"Instant" Fuel readings "overly fluctuating"?
#1
09-01-2018, 05:41 PM
"Instant" Fuel readings "overly fluctuating"?
I am eagerly awaiting my new OBD 2 reader which I need to give me fuel trims to try and diagnose my cat monitor not resetting.(1000mls and every drive cycle known to man) So I decided to take it for another long freeway drive and try to monitor the instant fuel readings on the dash to see what if anything was going on. What I noticed was a bit surprising but may be indicative of what I may find once I am able to read the fuel trims.... I think what I am seeing is a 15-20mpg fluctuation in the middle of when everything else is set steady. In other words I set the cruise to let's say 60mph find a long fairly straight freeway that is basically flat and the "instant" mpg seems to spike up and down by 15-20mpg without touching the pedals and the road conditions seemingly not changing.
Same thing also seems to happen on a long steady incline and a long decline (coasting). Probably even more pronounced on the coasting decline situation (larger fluctuations).
As usual car runs great, No codes or dash lights on. Is this a red herring or indicative of something?
Thanks
Alan
Same thing also seems to happen on a long steady incline and a long decline (coasting). Probably even more pronounced on the coasting decline situation (larger fluctuations).
As usual car runs great, No codes or dash lights on. Is this a red herring or indicative of something?
Thanks
Alan
#2
09-01-2018, 07:53 PM
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Deep RED herring.
Your reading of the mpg instant readout fluxuation is normal, given the state of electronics when your car was manufactured. I don't think you can tell anything about the fuel trim numbers from the mpg display.
Thefuel trim numbers are a positive or negative percentage number. If the number displayed is negative, it inticstes that a percentage of fuel was subtracted from the injector timing to compensate for the reading of the O2 sensor. Likewise, if it reads positive (minus sign absent), it means that % of fuel was added to correct a lean condition , as reported by the O2 sensor.
fluctuation is normal on the short term readings. The long term fuel trim is more valuable as a diagnostic tool. According to the shop manual for my year (2002 XKR), a plus or minus reading of up to 10% on either bank is to be considered normal and the system can easily adjust for that degree of flux. In general, my reading says the long term trim reading for the two banks combined should be no more than 10%. I'll go by the shop manual .
Z
Your reading of the mpg instant readout fluxuation is normal, given the state of electronics when your car was manufactured. I don't think you can tell anything about the fuel trim numbers from the mpg display.
Thefuel trim numbers are a positive or negative percentage number. If the number displayed is negative, it inticstes that a percentage of fuel was subtracted from the injector timing to compensate for the reading of the O2 sensor. Likewise, if it reads positive (minus sign absent), it means that % of fuel was added to correct a lean condition , as reported by the O2 sensor.
fluctuation is normal on the short term readings. The long term fuel trim is more valuable as a diagnostic tool. According to the shop manual for my year (2002 XKR), a plus or minus reading of up to 10% on either bank is to be considered normal and the system can easily adjust for that degree of flux. In general, my reading says the long term trim reading for the two banks combined should be no more than 10%. I'll go by the shop manual .
Z
Last edited by zray; 09-01-2018 at 07:59 PM.
#3
09-01-2018, 08:00 PM
The instant fuel trim readings are not read in mpg. They are a positive or negative percentage number. If the number displayed is negative, it inticstes that a percentage of fuel was subtracted from the injector timing to compensate for the reading of the O2 sensor. Likewise, if it reads positive (minus sign absent), it means that % of fuel was added to correct a lean condition , as reported by the O2 sensor.
fluctuation is normal on the short term readings. The long term fuel trim is more valuable as a diagnostic tool. According to the shop manual for my year (2002 XKR), a plus or minus reading of up to 10% on either bank is to be considered normal and the system can easily adjust for that degree of flux. In general, my reading says the long term trim reading for the two banks combined should be no more than 10%. I'll go by the shop manual .
Z
fluctuation is normal on the short term readings. The long term fuel trim is more valuable as a diagnostic tool. According to the shop manual for my year (2002 XKR), a plus or minus reading of up to 10% on either bank is to be considered normal and the system can easily adjust for that degree of flux. In general, my reading says the long term trim reading for the two banks combined should be no more than 10%. I'll go by the shop manual .
Z
#5
09-01-2018, 09:27 PM
Deep RED herring.
Your reading of the mpg instant readout fluxuation is normal, given the state of electronics when your car was manufactured. I don't think you can tell anything about the fuel trim numbers from the mpg display.
Thefuel trim numbers are a positive or negative percentage number. If the number displayed is negative, it inticstes that a percentage of fuel was subtracted from the injector timing to compensate for the reading of the O2 sensor. Likewise, if it reads positive (minus sign absent), it means that % of fuel was added to correct a lean condition , as reported by the O2 sensor.
fluctuation is normal on the short term readings. The long term fuel trim is more valuable as a diagnostic tool. According to the shop manual for my year (2002 XKR), a plus or minus reading of up to 10% on either bank is to be considered normal and the system can easily adjust for that degree of flux. In general, my reading says the long term trim reading for the two banks combined should be no more than 10%. I'll go by the shop manual .
Z
Your reading of the mpg instant readout fluxuation is normal, given the state of electronics when your car was manufactured. I don't think you can tell anything about the fuel trim numbers from the mpg display.
Thefuel trim numbers are a positive or negative percentage number. If the number displayed is negative, it inticstes that a percentage of fuel was subtracted from the injector timing to compensate for the reading of the O2 sensor. Likewise, if it reads positive (minus sign absent), it means that % of fuel was added to correct a lean condition , as reported by the O2 sensor.
fluctuation is normal on the short term readings. The long term fuel trim is more valuable as a diagnostic tool. According to the shop manual for my year (2002 XKR), a plus or minus reading of up to 10% on either bank is to be considered normal and the system can easily adjust for that degree of flux. In general, my reading says the long term trim reading for the two banks combined should be no more than 10%. I'll go by the shop manual .
Z
The car is pretty smelly from a cold start but I just put that down to a rich mix to get it started from cold.
#6
09-01-2018, 09:54 PM
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an air leak would cause the fuel trim number to be in positive range because the system is adding fuel (+) to compensate for the extra air.
Conversely, an excessive fuel condition, as in a leaky injector or lack of air, as in a plugged up air filter element, is going to show up as a negative (-) fuel trim number because the system is taking away fuel in an attempt to correct the problem.
However I don't think your car should be smelly rich at start up, even with the normal richening required.
Z
PS
with the engine warmed up completely, at idle with the car in neutral or park the short term fuel trim will be close to zero. But they will go up and down as the throttle is pressed and when the load on the engine is changed. For that reason, the long term numbers are going to be more helpful to learn what is going on.
Last edited by zray; 09-01-2018 at 09:59 PM.
#7
09-01-2018, 10:12 PM
an air leak would cause the fuel trim number to be in positive range because the system is adding fuel (+) to compensate for the extra air.
Conversely, an excessive fuel condition, as in a leaky injector or lack of air, as in a plugged up air filter element, is going to show up as a negative (-) fuel trim number because the system is taking away fuel in an attempt to correct the problem.
However I don't think your car should be smelly rich at start up, even with the normal richening required.
Z
PS
with the engine warmed up completely, at idle with the car in neutral or park the short term fuel trim will be close to zero. But they will go up and down as the throttle is pressed and when the load on the engine is changed. For that reason, the long term numbers are going to be more helpful to learn what is going on.
Conversely, an excessive fuel condition, as in a leaky injector or lack of air, as in a plugged up air filter element, is going to show up as a negative (-) fuel trim number because the system is taking away fuel in an attempt to correct the problem.
However I don't think your car should be smelly rich at start up, even with the normal richening required.
Z
PS
with the engine warmed up completely, at idle with the car in neutral or park the short term fuel trim will be close to zero. But they will go up and down as the throttle is pressed and when the load on the engine is changed. For that reason, the long term numbers are going to be more helpful to learn what is going on.
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