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2000 5.9 cummins- what would cause spike in temp from 186 to 210 for up to 10-15 seconds then back down, average temp is around 194? Doesn't matter if it's at idle or driving down the road. Any input would help, thanks
 

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It could be one of two things. Air in the system or faulty/sluggish thermostat. Have you done any work to the cooling system lately that would allow air into the system such as replace or remove water pump or replace a radiator hose? Is the radiator completely full of water? If you haven't done anything to allow air into the system then maybe replace the thermostat and see if that fixes it.
 

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Weak/old radiator cap can be all sorts of fun! I like to start off with a system pressure test including the cap.
 

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Sounds like the fluctuations are just the thermostat doing its job possibly, my rides with digital readouts do just this as the thermo opens and closes.
 

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a stant thermostat perhaps? if you look at a cardone, and I know coolant specialists who sware by cardone, they have a 1/8th bypass hole in the stat flange. I'll explane why. the thermal coupling needs to saturate with heat, top and bottom before it opens. the stant stats work and yours sounds like its active. I always drill mine it they don't have a hole. this also helps burp the flawed system of air.
 

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I planned on it. I just wanted to test temps with Prime95 to compare it with stock on the same curve. I didn't see (or hear) these spikes at stock idle, so it worries me and wanted to check on it before going any further.
snaptube vidmate
 

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prime95 whodat !
 

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for stant thermostats yes this is normal. don't get me wrong, I don't like it either. and there are ways to mitigate this, "BURP", action.. MANY threads have discussed this and the facts that a higher than normal heater core and stat housing tend to trap air. high spots are engineered improperly into the rams system. HAVE you ever been told or read to park with the nose Uphill to top off the coolant? HA! funny
 
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