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you mean geek .here's the less concise version ...
the role of the ecu/dme/car computer, is to provide spark and fuel.
for fuel, the ecu measures the velocity (mass/second, actually, which allows for changes in elevation) of air into the engine with the mass air flow (maf) sensor.
the velocity is multiplied by the area of the intake tube to get flow of air (this is why the diameter of the maf holder is important).
the ecu has charts where it looks up the air volume and adds an appropriate amount of fuel.
there are four main types of charts that the ecu uses:
- limp (maf is broken, so ecu uses throttle position sensor - tps - to determine fuel); - idle; - low load/cruise/closed loop/'taking it easy' mode; and - high load/wide open throttle (wot)/open loop/'giving her heck' mode.
in low load the engine is not working too hard; it is cool, rpms are in the mid-range. in this mode of operation the ecu tries to add 1 part of fuel to every 14.7 parts of air.
this is considered the ratio of air to fuel (afr) that is (a) most efficient, and (b) makes most power. in 'science' is called the stoichiometric ratio.
the o2 sensors in the exhaust monitor the products of combustion and if the results stray from the 14.7:1 afr (ie, there is uncombusted o2 in the exhaust) then the ecu 'trims' the amount of fuel added.
initially these are short term fuel trims (stft). if they persist then they become long term fuel trims (ltft). this the the fra number that your obd tool reads.
the act of the o2 sensors modifying the fuel mixture is called feedback or closed loop.
the o2 sensors used the m96 are called 'narrow band'; this means that they only operate in a small range around the 14.7:1 afr (as opposed to wide band which operate within a ... wide band).
this system was designed to adapt to aging components; small vacuum leaks, drifting maf values, leaky or clogged injectors, drifting fuel pressures, etc.
in high load the engine is working hard; it is hot, rpms are high, or throttle is wide open but not a lot of air moving (ie, going up a steep hill).
as such, the ecu decides to switch between low and high load maps based on engine temp, intake air temp (iat) tps, rpms.
in this case the ecu wishes to 'richen' the afr - add more fuel. this additional fuel serves to quench/cool the combustion chamber, otherwise it can get hot; detonation can occur and holes get knocked in pistons.
typical high load afr is 12.5: 1 or so.
the narrow band o2 sensors do not operate in this range of afr, so there is no feedback; this is open loop operation.
Is the following correct ?
The significance of these Fuel trim results is that
1. the :Silk" system has no fuel trim problems
2. the "Stelan" system may have a fuel trim problem.
If this is correct, we can focus on SIlk's mod to the AOS system to make it as practical as possible?
Is the following correct ?
The significance of these Fuel trim results is that
1. the :Silk" system has no fuel trim problems
2. the "Stelan" system may have a fuel trim problem.
If this is correct, we can focus on SIlk's mod to the AOS system to make it as practical as possible?
It sure looks that way. Notice the tubing size between the two. I wonder if that's the difference?
Maybe just as simple as -
Larger diameter tubing(5/8"?) with reinforced walls so it does not collapse under vacuum/looks stock/does not deteriorate with oil vapor/heat.
Use stock-looking hose clamps.(Norma?)
the difference is the diaphragm and how the aos responds to vacuum. on decel the engine wants to idle (throttle closed) however rpms are high and lots of splashing happening in the crank case. with throttle closed not much fuel is required. further, with throttle closed there is a lot of vacuum in the intake. on an oem aos, this vacuum would pull the diaphragm closed and limit the amount of cleg (air with combustible fuel and oil vapour) drawn from the crankcase. with the aftermarket catch can, even with a third party diaphragm (i think stelan used an vw/audi unit) too much of this stuff gets past and throws off the trims.
all silk is doing is attaching the aos to the intake at a different location. completely apples and oranges.
frao and frau will always read the same as fra unless in a turbo with wideband o2 sensors
the car does not trim at wot/open loop (no feedback from oem narrow-band o2 sensors to establish trims as ecu richens mixture when engine working hard)
for rkat, values are given as +/- % fuel added, and you will get cell at +/-4%
for fra, values are given as +/- fuel added from a baseline of 1, and you will get a cell outside of 0.7 to 1.32
your trims are fine
Great stuff! Thanks RK. What a community.
I made a decision to investigate this further after today's conversation. Bank #1 looks good.
Bank #2 is a bit iffy on this trial. Had a lot of trouble getting Durametric to work for me - at least the way I wanted it to work for me....
Had a few minor starts and stops and the engine didn't fire-up in its normal willing way the last go-round.
Not sure what to make of the 0.80 for idle compensation. I'll try to test again and see if it changes. I've got a lot of other things to look at as well.
sooo, first things first. rkat (idle) is supposed to be % at +/-4%, while fra is an absolute number centered on 1 (0.7 to 1.32).
durametric shows the opposite; your pics show fra as the % and rkat as an absolute number.
who's right?
the clue is in the negative rkat. negative % makes sense (add or remove a % of fuel) but a negative value doesn't unless centred on zero (but zero would mean no fuel added ...?). i think durametric has their scales wrong?
if so, an rkat trim of -0.8% where a cel trips at 4% is not a big deal.
if you are worried about variation from bank to bank (your other rkat is -0.94%; even higher but well within acceptable limits) the variable is probably one o2 sensor getting a bit tired. note that the ecu is pulling a bit of fuel which means less air than anticipated so no vacuum leak; perhaps a lazy injector (sending them to rce for cleaning is always good for an engine spa).
note your fra's are pretty much bang on and equal on both sides (nobody gets a 1 and you are well away from .7 limit).
sooo, first things first. rkat (idle) is supposed to be % at +/-4%, while fra is an absolute number centered on 1 (0.7 to 1.32).
durametric shows the opposite; your pics show fra as the % and rkat as an absolute number.
who's right?
the clue is in the negative rkat. negative % makes sense (add or remove a % of fuel) but a negative value doesn't unless centred on zero (but zero would mean no fuel added ...?). i think durametric has their scales wrong?
if so, an rkat trim of -0.8% where a cel trips at 4% is not a big deal.
if you are worried about variation from bank to bank (your other rkat is -0.94%; even higher but well within acceptable limits) the variable is probably one o2 sensor getting a bit tired. note that the ecu is pulling a bit of fuel which means less air than anticipated so no vacuum leak; perhaps a lazy injector (sending them to rce for cleaning is always good for an engine spa).
Note your fra's are pretty much bang on and equal on both sides (nobody gets a 1 and you are well away from .7 limit).
RK - thanks for the lesson. The units are swapped, aren't they? I was having a little trouble interpreting the 0.80, but with your explanation, the other bank may be more suspect. This is potentially related to another set of data I collected.
Cly#3 is substantially rougher than the other cylinders. This is just a comparison between #1 and #3. The injector does come into question.
Porsche's explanation of mixture control and adaption trims where RKAT is adding and subtracting Ti, and FRA is multiplying Ti (with a value of 1= no change)
Durametric seems to be displaying a different value system. I would run the adaptions to each limit to see where they are by artificially enrichen (propane) and lean (vacuum leak) too see what the limits are.
05-02-2019, 03:37 PM
