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Old 05-27-2016, 10:13 PM   #1
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P30 fuel trim and an old man's memory

so, I'd like to discuss, ok, remember more about fuel trim, for my p30 chassis. If any of you young whipper snappers out there can help, chime in!! I realize that this a chevy chassis, but fuel trim is pretty universal these days for all fuel injected vehicles.

So the way I under stand fuel trim is that 0 is perfect, - fuel trim means the computer ECM is subtracting fuel from the base line injector pulse width program to keep the fuel ratio at 14.7:1. A positive ratio indicates that the ecm is adding fuel to compensate for extra air being added to the intake. I've got a -2% fuel trim on one bank and a -4%(+-1%) on both. It is not bouncing between - and +. This would indicate to me that I have to little air going into the throttle body through the MAS and MATS. I've checked the air filter, new MAS, new MATS, still - fuel trim. I replaced all 4 O2 sensors, and most of the other sensors on the engine. I've replaced the exhaust manifold gaskets and checked for leaks, none. Good compression, 140 on 6, 130 on 2. Did leak down tests on the 2 low ones, all ok. Misfires are at 0 except number 8 cylinder, once per 32 cycles when it is cold. Fuel pressure near perfect with a new FPR. 8 new fuel injectors. I use a tech2 knockoff code reader, but believe I am getting reliable information.

Question? I'm at 3500 ft altitude, would this make it read negative like that? I am wondering if any one have experience with altitude concept?

I have not replaced the idle solenoid, but pulled it apart and cleaned it. Seems like I is functioning perfectly. Any experience with this being the problem?

Ok, so I know what the books say, what I'm looking for is some one who has practical experience with the fuel trim and any other ideas that I could look for. thanks.
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Old 05-27-2016, 10:15 PM   #2
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By the way, a positive fuel trim usually indicates a vacuum leak somewhere in ones intake system, bad O2 sensors or bad exhaust manifold gaskets.
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Old 05-28-2016, 08:33 AM   #3
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At 3500 ft. wouldn't the oxigen level in the air be low and cause this condition. Have you checked it at lower altitudes. The oxigen sensors read % of ox,
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Old 05-28-2016, 08:46 AM   #4
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The long term trim compensates for things like fuel injector differences, fuel pressure differences, minor deviations in a MAP or MAF sensor, throttle position sensors, timing, mechanical imperfections, etc, etc.

Short term compensates for minor everyday traffic stuff, outside air temperature, fuel differences, acceleration, speed, etc. The short term can roll into the long term trim if the short term numbers are consistent.

Generally, PLUS trim means that additional air (or not enough fuel) is entering the system and the system needs to add fuel to compensate and get the ideal mixture. This could be a vacuum leak, fuel pressure low lazy fuel injector, bad MAP or MAF sensor.

MINUS trim goes the other way, The O2 senses that not enough air (or to much fuel) is entering the system, so it cuts back on the fuel. This could be something like an injector thats stuck open, or fue pressure that to high, or a dirty MAF sensor wire.


If the long term trim gets to far out (Plus 25-30 or minus 18 to 20) the computer (ECM or whatever) will generally a fuel trim "LEAN" or "RICH" code.

I would not worry about the trim numbers unless you're setting these codes,
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Old 05-28-2016, 03:16 PM   #5
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Yes, 3500 ft is more than enough to cause negative "trim". I would say your ECM is "working as designed".
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Old 05-28-2016, 11:20 PM   #6
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Quote:
Originally Posted by Waiter21 View Post
The long term trim compensates for things like fuel injector differences, fuel pressure differences, minor deviations in a MAP or MAF sensor, throttle position sensors, timing, mechanical imperfections, etc, etc.

Short term compensates for minor everyday traffic stuff, outside air temperature, fuel differences, acceleration, speed, etc. The short term can roll into the long term trim if the short term numbers are consistent.

Generally, PLUS trim means that additional air (or not enough fuel) is entering the system and the system needs to add fuel to compensate and get the ideal mixture. This could be a vacuum leak, fuel pressure low lazy fuel injector, bad MAP or MAF sensor.

MINUS trim goes the other way, The O2 senses that not enough air (or to much fuel) is entering the system, so it cuts back on the fuel. This could be something like an injector thats stuck open, or fue pressure that to high, or a dirty MAF sensor wire.


If the long term trim gets to far out (Plus 25-30 or minus 18 to 20) the computer (ECM or whatever) will generally a fuel trim "LEAN" or "RICH" code.

I would not worry about the trim numbers unless you're setting these codes,
About the FPR, yes it has been replaced and I have taken readings, all is with in specs. The fuel injectors are brand new and I'm not getting abnormal 02 sensor readings(new). MAF and MATS are brand new too, voltages look well within specs. That is why I can't figure out the negative trim unless it is the elevation. thanks for the verification of my memory....
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Old 05-28-2016, 11:22 PM   #7
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Quote:
Originally Posted by Gary RVRoamer View Post
Yes, 3500 ft is more than enough to cause negative "trim". I would say your ECM is "working as designed".

Have you dealt with these kind of altitude readings before? Nothing is mentioned in any of my reference materials, but they are really old. Not questioning you credentials, just hoping for some practical experience.
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Old 05-30-2016, 10:12 AM   #8
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As a matter of fact I worked as a computer consultant to GM, Chrysler and Ford back in the days when these systems were newly designed. Did a lot of analysis because techs were forever replacing computers and sensors for invalid reasons. But that's not really the point here.

A "naturally aspirated" gas engine loses about 3% of its power for every additional1000 ft in altitude. That's because of the reduction in available oxygen to feed combustion, just basic phyics. The ECM adjusts air fuel mix to maintain the stochiometric volume requirement for efficient fuel burn, which is about 14.7:1 The base calibration is done at sea level, and the ECM "trims' that as needed to maintain the proper mix when the air gets thinner. That's what the MAP or MAF sensors are for, to determine the density of the incoming air charge. It can't change the air, so it adjust the fuel to match, thus reducing power. The driver responds by pushing down more on the accelerator, bringing in more air so fuel consumption can go back up again and once again more power gets produced. At some altitude, there isn't enough air and you simply cannot get full power because of the negative trim. Turbocharging and superchargng can overcome that to some degree, though.

The ECM may also do some negative trim to help the engine run a bit cooler under heavy load.
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Old 05-31-2016, 07:32 AM   #9
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Quote:
Originally Posted by Gary RVRoamer View Post
As a matter of fact I worked as a computer consultant to GM, Chrysler and Ford back in the days when these systems were newly designed. Did a lot of analysis because techs were forever replacing computers and sensors for invalid reasons. But that's not really the point here.

A "naturally aspirated" gas engine loses about 3% of its power for every additional1000 ft in altitude. That's because of the reduction in available oxygen to feed combustion, just basic phyics. The ECM adjusts air fuel mix to maintain the stochiometric volume requirement for efficient fuel burn, which is about 14.7:1 The base calibration is done at sea level, and the ECM "trims' that as needed to maintain the proper mix when the air gets thinner. That's what the MAP or MAF sensors are for, to determine the density of the incoming air charge. It can't change the air, so it adjust the fuel to match, thus reducing power. The driver responds by pushing down more on the accelerator, bringing in more air so fuel consumption can go back up again and once again more power gets produced. At some altitude, there isn't enough air and you simply cannot get full power because of the negative trim. Turbocharging and superchargng can overcome that to some degree, though.

The ECM may also do some negative trim to help the engine run a bit cooler under heavy load.
Thank you, it is exactly what I wanted to know. At no time has my ECM set a code but from what my literature indicates that doesn't happen until we get to the 25% level? correct? Or have my books been updated and I haven't been

I haven't taken any readings at the higher or lower elevations, that why I was wondering. So, basicly if I take a reading at sea level then it should show that the long term trim will be almost 0% and if I take one at 7000 feet it should almost double? Appreciate the time on this.
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Old 06-01-2016, 10:30 AM   #10
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Ignore it. Fuel trim numbers under 10%, positive or negative are not significant. And fuel trim indicates a deviation from the anticipated amount of fuel given the current conditions, meaning that barometric pressure and altitude are already taken into account. In other words, the vehicles that live at high altitude also start at a "0" fuel trim and the pcm corrects from there. You do not see them with negative trim numbers to compensate for altitude. Here in Phoenix fuel trims should be the same as up in Flagstaff (6-7k higher altitude).

I am, or was, a drivability technician for more than 20 years, part of that time for General Motors, and now I teach it at the college level as well as own a repair shop. I teach my students and employees to ignore fuel trim under 10%, and in some cases it is not practical to address issues under 15%. Trying to correct a 2-4% fuel trim issue is trying to "correct" 80% of the perfectly good running cars that I see every day. There is no such thing as perfect fuel trim, we see cars donated by the manufacturers at the college with less than 100 miles on them that have worse fuel trim numbers than 5%.

And keep in mind, you are chasing idle fuel trim, apparently. Fuel trim cells change as soon as you break idle and you are looking at a whole other set of numbers. Affected by a whole other set of variables.

If negative fuel trim continues to increase, to over 10% total fuel trim (short and long term added together) you would start looking at maf and baro sensor problems as most likely culprits but egr contamination will also cause negative trim readings.
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Old 06-02-2016, 08:32 AM   #11
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Ignore it. Fuel trim numbers under 10%, positive or negative are not significant. And fuel trim indicates a deviation from the anticipated amount of fuel given the current conditions, meaning that barometric pressure and altitude are already taken into account. In other words, the vehicles that live at high altitude also start at a "0" fuel trim and the pcm corrects from there. You do not see them with negative trim numbers to compensate for altitude. Here in Phoenix fuel trims should be the same as up in Flagstaff (6-7k higher altitude).

I am, or was, a drivability technician for more than 20 years, part of that time for General Motors, and now I teach it at the college level as well as own a repair shop. I teach my students and employees to ignore fuel trim under 10%, and in some cases it is not practical to address issues under 15%. Trying to correct a 2-4% fuel trim issue is trying to "correct" 80% of the perfectly good running cars that I see every day. There is no such thing as perfect fuel trim, we see cars donated by the manufacturers at the college with less than 100 miles on them that have worse fuel trim numbers than 5%.

And keep in mind, you are chasing idle fuel trim, apparently. Fuel trim cells change as soon as you break idle and you are looking at a whole other set of numbers. Affected by a whole other set of variables.

If negative fuel trim continues to increase, to over 10% total fuel trim (short and long term added together) you would start looking at maf and baro sensor problems as most likely culprits but egr contamination will also cause negative trim readings.
Wow, thanks, exactly what I was looking for! New maf and baro sensor's, new egr valve as well. So, it sounds like I have my problems solved.

So, do you don't think the Air Idle Solenoid could cause this? When ac is activated and/or put in drive it seems to maintain the idle correctly at about 650 +-25rpm's
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Old 06-02-2016, 08:56 AM   #12
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If you think you may have vacuum leak just get a spray bottle full of water and on cold engine at idle just spray every joint and hose.

If any response or change a leak is there.

No change then likely no leak.
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Old 06-02-2016, 10:09 AM   #13
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Not really sure why you are worrying about this. It doesn't sound like you have a drivability problem and if the numbers you are quoting are the long term fuel trim numbers then the engine is running as close to stoichiometric (14.7:1) as possible. Short term is probably bouncing around a lot more but still within specs. Barometric pressure should reset each time you start the vehicle so it should be compensating for the altitude and it sounds like it is due to the fact that it feels that there is not enough O2 and is slightly removing fuel. The engine will have slightly less power but not enough that you would notice much.
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Old 06-02-2016, 12:06 PM   #14
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Sounds like the ECM is doing exactly what it was designed to do, adjusting trim for minor deviations in the engine, sensors, and components.

If the required long term adjustments become to large, then the ECM will set the trim error codes.

As long as the trim error codes are not being set, there isn't anything wrong, and IMHO - you're wasting your money attempting to get the long term trim to zero.
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