Temp drops when thermostat opens?

[servion]

2002 F350 7.3, 145k. Running CAT ELC and a 203* tstat.

Its starting to get cold around here, and the truck sat overnight and all day without being started. This is the coldest its been that I've used the truck since I've owned it (only owned it for 3000 miles).

I plugged the block heater in about 2 hours before starting it this evening, and I fire it up and it starts right up (probably 20* out). After running for 3-5 minutes, I get outside and in the truck and the stock temp gauge is at full operating temperature. I thought this was weird, because its been taking forever for this thing to get up to temp since its been getting colder out - usually at least 5 minutes of idling and then 5 miles of driving.

I could hear the EBPV still closed while idling. I hopped in and started driving, and I can still hear the EBPV closed at light-to-no throttle - as soon as it gets over a little bit of throttle, it opens up, but as soon as I go back to light-to-no throttle, it closes and I can hear the hissing again - all while the temp gauge is still up to full operating temp.

After driving maybe ~5 miles (the usual time to get it warmed up fully), I look down at the temp gauge and it has moved DOWN to about 1/2 temp. It wasn't until after the engine started to reach full operating temp again that the EBPV hiss went away completely.

What's the deal, any thoughts? This is the first time I've used the block heater, so my theory is that the block heater was very effective in warming up the coolant in the block, so it showed full temp right away... then the thermostat opened, and all that coooold coolant rushed from the radiator and into the block, causing it to register less than full temp? Not sure if that makes sense though. Also, is the EBPV operation based on the EOT or the ECT (or both)? I thought it was weird that the EBPV stayed closed when the stock ECT gauge was at full temp, unless the block heater only heats the coolant in the block and not the oil in the pan, and the EBPV operates based on the EOT?

[Zaairman]

EBPV is based off of EOT, and the stock temp gauge is about as useful as tits on a bull. Your EBPV is operating normally for a short in town drive. Now, if it was still kicking on during an hour long highway drive, you'd have some issues. If the truck won't warm up in the cold while driving, it might be time for a new thermostat.

[servion]

I did a complete coolant flush ~1000 miles ago. Put in a new dieselsite 203* tstat and billet housing. I pulled it out (to fix a leak) and verified that it is in a closed state, but it able to be held open by hand.

So, I haven't seen where the block heater actually warms the block, but am I correct in assuming that it would heat the coolant in the block, but not the oil in the pan, hence the warm dummy gauge with closed EBPV?

Does anyone else's dummy gauge go up quickly after using the block heater, but drop back down later on (I'm assuming when the thermostat opens)?

[Zaairman]

On warm days, I've noticed that my temp gauged will be moved by the block heater. Haven't really seen it move like you're describing, but I also have no idea how old my thermostat is.

[Lilfarmrboy]

I had a 2002 7.3 and I had a 2004 F150 that I changed out the Gold Coolant to the Rotella ELC coolant and in both trucks I would notice that if I let them set and idle in the yard they would get up to normal operating temp within 5 minutes and then after I got down the road the temp gauge would drop until about 5 miles of driving. I never did figure out if it was the different coolant doing it or what but it never seemed to affect my truck. As far as the block heater I'm not sure what it heats up but the 7.3 would be blowing hot air out of the ducts within 3 min if let set to idle. I don't have either truck now, I just bought a 2005 Crew Cab dually, mainly wanted to avoid the pollution controls on the newer trucks.