Chevrolet Repair: 91 Blazer Rough cold start, coolant temp, vacuum leak

Question
First thanks for your time...  Anyway  I have a 91 blazer 4.3 tbi only 30k on it.. when i start my truck after sitting for awhile the idle lopes, today it cut off on me and I had some difficulty getting to stay started. Always though after a couple minutes of warm up it runs like a dream.  Smooth as can be, it has never cut off or even the slightest lope any other time except when first starting. Its just in the initial start up. At first I thought it was due to the real cold weather up here in Ohio but after it kept doing it I realized there must be more to it. i was thinking EGR but just not sure. I know I'm due for a tune up esp fuel filter.  i think everything else on it except PCV is factory.  Any help would be greatly appreciated  

Answer
Hi Anthony,

Only 30K, what a nice truck.  I had a '93 4x4 and loved it.  Easy to park, ran great 99% of the time.  It rusted out though (Michigan), and had an annoying squeak from the spare tire mounted outside on the back.  That drove me nuts.

Anyway, your question.  Tune ups never hurt.  Only 30K, but 15 years.  Other than that, idle is controlled by an idle control motor.  But that puts a code and will act up all the time.  It gets its signal from somewhere though...the coolant temp sensor.

The reason why it runs great after warm up is the O2 sensor.  This can 'tune in' the system to run right.  It also gets a signal from the coolant temp sensor.  It considers it's input (opinion) and decides if it wants to use it or not.  

With 30K on the truck, the electronic parts should be good.  Doesn't mean they are.  Coolant sensor should also put a code.  But, not always.  Also if the coolant is low, the coolant sensor can give bad information.  It is not in the liquid it trying to sense, so the temp range is off.  Without the O2 input (at cold start up), the computer does the best it can.  

However with 15 years of mostly sitting, things dry up.  Things like gaskets.  Intake can cause vacuum leak.  Too much air can cause erratic cold idle.  This can be corrected by the O2.  

If the coolant level is up in the radiator, then it is best to get the O2 read by a scanner.  I will tell you a lot about why it is able to make it idle right, warmed up.  This will lead you to pinpoint the problem.  On the older trucks, this cannot always be done.  That is when you have to look for air leaks, then tune up.  Then look at sensors.  Think of older cars.  The idle control motor is the old idle cam on the carburator, the fuel pressure relay is the old float, and the coolant sensor is the choke.  

Hope this helps.

C J S