Here is something that has VERY confused, here is the info first:
2004 Silverado 1500 4x4 Z71 5.3L 240,000 miles

Yesterday, went out of town to visit family 6 hours away, I had it on cruise control at 75. About 4 hours into the trip, the engine like "stalls", but it did like a fast stop and start. I started looking at my gauges and notice that my temperature was below 210.

I started watching it and noticed it would go up and down and when it would hit 160, the engine would like "restart" and the temperature would shoot back up to normal at 210.

So then every time I would see it go down, i would let go of the gas and it would crawl up to 210 again.

Has anyone seen this before? water pump AND thermostat were replaced 2 years ago when it had 192,000 miles. The truck is not overheating. The temperature was 41 degrees outside, could it be that the fan clutch won't turn off? water pump not flowing water?

I've never seen this before, I am at a loss here. I have included the pictures when the temp started to go down and another back to normal.

 

Your Thermostat is sticking.

Just because it was replaced a couple years ago doesn't mean it won't malfunction...Also looks like the wrong one was installed.

Should be a 195* degree thermostat and if your gauges are reading correct it looks like you got a 205* thermostat...or it is sticking closed and sticking open.

The 40* ambient temps at freeway speeds will reveal any flaw with the Stat trying to do it's job.

Additionally,the Computer is "seeing" these temp fluctuations and probably going from closed loop to open loop repeatably and freaking out trying to stay running correctly.

 

Thanks for the info, I will replace it, this makes sense.

I looked at the thermostats and see two different ones:
1. OE recommended is 187 degrees AND
2. Alternate Temperature 160 Degrees Fahrenheit

Which one of these do I need? The 187?

 

187 lower temp thermostats are a joke and hurt more than they help

 

where can i get longer bolts? the threads in the beginning are gone, but towards the back, they still grip, but i need longer bolts, any recommendations? or do i need a "tap" thread maker?

 

First thing I would check is the coolant level. I had a similar problem and I was low on coolant.( I have the same truck)
As far as the threads...............if a longer bolt wont work, then you need a heli-coil kit. They work great.

 

Went through three thermostats from local GM dealer before i got one that worked properly in our 06 tahoe. I alway use a 195' - never had a 205 that lasted very long.

 

OK, got a helicoil and fixed the threads. new thermostat is in, drove around town and got on the highway for about 15 miles. Temperature did not budge from 195. I will travel back Wednesday, a 6 hour trip, I will report if all went good or not.

 

I don't know if it is possible on newer vehicles, but it was not uncommon for drivers of 1970s GM vehicles to install the termostat upside-down. I doubt that is your problem, but it might be worth investigation.

More likely than not, you do have some kind of thermostat issue.

 

It did it again, after about 5 hours, i saw it temperatures drop and engine would stall at the 160, here is an image of all the gauges.