Kia Troubleshooting Questions? Ask a Mechanic
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When there is a very high POSITIVE fuel trim as you have indicated, it is usually a sign of a vacuum leak. It is important to remember that the fuel trim is a result of the engine computer's interpretation of the oxygen sensor information. In other words, if the oxygen sensor detects that there is not enough oxygen content in the exhaust, the computer determines based on this that there is either too much fuel or not enough air, and it does the only thing it can to control it: reduce fuel injector pulse width to compensate. The same is true in reverse if there is too much oxygen content in the exhaust, the computer compensates by leaving the fuel injectors open a little longer to add fuel. The problem with this is that usually, it is not a problem of not too much fuel, but too much air (*hence the vacuum leak assertion*) which cannot be accounted for by adding fuel.
*in any event, you should start by checking for a vacuum leak. My favorite method for doing this is using a can of FLAMMABLE brake cleaner while monitoring literal oxygen sensor wave forms to look for changes when I cover a particular area.
Another quick check that you can do even before the vacuum leak check is to look at the coolant temperature pid and see if it seems implausibly low, which can also cause a condition in which the computer compensates for perceived low temperature by running richer than normal.
I have never tried this method, but it is conceivable that if you maintain partially open throttle for long enough, it could rule out a vacuum leak if the fuel trim were to change, but that seems like a stop-gap method of checking for a leak and not very accurate. Are you able to try the two things that I have suggested?
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