![]() If the value has come back down into a more normal range, it confirms the engine has a vacuum leak because a vacuum leak has more of an impact at idle than at higher speeds. Rev the engine and hold it at 1,500 to 2,000 rpm and look at fuel trim again. If fuel trim is running 10 or above, the engine is lean. Normal fuel trim should usually be plus or minus 8. You can also plug a scan tool into the data link connector (DLC) and look at fuel trim. It may be necessary to remove, inspect and clean the EGR valve. No vacuum leaks? It’s possible the EGR valve is not closing due to a buildup of carbon around the base of the valve. You should also inspect all the vacuum hose connections, especially the PCV and EGR plumbing, and the large vacuum hose to the brake booster. Or, you can shut off the engine and use a smoke machine to check the intake system for leaks. You can spray carb cleaner or propane at suspected leak points while the engine is running to see if it changes the idle. Plastic intake manifolds with upper and lower plenums are notorious for developing leaks. If you hear any high-pitched whistling noises, the engine may be sucking air though a leaky hose or intake manifold gasket. The first thing you do with a P0300 code is to look and listen for possible vacuum leaks while the engine is idling. But as we just said, it might also be due to low fuel pressure, dirty fuel injectors, worn or fouled spark plugs, bad plug wires or a weak spark (low coil voltage). The cause could be fuel, ignition or compression, so to diagnose this fault there are number of things you have to check.Ī random misfire code means multiple cylinders are misfiring, and in many cases the underlying cause is a vacuum leak. P0300 Random Misfire Code (Ford, GM or Chrysler) Here’s a code that tells you the engine is misfiring, but it won’t tell you why. Keep that in mind as you read about the codes below and the fixes that may solve these faults. But no code by itself can tell you which part to replace, because in many instances any one of several causes may have set the code. A diagnostic trouble code (DTC) certainly helps, but it only tells you that some kind of fault has occurred, and maybe the circuit or component associated with the fault. Some people think a code is all you need to fix a fault and turn off the Check Engine Light (or Malfunction Indicator Lamp or MIL).
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