Ford Repair: cold engine, temperature gauge, cold engine

Question
My engine does not heat up. It stays below the Cold outside of the lines for Normal range for quite a while (about 20 minutes). I had a new cylinder head, thermostat,  serpentin (sp) belt, water pump, and hoses put on in May. (It had gotten too hot and cracked the head.) Six weeks ago I had to replace the thermostat again. They said the first one was defective. (Two different shops.)I recently tried a fuel filter, adding a gas treatment, and using the highest grade gas for a tank as it rattled some too, but that was not it. I read something about a PCV valve I think it was. Oh, after the 20 minutes, it is just barely over the line into the Normal range. Once it gets warm enough runs fine, but just takes way too long even for a 1992 Ranger with 216,000 miles on it!

Answer
The only thing that can cause an engine to take too long to warm-up is a bad thermostat.  The thermostat holds the coolant in the engine, and doesn't allow it into the radiator to be cooled.  The temperature at which the coolant is released is determined by the thermostat.  In cold weather it is normal for the temperature gauge to read on the low side of normal, and it usually takes about 15 minutes of idling to reach normal operating temperature.

I hope this helps,
Dave