Car shudders in N or P with engine running...

[taylormah]

Whenever I leave the engine idling in N or P gear, my 1996 escort starts having this problem with it. It starts taking these deep 'breaths' followed by a shudder, as if the engine is going to stall just now, and sometimes it does stall. It feels like suddenly somebody is strangulating it, and it is struggling to get air and sometimes it dies out. Can you speculate what is wrong with it??
BTW, I checked the air filter, and its not that dirty, however in the air filter compartment, I noticed there was this rectangular black foam in the corner which had greasy dirt on it. What is it anyway? I dont know what is that, but I took it out, blew some air on it and slid it back in place, but problem still persists.

[BroncoJoe19]

Check and clean your IAC
Ford Fuel Injection » Idle Air Bypass (IAB)

Checking for vacuum leaks.
Some of the sensors and some of the actuators either read the amount of vacuum, or use vacuum to move controls, therefore it is important to the proper functioning of the engine that your vacuum system is fully intact. Otherwise one may get codes falsely indicating that there is a problem with a controller/actuator, or a sensor when in reality it is caused by a lack of vacuum.

Look at your rubber vacuum hoses (they are about 1/4 - 3/8 inches in diameter, if they are dry rotted or cracked, replace them. IF they slip on/off of their connectors too easily, they may have stretched out a little bit, cut the ends off, and put them back on.

You may listen for a leak. Sometimes using a cut off piece of garden hose is a good aid to listen with. Sometimes a cardboard tube (like from gift wrapping paper) is helpful. Just remember that whenever you stick your head inside a running engine compartment, that you are not wearing any loose clothing or jewelry that can get caught.

Another suggestion is to use a spray can of carb cleaner and spray around the bottom of the intake manifold, and the base of the throttle body assembly. IF there is a leak, you should hear a change in your engine.
Keep a fire extinguisher handy, or use water instead of carb cleaner (it works, but not as well.) Be aware that if you use water and the wires get wet and it runs worse... your wires may be bad, and the water is allowing them to arc instead of jump the gap of the spark plug.

Another suggestion is: to take a propane torch, turn it on (but don't light it) and move it over the vacuum hoses and connections. The engine RPM's will increase if there is a leak and the propane gets sucked in.

Courtesy of TheOldWizard ford-forums... Don't forget to remind folks to check the PCV valve, hose and grommet !

Courtesy Ramnasal ford-forums... Clamp off the hose that runs to the vacuum booster (brake booster) or pull it and plug it. It is possible that the diaphram on the inside of the booster may go bad and cause a vacuum leak.