Been seeing some folks having problems with multiple misfires where vacuum leaks are a potential concern. Wanted to share one few minute test where you can pretty much rule out all the HVAC and 4WD vacuum components and hoses.

If you have noticeably felt misfires, you can do this with nothing more than a set of needle nose pliers. If the misfires aren't really felt that well, then some assumptions have to be made:

Assumptions are:

1. You have an OBD II dongle and an app such as Torque Pro or another that allows you to see fuel trims and misfire counts (pretty inexpensive device (<$100)). Or even better you have a full blown bi-directional scan tool (prices have really come down for some pretty capable DIY ones (<$500).

2. You believe you have unmetered air (MAF doesn't see) getting into the engine by watching fuel trims and misfire counts improve as you brought idle up to ~2500RPM.

With engine warm, in closed loop and at idle...pinch off (use a tool) the vacuum hose where shown in the pic while watching fuel trims and misfire counts.




If fuel trims and misfire counts don't noticeably improve (give it a minute or so) you can have a good degree of confidence that you don't have a vacuum leak problem in the HVAC, 4WD system or the vacuum tank to include all vacuum hoses and lines for those systems. You can now focus your attention to the left of the pinch (engine, EVAP, brake booster, etc.

If fuel trims and misfire counts do improve, then you know you have unmetered air coming from the HVAC/4WD system/hoses/vacuum tank. You have a direction and can start focusing further on finding the leak(s) in the HVAC and/or 4WD.

With this test you haven't disconnected anything, got a direction, and can move on to further diagnosing.

 

Good idea!