At idle the engine rpm fluctuates and surges from, lows of 300 to highs close to 2000 and all points in between. I am at a loss to understand what’s causing this and how to make it go away. After anywhere from 5 to 20 minutes of idling or driving the engine will die out and has to sit for about two minutes before restarting. When the idle stays somewhat steady there is an audible popping sound from the exhaust, like tiny backfires. I checked the fuel pressure and it stays about 36psi. I cleaned the throttle plates and changed the IAC because the old one was coked up. I cleaned the cap, rotor and checked the wires for corrosion. It seems almost as if the computer is retarding and advancing the engine speed in some sort of juggling act. Which I understand is how the computer controls the engine however, not this radically. Also when it is started and when I rev the engine there is a good size gray/ black cloud of smoke that comes out of the exhaust.
Has anybody had this type of problem?

 

people have this problem all the time. if the IAC was that bad, then you need to clean the passages in the throttle body as well. autozone sells a gasket kit for this that is 8 or 10 bucks. then do the minimum air procedure .https://www.thirdgen.org/newdesign/tech/tpimod2.shtml

it could also be a vacuum leak somewhere. have you done any work to the engine at the same time the symptoms started? these are simple things to check. you might check distributor cap/plugs/wires/coil to make sure these are functioning properly. also a faulty oxygen sensor can cause these symptoms.

 

I checked for vacuum leaks using the spay water technique and didn’t find anything. I will do the minimum air procedure and double check the rest of those items.

 

Most hunt for idle issues is an issue with the IAC and/or a vacuum leak. When checking for a vacuum leak, you can use a propane torch, move the tip around to all joints, if the RPM increases suddenly, you have found your vacuum leak.

Next, think back, what was done to the engine or what was changed prior to the hunt for idle problem showed up.

A good general tune up never hurt anything, plugs, plug wires, cap and rotor. Also change the fuel filter and run fuel injector cleaner thru the fuel system (and do that every 3,000 miles).

Then complete the minimum air routine and set the static timing at 6 to 8 degrees. Remember to disconnect the EST wire to set static timing.

Clean the MAF sensor (if you have one) with brake cleaner, be carefull not to be too aggressive and break a wire.

Check the TPS with a volt meter. As you swing the TPS thru full arc, does the voltage increase accordingly and smoothly.

 

Smoke = rich. The surging is just a symptom of the problem. Check your fuel system and your pressure regulator, make sure the diaphragm isnt ruptured and its working properly....make sure engine holds fuel pressure and the injectors arent leaking down on shutdown. Also check for a weak ignition system.

This is a VERY VERY common Mustang problem and I've dealt with it many many times and 90% of the time it's a fuel delivery issue on our cars. Other common causes, like people have mentioned, are vacuum leak, bad TPS reading, bad MAF......