Temp sensor changed, new problem now

So I got a new sender, wired it all in, let the goop dry for sealing the threads. I went to start the truck up and let it get up to tempature and the guage will climb into the red. The scan tool reads 194-205 the entire time. I verifed it with a hand held temp gun.

I filled the system back up, put the cap back on, and no pressure in the upper hose. The hose gets hot but never gets hard like it's full of fluid. I even tried it with the cap off and nada. This was during a 30 minute period of letting the truck run. Even when I placed the cap on while things were hot there was still no evidence that the t-stat would open.

I never had a cooling issue before. I lost maybe 1/4 gallon of fluid (maybe 1/2) if that doing the sensor swap. I thought I'd just refill it and off I go but no matter what I do that upper hose doesn't get hard whatsoever.

The cap is 3-4 years old, 16psi. I've never changed the t-stat but I've never had a problem with either one so no reason to. I am stumped on this one. I think the cap might be bad because it isn't building any pressure and I know the system is full of fluid.

Any ideas?

To build up pressure you have to have the cap on when it is cold and let the pressure build up. The temp sounds just right. It doesn't sound like you have a problem.

I think you've got air in the system. It may take several drive cycles to get it out. Make sure to keep the overflow reservior full as each time the motor cools (overnight) it will draw coolant out of the reservior. Be patient and it will work the air out.

So that sender went out as my cooling gauge stopped working. I changed it with one from the parts store and the gauge now works.......it's just ahead of what the scan tool is telling me the temp is. Say the tool says it's 195 (which is normal) the gauge reads 260 and flirting with making the service engine soon light come on.

I know the sensor works, but I am starting to think it is the wrong sensor as if the values were different say for a different engine......it'd feed my gauge a different value. It was the correct part # but I dunno........it seems too weird. All the gauges worked fine before this happened. Now just the coolant gauge is off.

If I had air in the system would it cause the gauge to read funny? I don't know exactly how much air would be in the system but I guess I can keep trying to let the air cycle through the system and take from the over-flow and watch it. I know I have lost some coolant and have replaced some but it seems weird. I'm almost at my ending point with this.

Any other ideas? In other words the gauge does work, it reads now, just reads the wrong temp vs. what the scan tool is reading. I've verified it with a temp gun and it's right with the scan tool. So I know the scan tool is right and the gauge is wrong.

What year is this truck? On the TBIs (88 - 95) the temp sensor for the gauge is in the drivers side head. The sensor in the intake is for the computer. I believe the same is true for the vortec engines.

Which one did you change?

//2000CAYukon

I filled the system back up, put the cap back on, and no pressure in the upper hose. The hose gets hot but never gets hard like it's full of fluid.

This is the phrase from your 1st post that leads me to believe there is air in the system. As to if the correct sensor was replaced?? If there is air around the sensor, and not coolant, it's going to read a higher temp.

One other thing is the "goop" you put on the sensor threads. Sometimes this will not allow the sensor to see a good ground at the engine for a good reading to the guage.

What year is this truck? On the TBIs (88 - 95) the temp sensor for the gauge is in the drivers side head. The sensor in the intake is for the computer. I believe the same is true for the vortec engines.

Which one did you change?

//2000CAYukon

The one in the head, it's a 91 k1500 pickup with the 350.

I had bought one from Kragen autoparts for $10.99 and it's almost 50 degrees off from what the computer says. I have on order for tomorrow a new Delphi one (GM parts) that was like 3 times the cost but hopefully it will rectify my problem.

This is the phrase from your 1st post that leads me to believe there is air in the system. As to if the correct sensor was replaced?? If there is air around the sensor, and not coolant, it's going to read a higher temp.

I now have fluid in the system. There was air in there initially but I managed to get that out. The over-flow tank is always hot when hot and cold when cold, plus the radiator is filled to the top so I know there is no air in the system. I replaced about a half gallon (which is what I had leak out) + a little more to count for the drips and drops that spewed all over the place. I let it run, get pressureized, and let it sit overnight, refill next day, same thing, and today it was back to normal. I can see when the T-stat opens as the gauge moves, but again it's like 50 degree's off of what it should be.

Well good news guys!

I had a faulty temp sensor from Kragen. I bought a $30 Delphi one (GM part) and lone behold it works fine. My computer says it's 195.......and the gauge says it's 210 (so a little off still) but I have a 180 t-stat to put in this weekend. I want to do that just for a piece of mind :)

I installed the new one no different than the old one. A little teflon tape for sealing and screwed it in. I ran the motor and when the T-stat opened it read right smack in the middle. I figured though with a lower T-stat it would read below the half way point and I wouldn't suffer too much in any engine related field.

Thanks for all your help though guys, it did lead me in the right direction. Now I just have to deal with Kragen on getting refunded on a sensor that's faulty.