Thermostat, Coolant temp. sensor, other?

I have a '99.5 Jetta GL with a 2001 engine (long story). When the engine was replaced in the spring of '07, the mechanic replacing the engine recommended installing a new thermostat. He used a NAPA thermostat. About 6 months after the engine replacement, my engine temperature started bouncing around. I replaced the coolant temperature sensor (CTS) with a new blue one from TDIParts.com. The temperature ran steady at the 190 deg. mark until about a month ago. The temperature began topping out at one hash mark before the 190 deg. mark. It consistently topped out at this same point and after doing some forum searches, I felt confident it was the thermostat. I replaced the thermostat with a Wahler from TDIParts.com. Now the temperature is consistently running at two hash marks below the 190 deg. mark or the first hash mark past the 3 close ones. I measured the termperature in the coolant reservoir with a meat QA meat thermometer and it read 182 deg after a 10 mile drive. The consistent temperature reading makes me think it is not the CTS. Any opinions, suggestions, etc.? By the way, my fuel economy has dropped about 3 mpg since the operating temperature change.

Are your fans running constantly?

No, the fans are not running continuous.

The thermostat is a little tricky. Of the couple I've changed, I broke the housing popping it out. If the tabs on the housing are broken, the thermostat may not be sealing properly, resulting it in running cool.

You may be on to something here. The tabs were broken on my old housing, but still partially attached. I did buy a new housing when I purchased my new thermostat. I bought it in case I broke the neck of the original one while removing the hose. I read a few threads that said it wasn't necessary for the tabs to lock into the thermostat. When I removed the bad thermostat, it was not inserted into the tabs. Bottom line, I did not lock the new thermostat in the tabs. Is it necessary? If so, what is the best method to lock it in place? Can you lock it in place in the housing, with the o-ring between the thermostat and housing and then bolt in place or do you have to install the thermostat in the engine, then the o-ring, and then rotate the housing 90 deg. and lock it into the thermostat before you bolt it down?

The tabs aid in alignment of the thermostat for installation purposes. Put the thermostat and seal on the housing prior to putting it on the car. I would give a new housing a shot if you have one.

aNUT said:

The tabs aid in alignment of the thermostat for installation purposes. Put the thermostat and seal on the housing prior to putting it on the car. I would give a new housing a shot if you have one.

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I removed the thermostat again last night and there is now way the thermostat will lock into the tabs on the new housing. The tabs touch the top disk on the thermostat. I wish I had a camera. A photo would have explained better. From what I saw, there is no other way to install the thermostat than place it in the block with the top cross bar positioned 90 deg. from the housing bolt holes, insert the o-ring to hold the thermostat in place, then bolt the housing in place. Same result after re-installation. The temperature is running at the first hash mark past the 3 initial marks. Unless someone posts a different suggestion, I'm replacing the coolant termperature sensor again.

Problem solved. I replaced the coolant temperature sensor - again.