# Fuel trim **URL:** \ **Category:** General Discussion\ **Created:** [July 29, 2011, 6:38pm UTC](https://community.cartalk.com/t/fuel-trim/66697 "2011-07-29T18:38:27Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![automechanic1993](https://avatars.discourse-cdn.com/v4/letter/a/a9a28c/32.png) [@automechanic1993](https://community.cartalk.com/u/automechanic1993)\ **Post date:** [July 29, 2011, 6:38pm UTC](https://community.cartalk.com/t/fuel-trim/66697/1 "2011-07-29T18:38:27Z")
what is fuel trim and how does it work ---
**Author:** ![ken\_green](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/ken_green/32/816_2.png) [@ken\_green](https://community.cartalk.com/u/ken_green)\ **Post date:** [July 29, 2011, 6:42pm UTC](https://community.cartalk.com/t/fuel-trim/66697/2 "2011-07-29T18:42:38Z")
Automotive ? That sounds like a term from my Cessna 172. The air/fuel ratio was user adjustable and you watched your exhaust temp guage to see that the optimal ratio for altitude had been attained. ---
**Author:** ![automechanic1993](https://avatars.discourse-cdn.com/v4/letter/a/a9a28c/32.png) [@automechanic1993](https://community.cartalk.com/u/automechanic1993)\ **Post date:** [July 29, 2011, 6:44pm UTC](https://community.cartalk.com/t/fuel-trim/66697/3 "2011-07-29T18:44:45Z")
yes fuel trim for the computer what is it ---
**Author:** ![cigroller](https://avatars.discourse-cdn.com/v4/letter/c/c68b51/32.png) [@cigroller](https://community.cartalk.com/u/cigroller)\ **Post date:** [July 29, 2011, 6:46pm UTC](https://community.cartalk.com/t/fuel-trim/66697/4 "2011-07-29T18:46:34Z")
The fuel trim is the air/fuel mix. On contemporary cars it is computer controlled with input from the oxygen sensors. ---
**Author:** ![automechanic1993](https://avatars.discourse-cdn.com/v4/letter/a/a9a28c/32.png) [@automechanic1993](https://community.cartalk.com/u/automechanic1993)\ **Post date:** [July 29, 2011, 6:58pm UTC](https://community.cartalk.com/t/fuel-trim/66697/5 "2011-07-29T18:58:24Z")
basiclly it is the fuel feed back system ---
**Author:** ![automechanic1993](https://avatars.discourse-cdn.com/v4/letter/a/a9a28c/32.png) [@automechanic1993](https://community.cartalk.com/u/automechanic1993)\ **Post date:** [July 29, 2011, 7:02pm UTC](https://community.cartalk.com/t/fuel-trim/66697/6 "2011-07-29T19:02:47Z")
so the oxygen sensor tells the computer how much oxygen content is in the exhaust and then the computer tells the injector how long to stay open. ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [August 2, 2011, 5:59pm UTC](https://community.cartalk.com/t/fuel-trim/66697/7 "2011-08-02T17:59:39Z")
Not exactly. A modern car’s injector pulse width is determined by the computer reading signals from sensors telling it the position of the throttle, the amount (mass) of air flowing into the engine, the crank speed, the crank position (multiport systems meter by cylinder), the engine temperature (cold engines actually bypass the oxygen sensor signal), and the oxygen sensor. I may be missing one, but the point is that the oxygen sensor signal is just one in a number of signals that the computer considers in adjusting the injector pulsewidth. It’s actually less major than the others, and is even not considered at all if the engine is cold. ---
**Author:** ![circuitsmith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/circuitsmith/32/40376_2.png) [@circuitsmith](https://community.cartalk.com/u/circuitsmith)\ **Post date:** [August 2, 2011, 7:10pm UTC](https://community.cartalk.com/t/fuel-trim/66697/8 "2011-08-02T19:10:20Z")
Given information mountainbike listed the engine computer uses a data table (map) to calculate the injector pulse width that should give the correct amount of fuel under those particular conditions. The oxygen sensor confirms whether that is indeed the case. Then the computer makes an adjustment until the oxygen sensor says it’s just right: generally wandering from slightly rich to slightly lean. The difference between the calculated fuel delivery rate and what actually makes the oxygen sensor happy is the fuel trim. If there’s a problem like a vacuum leak the difference becomes big. ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [August 2, 2011, 11:55pm UTC](https://community.cartalk.com/t/fuel-trim/66697/9 "2011-08-02T23:55:02Z")
Excellent clarification, Circuitsmith.With your permission, I just might borrow it sometime. ---
**Author:** ![circuitsmith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/circuitsmith/32/40376_2.png) [@circuitsmith](https://community.cartalk.com/u/circuitsmith)\ **Post date:** [August 3, 2011, 1:32am UTC](https://community.cartalk.com/t/fuel-trim/66697/10 "2011-08-03T01:32:05Z")
Have at it. ---
**Author:** ![automechanic1993](https://avatars.discourse-cdn.com/v4/letter/a/a9a28c/32.png) [@automechanic1993](https://community.cartalk.com/u/automechanic1993)\ **Post date:** [August 3, 2011, 3:06am UTC](https://community.cartalk.com/t/fuel-trim/66697/11 "2011-08-03T03:06:07Z")
can i use soapy water to find a vacuum leak ---
**Author:** ![cigroller](https://avatars.discourse-cdn.com/v4/letter/c/c68b51/32.png) [@cigroller](https://community.cartalk.com/u/cigroller)\ **Post date:** [August 3, 2011, 3:49am UTC](https://community.cartalk.com/t/fuel-trim/66697/12 "2011-08-03T03:49:10Z")
Well, I guess you could sort of use soapy water to find a vacuum leak - but it wouldn’t work all that well, you’re thinking backwards, & you will probably damage something - like the engine, or at least O2 sensors & the catalytic converter. You’re thinking backwards because a vacuum sucks in, it doesn’t blow out. So if you want to suck soapy water into the engine then you could try it. But I think you’d rather not suck soapy water in. So, use something that will burn when it get sucked in. My favorite “shade-tree” method is to rig a piece of rubber hose to the end of a propane torch. Turn on the propane but don’t light it (!) - feed the until propane around vacuum lines & all over the intake. If it hits a leak the idle will respond. Perhaps now would be a good time to describe your entire issue. ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [August 3, 2011, 5:21pm UTC](https://community.cartalk.com/t/fuel-trim/66697/13 "2011-08-03T17:21:14Z")
A can of starter fluid works well for this too. ---
**Author:** ![irlandes](https://avatars.discourse-cdn.com/v4/letter/i/b5e925/32.png) [@irlandes](https://community.cartalk.com/u/irlandes)\ **Post date:** [August 4, 2011, 4:09am UTC](https://community.cartalk.com/t/fuel-trim/66697/14 "2011-08-04T04:09:40Z")
I just bought an Equus 3140 scanner, and the Haynes OBDII book. They described trim just exactly as Circuitsmith did. Live data reports the long term and short term trim, I think. Good job, circuit. ---
**Author:** ![oblivion](https://avatars.discourse-cdn.com/v4/letter/o/3ab097/32.png) [@oblivion](https://community.cartalk.com/u/oblivion)\ **Post date:** [August 4, 2011, 10:06am UTC](https://community.cartalk.com/t/fuel-trim/66697/15 "2011-08-04T10:06:36Z")
Fuel trim is the amount the engine computer has to compensate for conditions not being ideal by enriching or leaning the fuel mixture for best combustion. This is necessary to compensate for atmospheric conditions, vacuum leaks, engine wear, fuel pressure not being quite at spec, a cylinder not firing right, etc. The more oxygen sensors the car has, the more precise the computer will be able to control the mixture–if a car only has one sensor, and a cylinder is misfiring, the computer will lean out the fuel mixture based on unburnt fuel being ‘seen’ in the exhaust. Unfortunately, this means that it will be lean for all cylinders, not just the misfiring one, so it will make the engine run worse than just one cylinder not doing its job. Same with a vacuum leak–the computer will try to enrich the mixture, causing the whole engine to suffer. The computer will then see an over-rich mixture and lean it out—lather, rinse, repeat. Ideally, an engine will have very little “long term” fuel trim, and only slight “short term” fuel trim. The short term fuel trim will vary as the engine warms up, the barometric pressure changes, etc. If either long or short term fuel trim is consistently at or near the + or - limit, you likely have an engine problem that needs to be addressed.