Fuel trims explained (the easy way)

[Music] hi everybody and welcome back to a new episode of diagnosed damn despite being around for a very long time i've noticed that a lot of you are still struggling understanding fuel trims that's why for this episode i decided to share a training video i've made for my program ddtsb in this video i explain fuel trims the easy way i really hope that this video helps some of you understand fuel trims better and if you already do understand i guess a little bit of repetition won't hurt anybody anyway please enjoy the video [Music] i'm diagnosed dan and welcome to this training video on field trims as a diagnostician the most common fault codes you're going to encounter are fault codes like mixture to lean mixture to rich or mixture adaption limit exceeded to diagnose mixture fault codes it's essential to know how fuel trims work to get a better understanding about fuel trims first we're going to make a leap back in time to the time when carburetors were still commonly used on vehicles as the car left the factory of course it ran perfectly or at least it was supposed to but over time maybe because of wear or maybe because your carburetor got a little bit dirty the mixture changed a little and the car didn't run as it used to so when the car came in for service we had to manually adjust or trim the mixture to compensate for this on the carburetor there's a mixture adaption screw and by turning this screw we can make the mixture leaner or richer whatever we desire now we can check our results by hooking up a gas analyzer to the exhaust of the vehicle on today's vehicles we no longer use carburetors and we no longer need to manually adjust the mixture instead the engine computer does all the work for us but we can still see what the computer is doing by taking a look at the fuel trim data pits before a new vehicle leaves the factory the engineers that designed the engine has first calculated and later tested on a test bench the exact amount of fuel that needs to be injected at a certain rpm under a certain engine load so in theory when a new vehicle leaves the factory and everything is new and perfect there's no need to deviate from the programmed amount of fuel so in theory the fuel trims should be at a perfect zero but you will almost never see a perfect zero fuel trim on a new car and this is because although all the cars have got exactly the same software there are small tolerances in the production process of all the engine parts so no two engines are exactly identical that's why it's perfectly normal to see up to five percent positive or negative fuel trim on a brand new engine now this is still a fairly new car but let's imagine this car develops a problem let's imagine because of wear the fuel pump delivers 10 percent less pressure compared to when it was new now when the injector opens it injects less fuel because there's less pressure than before resulting in a lean mixture if we don't do anything about the lean mixture the owner of the car might notice a lack of performance and he or she might notice that the engine doesn't run as smooth as it used to now in the old days we would have to hook up a gas analyzer to the exhaust of the vehicle and manually adjust the fuel trims but in modern vehicles there are two sensors in the exhaust that will report the lean mixture back to the engine computer and the engine computer will try to correct the problem by adding more fuel the computer can only add fuel by opening the injectors longer if we're adding fuel we call this a positive fuel trim if for some reason the mixture is too rich and we need to take away fuel we call this a negative fuel trim hi i'm diagnosed dan ddtsb stands for diagnosed then tech support base and the goal of me my team and our partners is to assist workshops with beyond dealer level support for an affordable price in fact for less than a euro a day sure independent workshops can access oe websites but you need a subscription that will cost you hundreds or even thousands a year and that's for one brand only factory websites usually update their bulletins in the first four years after a new model has been launched after that they usually focus on a new model and of course when the cars get older they'll find their way to the independent workshops and they just get a lot less feedback together with our partners we keep updating our database as long as we see certain models on the road we gather information from our european health desks and we start an investigation when we see a trend for a certain fault on a certain model after a thorough investigation our team creates a bulletin that tells you what to look for and offers you a possible solution this all is based upon over 50 000 fixed cases our hotline see annually that means that if we have seen the fault you're working on frequently in our help desk we've got a bulletin for it with a possible solution no matter the age of the car the process is very simple just select the car you're working on enter the fault or symptom and read the bulletin it's that simple if you need more help just type your question in the hotline section and we'll make sure you solve the case you're working on together with one of our brand specialists we upload hundreds of new bulletins every month and from time to time we do a major update to make the program even better this week we have done the first major update of the program since the launch this january we have added the measuring method section and we have now included the educational section in the educational section you can watch exclusive diagnosed and training videos that are not available elsewhere we've all heard the stories or maybe you experienced it yourself you've replaced a lot of components but the car is still not fixed in the measuring method section we'll provide you with a lot of self-study material and will tell you how to measure a component if a bulletin tells you to do a measurement by doing a thorough measurement we can drastically reduce the amount of parts that are being changed without fixing the car recently we also opened the ddtsb professional facebook group that's only accessible with a valid subscription to ddtsb on behalf of the entire ddtsb team i want to thank you for massively subscribing to ddtsb i want to tell you that we are ready to assist you in any way possible if you're interested but not yet subscribed to ddtsb please check out ddtsb.com for more information we still need to do the giveaway from last video if i hit enter and your name shows up on the screen please contact me on the email address in the description box of this video and i'll hook you up with a free subscription to ddtsb when you take a look at your scan tool you will notice you've got a short term and long term fuel trim the short term can quickly react to sudden and temporary changes in the mixture when you've got a more permanent problem and the computer needs to take away or add fuel for a longer period of time it will take that value and place it into long term when you see a long term value you will know you've got a more permanent and long term problem it's perfectly normal to see positive or negative field trim numbers even on brand new cars but even this system has got its limitations when there's something really wrong like a stock open injector or a vacuum leak even the fuel trends can't compensate anymore and over time the computer will flag a fault code and illuminate the middle light at that point the car needs to come into our shop and we need to diagnose the problem when we hook up a scan tool we will find fault code saying something like mixture to lean makes you too rich or mature adaption limit reached now let's put some of the theory into practice on this jeep i'm going to drive the system rich by introducing some brake clean into the air filter of this jeep now if my theory is right we should see the short-term field trim go negative immediately so taking away fuel to compensate for the rich mixture right now we're going to do exactly the opposite we're going to drive the system lean by disconnecting a vacuum hose right away we should see the short term fuel trim go positive so adding fuel to compensate for the lean mixture so if you have diagnosed and fixed a mixture problem you can confirm your fix by taking a look at fuel trims positive or negative 10 to 12 percent is considered the limit on most brands if you're above that you might still have another problem i really hope you enjoyed this video thanks for watching and see you next time [Music] in this video we're gonna do another giveaway but this time you need to answer a question the question is what's the date of first registration of the car that's being used in the demo version of ddtsb to find the answer visit ddtsb.com and enter the demo version then answer the question in the comment box below this video and in the next video i randomly pick a winner and hook up the winner with a one year free subscription to dd tsb i really hope this video helped some of you understand fuel trims better and remember when you need help and you've got a professional workshop please subscribe to ddtsb and we will assist you in any way possible anyway for the next time i've got a cool case study coming up but until then please remember diagnose dan fix it again see you next time guys you

Channel: DiagnoseDan

Views: 37,693

Rating: 4.9188323 out of 5

Keywords: Autodoktoren, fuel trim, Car repair, BMW, jeep, faultcode, fail, Volkswagen, Explained

Id: iYmOnXPBrB0

Length: 12min 26sec (746 seconds)

Published: Tue Mar 23 2021