Truck Repair: 1994 Jimmy Surging Idle, mass airflow sensor, air control valve

Question
My daugheter in-law has a 1994 Jimmy 4x4 with the V6 Vortex engine with 80k miles.  Last year it started what I'd call surging at idle: RMP's dropping low,  nearly dying, then recovering - repeating  about every 2 seconds.  Then at one point last year it died and would not start.  The car was towed, an  analysis pointed to a bad fuel pump, and it was replaced. But since then the surging still comes back every couple of months - the check engine light also comes on.  Borrowing a code read  son was determined it was a faultly EGR valve.  It was replaced and ran better but then in a month or 2 later the surging idle returns.  He used to fix it by removing and cleaning the EGR valve - a real pain every month or two.  Lately he has been "fixing" it by pouring 1/2 can of Seafoam into the engine through the PVC valve while the engine is running - generally with good luck.  His assumption is that this is clearing out carbon from the EGR valve.  But last night the surging again returned without any improvement by using Seafoam.  I read about cleaning the throttle plate and idle air by-pass hole - we  need to try this.  I also read that the engine goes into a "limp" mode when the check engine light comes on - is this correct?  If so then the problem could be anything and only an engine analysis would point to the probably issue (?).  IF it is carbon in the EGR, why this happening and how can it be cured once and for all?

Answer
Hi Mark,
If cleaning the egr fixes it, then the carbon is probably the problem, and the only fix is getting all of it out, and preventing it from forming again.
There used to be a system of injecting water to knock the carbon loose, and hopefully out.
Do a Google search for "water decarbonization".

Cleaning the idle air control valve and port is a good ides.

Then there is the temperature sensor, and the Mass airflow sensor that could be cleaned and checked.

http://www.chevytalk.org/threads/showthreaded.php?Cat=0&Number=1366279 for the temp sensor info...very interesting.

And then
http://www.fordscorpio.co.uk/cleanmaf.htm
for info on the mass airflow sensor. It is a Ford link, but they work the same.

Van