A vacuum leak usually results in higher idle speed than normal. This is because air is leaking into the low pressure intake manifold and bypassing the mass airflow (MAF) sensor. The MAF sensor feeds the mass airflow rate to the ECM. However, with an intake air leak, the ECM is calculating fuel injection quantities that are too low for the actual air entering the engine. The engine will run lean if the feed back fuel control system cannot adapt to sufficiently account for the air leak. You may also see fuel system lean codes stored in the ECM.
Vacuum leaks are usually found with a visual inspection. The most common vacuum leak is a disconnected vacuum hose.
On the other hand, if your car is experiencing a lower idle speed than normal and/or stalling, then the problem usually lies with the idle air control valve. It may need cleaning and there is much written here and on the net to help you with that.
Hope this helps,
Tom
I took the Explorer (2000 V8 5.0 AWD) to AutoZone to check the error code.
> (My vehicle has problems starting when the engine is cold.)
>
> The error code that came back was P1507.
>
> I checked the internet, and this info came up:
>
> QUOTE
>
> Idle Air Control Underspeed Error
> A vacuum leak, a damaged IAC assembly, a stuck IAC valve or throttle valve
> typically sets one or both of these codes. The IAC can be checked with a
> scan tool, sensor tester or multimeter. A vacuum leak is one of the most
> common causes and can be checked with a vacuum pump.
>
> END QUOTE
>
> How do I test for a vacuum leak, and with what tools?
>
> Thanks