Coolant temperature flucuates

[MNorton]

I purchased a 1994 GMC Sierra C1500. The engine has been replace with a rebuilt 350 from a 1999 suburban. Also the throttle body injection has been replaced with an Edelbrock carburetor. I am in the process of trying to get all of the cluster gauges to work. When I hooked up the temperature gauge it fluctuates just like a sticking thermostat. Warms up but goes to like 220 on the gauge before it seems to open and cools down to 140 or so. Then it just repeats over and over. I replaced the thermostat with a 195 and the upper radiator hose due to the alternator had nicked it. I thought maybe there was an air bubble and there still might be. I do think the head gasket is ok, there is no water in the oil. I thought maybe the gauge was bad so I added an aftermarket water temp gauge and it pretty much tracks the cluster gauge. What do I look at next? Possible bad water pump?

 

[RichLo]

Could have gotten a dud new t-stat, Or the gasket is the wrong size and is interrupting its movement.

 

[454cid]

What water pump and accessories are you using? Between TBI and Vortec, one has an internal coolant bypass and the other doesn't.

 

[udidwht]

Could be a flaky ground wire issue.

 

[Erik the Awful]

What 454cid said. If you used the TBI water pump on the Vortec block, the coolant bypass is missing. You can either reroute the hoses to use the Vortec's on-pump bypass, or you can pull the water pump back off and drill the bypass hole in the block. There's also a bypass hole in the passenger head, but some say you don't need to worry about it.

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[MNorton]

I’m not sure about the heads but I checked and the port on the intake is plugged as is the one on the water pump!

 

[454cid]

I’m not sure about the heads but I checked and the port on the intake is plugged as is the one on the water pump!

It's not the intake, it's the front of the block where the waterpump bolts on.

 

[GoToGuy]

Since we see what you see, unless you post photos, you have to specify what you see. " The intake is plugged".
Uh... Air intake? Water intake? Thermostat housing water outlet? Heater QD fitting, push to connect on rear of intake manifold?
Think we're all blind then go from there.

 

[MNorton]

I finally took a picture. Here you can see the intake is plugged along with the water pump. Dine my engine is now a 99 suburban 350 I guess it had vortex heads. Should it be set up this way?

 

[udidwht]

I finally took a picture. Here you can see the intake is plugged along with the water pump. Dine my engine is now a 99 suburban 350 I guess it had vortex heads. Should it be set up this way?

That's your CTS to the right just below the wire harness with yellow & black/white wiring to it? Check to make sure the connection is solid.