Engine Performance problem
1994 Mercury Marquis V8 Front Wheel Drive Automatic 120000 miles
I am trying to diagnose a idling problem on a 1994 Mercury Grand Marquis. The car operates smoothly with no performance issues except at idle. At low rpm there is a whistle / howl from the rear of the engine. My first repair was to replace the Intake Air Control Valve but that didn't stop the noise. A matter of fact it added a second problem, the idle speed now slowly oscilates between 500-1000 rpm every 5 seconds. Still no performance issues.
When I pinch and hold the hose to the EGR valve at the intake manifold, the idle becomes steady and the noise goes away. There is a vacuum on the EGR side of that connection too.
Is there an issue with the EGR valve and its controls o the IAC Valvle and its controls?
You have a severe vacuum leak somewhere. The best way to find this is smoke testing but you can also try using a can of carb cleaner and spray a little in suspect areas and listen for engine change to indicate a leak.
There is a rubber " T" fitting at the far rear of the engine that is a very common source of leaks.