Coolant temperature fluctuations

[66gmcguy]

Hey all. I have an 82 K2500 with a newer vortec 350 from Blueprint engines (around 4000 miles on it by now), and just today started getting massive temperature fluctuations. It only happens when I’m cruising above 50mph or so. It will swing from 200-240 then back down to 190ish in a matter of seconds, and it will do it the whole time I’m cruising. It doesn’t do it if I’m just driving around town or idling. The sending unit is in the drivers side head. Now here’s where I’m stumped. I have a holley efi system and it shows you the coolant temperature on the display. Whenever the temperature swings hot on the gauge, the EFI display simultaneous shows a temperature DROP of only about 15 degrees (195-180), and once the gauge (in the cluster) hits about 240, it will start to drop again, while the temperature on the EFI display climbs back up to 195. The sending unit for the efi system is in the intake just below the thermostat.

Everything on this engine is new. I have a griffin radiator with electric fans, high flow water pump, new 195 high flow thermostat (with bypass), edelbrock aluminum intake, and I’m using the heater core (which is also brand new) as a coolant bypass. Is it possible that the thermostat is sticky (but not stuck) since the temperature is fluctuating around the opening/closing point of the thermostat? The coolant level is completely full.
Posted via Mobile Device

[Pontiac1976]

On the highway coming to low speeds you will get heat soak for a while since the air flow not the same under 40MPH. If you been running

I have a Holley EFI system and it shows you the coolant temperature on the display

the sensor maybe a little more tight for the reading then the factory gauge, but have you had the Holley system long enough to find out all the normal reading of all types driving you do yet most of the speed you do highway and city.
the temp sensor not in the closes of both means the reading are going to be off 4 to 10 DEG's on avg. It could be the factory gauge sensor gone bad or connector check first. But if they seam to to be the same temp on the factory gauge as before about then that the normal temps that all the gauges are going to run. What normal temps was the old gauges old parts running, that made you change out the old parts. If you run 4 season and all the same driving types on a Avg. year highway and city. If the new display is working well then it seem to ok. But heat sensor gun and a test drive a few temp pull from it @ the sensor as soon as you can safely stop and pop the hood and the rad top and bottom hoses under the same type heat up as you can and see if the temp are close if the gun read a lot higher or a lot lower then may be the sensor or the thermostat not working right or you have the wrong rating for your set up now the you got a better rad.
Two many changes makes it hard find out the base temps driving habits.( some of the hi-flow thermostat are cheap) Maybe just a regulator thermostat since you have a electric fan and high flow water pump. Check your sensor to the electric fan to see what it cut in and cut out is set up for. You may need to tweak that some. Now back to (some of the hi-flow thermostat are cheap have bypass) they made mostly for the brass radiator early factory early alum plastic rad of the 88-93 radiator. But since you have better fan and rad finetune the temp at what it turns on/off the fans, 195 thermostat hold the temp there but if the motor hotter and the rad sending cooler water before the stat cane open it by passing because motor heat.

[66gmcguy]

Originally Posted by Pontiac1976

On the highway coming to low speeds you will get heat soak for a while since the air flow not the same under 40MPH. If you been running the sensor maybe a little more tight for the reading then the factory gauge, but have you had the Holley system long enough to find out all the normal reading of all types driving you do yet most of the speed you do highway and city.
the temp sensor not in the closes of both means the reading are going to be off 4 to 10 DEG's on avg. It could be the factory gauge sensor gone bad or connector check first. But if they seam to to be the same temp on the factory gauge as before about then that the normal temps that all the gauges are going to run. What normal temps was the old gauges old parts running, that made you change out the old parts. If you run 4 season and all the same driving types on a Avg. year highway and city. If the new display is working well then it seem to ok. But heat sensor gun and a test drive a few temp pull from it @ the sensor as soon as you can safely stop and pop the hood and the rad top and bottom hoses under the same type heat up as you can and see if the temp are close if the gun read a lot higher or a lot lower then may be the sensor or the thermostat not working right or you have the wrong rating for your set up now the you got a better rad.
Two many changes makes it hard find out the base temps driving habits.( some of the hi-flow thermostat are cheap) Maybe just a regulator thermostat since you have a electric fan and high flow water pump. Check your sensor to the electric fan to see what it cut in and cut out is set up for. You may need to tweak that some. Now back to (some of the hi-flow thermostat are cheap have bypass) they made mostly for the brass radiator early factory early alum plastic rad of the 88-93 radiator. But since you have better fan and rad finetune the temp at what it turns on/off the fans, 195 thermostat hold the temp there but if the motor hotter and the rad sending cooler water before the stat cane open it by passing because motor heat.

I have the fans programmed to come on at 200 degrees and off at 195. The EFI system controls them, so they never came on at all during any of this, since the Holley CTS never read above 195. And I’ve had this entire setup for months and never seen any of this happen til today. I’m going to rule out gauges or sending units since I effectively have two gauges that starting showing anomalies at exactly the same time that seem to correspond with one another. I do have a sending unit for a different gauge in the passenger side head which I might hook up just to check, but I can pretty much say with certainty that it will show the same type of fluctuation.
Posted via Mobile Device

[Pontiac1976]

First have you did this drive before under the same speed as possible, what air temps was it out side this day. It looks like from the temp set the fans turning off a little soon I think it should be 170 or 165, But I would do some checking in the Holley EFI board to see what fan ranges with a 195 regulator thermostat not a high flow bypass type thermostat