return spring can still cause a rich condition, but it probably ~~won’t~~ if it is only leaking.		An injector with a weak return spring can still cause a rich condition, but it probably won't if it is only leaking.
	If dealing with a lean condition at higher rpm, the MAF should be at the top on your list of suspects.		If dealing with a lean condition at higher rpm, the MAF should be at the top on your list of suspects.
	Also, you may have a fuel delivery problem. Check fuel volume as well as fuel pressure, particularly with return-less systems.		Also, you may have a fuel delivery problem. Check fuel volume as well as fuel pressure, particularly with return-less systems.
	All of these fuel trim numbers are based on O2 sensors that are working well and promptly.		All of these fuel trim numbers are based on O2 sensors that are working well and promptly.
	They should be able to switch rich-lean-rich at least once a second.		They should be able to switch rich-lean-rich at least once a second.
	Then, race the engine and you should see a sudden rich condition, a result of acceleration enrichment ~~(as would’ve been provided by the accelerator pump in a carburetor -- remember those?)~~.
			The O2 sensor readings can be plotted under the [[DAS]] menu "ME-SFI 2.8 - Actual values - Lambda control".
			[[File:W220_DAS_lambda_control.png\|thumb\|none\|Here, you are looking at two pre-TWC and two post-TWC O2 sensors. The red and green traces are the two front O2 sensors, which are switching normally.]]

			Then, race the engine and you should see a sudden rich condition, a result of acceleration enrichment.
	If yes, you have a good O2 sensor. If you have multiple codes, try clearing them, then clear out fuel adaptations.		If yes, you have a good O2 sensor. If you have multiple codes, try clearing them, then clear out fuel adaptations.
	Let the engine idle for five minutes and look at the idle (Additive) fuel numbers. Follow this with a short road test and look at the part throttle (Multiplicative) numbers to see if it looks like codes will set again.		Let the engine idle for five minutes and look at the idle (Additive) fuel numbers. Follow this with a short road test and look at the part throttle (Multiplicative) numbers to see if it looks like codes will set again.

	Of course, there are many causes of fuel trim codes, but using your ~~SDS~~ and fuel trim numbers should help reduce your diagnostic time~~. It also leads to accurate diagnosis and satisfied customers~~.		Of course, there are many causes of fuel trim codes, but using your [[DAS]] and fuel trim numbers should help reduce your diagnostic time.

Pokseva: initial version

2021-11-10T20:24:45Z

initial version

New page

If the DTC (Diagnostic Trouble Code) that's present is related to the fuel system, there are often many twists and turns in the trail that leads to the source of the trouble. What tools do we have to identify the problem?

'''First Things First.'''
Since the fuel system is one of OBD II’s three continuous monitors, if a problem exists it is probably going to set a DTC.
Of course, attend to the basics first, such as measuring fuel pressure and doing a visual check of electrical connections, wire integrity, hoses, etc.
Then there are the many sensors and solenoids that determine how much fuel is actually injected.
If you have an SDS, you will have access to the most comprehensive data available.
For example, look at coolant temperature as the engine warms up to verify that the thermostat is keeping the coolant within the proper range.
Typically, if the thermostat is opening too soon the PCM will flag a Code P0128.
Engine speed and load signals determine the base pulse width, so have the largest affect on mixture formation. This is providing the Mercedes-Benz self-diagnostics have not found a problem with other input sensors.

'''Feedback, Feedback, Feedback...'''
Believe it or not, the oxygen sensor arrived on the automotive scene over three decades ago.
The PCM looks at the signal voltage it generates to tell if the mixture is rich or lean.
If the signal indicates lean, the PCM will add fuel by increasing pulse width, and vicesa versa.
You can see the fuel trim indication in the data stream. Mercedes-Benz breaks down these fuel trim numbers into two readings.
The first is “Additive” fuel trim. This reading tells you what the PCM is doing to the mixture at idle and just off idle.
In the SDS, it is also referred to as the “Idle” trim reading, and, on older systems, "lower part load."
If you see this number deviating from the center point, you should perform testing while the engine is at idle.
If the problem is above idle and under vehicle load, you should look at the "upper part fuel trim," or "Multiplicative" number.
This represents fuel trims throughout the rpm range.

'''A Rose By Any Other Name.'''
These numbers can be represented on several different scales.
The SDS typically displays them as either a millisecond adaptation (ms), or in Lambda.
If you’ll recall, Lambda is a scale based on stoichiometry, the ideal ratio of 14.7 parts air to 1 part fuel by weight.
This is represented by the number 1.00. If the adaptation number is below this, the computer has seen a rich condition from the oxygen sensor, so is reducing fuel.
If the number is above 1.00, the mixture was lean and the PCM is adding fuel.
The ms scale is simple. This is how much time in milliseconds the injector is being held open or closed.
A negative ms number means fuel is reduced. If it's a positive number, then fuel is being added.
You should compare Bank 1 and Bank 2 readings where applicable, but you can also tell if the engine has a problem at idle or at part throttle.
Use this information to isolate the problem.

'''One Bank, Or Two.'''
When dealing with an engine with two banks, compare the fuel trims from one to the other.
If you have a problem with only one bank, focus on that side's O2 sensor.
Once you have isolated the problem bank, use the Additive and Multiplicative fuel trims to determine if it occurs at idle or part throttle.
If the problem is at idle, you need to perform additional testing. If you have a lean condition and a P0171/4, check for vacuum leaks.
This can easily be done with a smoke machine, but keep in mind that the oil is being burned, which can contaminate the MAF element, so remove the MAF first.
If you have a rich condition, you may have a leaking injector. If the problem is with both banks, you should look at sensors that affect both.
A lean condition can still be a vacuum leak, but it can also be low MAF output. At this time you may want to start looking at the MAF.

'''Problems Over The Road.'''
Small problems like a vacuum leak usually do not affect the fuel trim much above idle.
An injector with a weak return spring can still cause a rich condition, but it probably won’t if it is only leaking.
If dealing with a lean condition at higher rpm, the MAF should be at the top on your list of suspects.
Also, you may have a fuel delivery problem. Check fuel volume as well as fuel pressure, particularly with return-less systems.
All of these fuel trim numbers are based on O2 sensors that are working well and promptly.
They should be able to switch rich-lean-rich at least once a second.
Then, race the engine and you should see a sudden rich condition, a result of acceleration enrichment (as would’ve been provided by the accelerator pump in a carburetor -- remember those?).
If yes, you have a good O2 sensor. If you have multiple codes, try clearing them, then clear out fuel adaptations.
Let the engine idle for five minutes and look at the idle (Additive) fuel numbers. Follow this with a short road test and look at the part throttle (Multiplicative) numbers to see if it looks like codes will set again.

Of course, there are many causes of fuel trim codes, but using your SDS and fuel trim numbers should help reduce your diagnostic time. It also leads to accurate diagnosis and satisfied customers.