Hi, I have a 99Z28/LS1 that just recently developed an issue. My car has 125,000 miles on it and it threw the P0405 code which is something about the EGR valve and voltage. At the same time this happened, my car has started to run hot. It would always warm up to 210 on the gauge and stay there, but at the same time the EGR issue happend, it would go to 210 for awhile, but then (usually when on the freeway) start creeping higher and fluctation some between 230-250 and never going back to 210.

I disconnected the EGR altogether but the temps still rise the same way.
Any ideas ? I thought maybe the EGR is sticking open even when closed, but I haven't really noticed any kind of rough idle/running conditions. Just the temps.

I thought the cooling system thermostat may have stuck closed, but that would be ruled out by the car heating to 210 and staying there awhile before creeping past that (usually takes 10 min at 210 before it goes over)

Thanks for any insight.
Car is pretty much stock as far as upgrades.

 

Also, this is happening under all conditions, even in cool temps (60degrees) with the ac off.

 

I replaced the thermostat last night and that fixed it.