Temperature issue

I got my 98 Z71 a few months ago with 83K on it. Ran fine, no problems what so ever. Then the intake gasket cracked and the mechanic replaced the head gaskets as well as everythink else. Why I don't know.
Anyway, after getting the truck back, my temp gauge would go from cold, to 260, then back down to normal temp around 190, then back up to 250 or so , then back down to normal. I replaced the thermostat and re-filled coolant to proper level and it kind of straightened out, Now when cold, goes from cold, to 245 and then back to normal when its warming up, and then sits at 190 the rest of the time at normal running temp.

Why would it have started this after getting it back from the shop?

Check the connection to the temp sensor in the cylinder head. I also heard (never seen this) that if teflon tape is used (instead of thread sealant) on the sensor, it can mess up the reading.

//2000CAYukon

I've checked everything several times. except for the actual temp sensor itself. I'm going to hook it back up to the computer and see what the actual temp readings are compared to what the gauge shows. It went all the way to 260 today before finally going back down after a few minutes.

Caused by air in the system. Once it all works out everything fine.

My temp was erratic when I had air in there too but never got near
that hot.

Well how long should it take? And is there a way to bleed it out??

It should only take one or two warmups to get it all purged. If your temp is still going to 260 the thermostat could be faulty even though new. What knid of shape is your waterpump in?

It should only take one or two warmups to get it all purged. If your temp is still going to 260 the thermostat could be faulty even though new. What knid of shape is your waterpump in?

I'm going to guess that its in pretty good shape? Only has 87K on it, and I never had this problem until the intake/head gaskets were replaced a few months ago.

So to bleed the system, I need to start when it's cold, remove the cap and let it warm up with the cap off correct?

You need to leave the cap on or you'll end up with a big mess. The air will work it's way back to the radiator, then when the truck cools off top it off. But, if you have been having this issue for awhile now, the air should already be out. I'd then check to be sure that you have the correct thermostat in the thing.

if they changed the head gaskets it might be a good idea to check coolant temp sending unit becuase on my 1994 sierra 5.7 liter my guage went straight up to past 260 even when cold and when i changed out the sending unit it went back down to normal. just a though it might be bad and have wacky readings.

if they changed the head gaskets it might be a good idea to check coolant temp sending unit becuase on my 1994 sierra 5.7 liter my guage went straight up to past 260 even when cold and when i changed out the sending unit it went back down to normal. just a though it might be bad and have wacky readings.

This is the one on the driver side head, correct?

You need to leave the cap on or you'll end up with a big mess. The air will work it's way back to the radiator, then when the truck cools off top it off. But, if you have been having this issue for awhile now, the air should already be out. I'd then check to be sure that you have the correct thermostat in the thing.

I'm positive its a 190*, or whatever is factory. I double checked everything when I installed it. Its installed correctly, new gaskets, etc...

I'm positive its a 190*, or whatever is factory. I double checked everything when I installed it. Its installed correctly, new gaskets, etc...

Do you still have the thermostat you took out? If so, I'd throw it back in to see what happens. Even though you have a "new" one in there now, doesn't really mean it is functioning properly.

yea there is a sending unit on the drivers side right next to the #3 spark plug wire. it could possible be causing the problem but the only way to find out is to get a new one and put it in

Do you still have the thermostat you took out? If so, I'd throw it back in to see what happens. Even though you have a "new" one in there now, doesn't really mean it is functioning properly.

The old one did the same thing. Thats why I replaced it.

Well, now you got me. Sorry I couldn't help further.

I replaced the tstat again and even a new temp sensor.
No dice. I still dunno wtf.

I got my 98 Z71 a few months ago with 83K on it. Ran fine, no problems what so ever. Then the intake gasket cracked and the mechanic replaced the head gaskets as well as everythink else. Why I don't know.
Anyway, after getting the truck back, my temp gauge would go from cold, to 260, then back down to normal temp around 190, then back up to 250 or so , then back down to normal. I replaced the thermostat and re-filled coolant to proper level and it kind of straightened out, Now when cold, goes from cold, to 245 and then back to normal when its warming up, and then sits at 190 the rest of the time at normal running temp.

Why would it have started this after getting it back from the shop?

I don't understand why the head gaskets were changed if all that was happening was an intake gasket leak. I would investigate what gaskets were used, and how were they installed (sealant?). What does everything else include? Water pump, radiator, cap? A truck I had once was plugged up with leaves and trash between the radiator and the A/C condensor. I also used a vortec block with TBI heads in a 90 model truck once, and had temp issues, found there are holes in the deck of the earlier model motors that are not present on the vortec blocks. I never did confirm this to be the problem but I changed everything related to the cooling system and the problem still existed, a helper wreck the truck before the cause was found. There may be differances in the head gaskets from the vortec to older model blocks I can't confirm this. I would like to hear what fixes the problem if and when its repaired. :uhoh:

I replaced the tstat again and even a new temp sensor.
No dice. I still dunno wtf.

I had the same problem with my 97 after I replaced the im gaskets. I replaced the t-stat a couple of times, and bled the system a couple of times no dice.

I ended up drilling a tiny hole in the t-stat, like 1/16 or 1/32. Just enough to let the air out, but not enough to bypass the t-stat too much, or the engine will never warm up.

It ended up solving the problem (or at least bandaiding it). My temp gauge now comes up to temp, and stays rock solid.

I would say there is a small leak somewhere that's still letting a touch of air back in the system when the truck cools down? I don't know I'm just guessing, I stopped troubleshooting once I got the temp stable.

I had the same problem with my 97 after I replaced the im gaskets. I replaced the t-stat a couple of times, and bled the system a couple of times no dice.

I ended up drilling a tiny hole in the t-stat, like 1/16 or 1/32. Just enough to let the air out, but not enough to bypass the t-stat too much, or the engine will never warm up.

It ended up solving the problem (or at least bandaiding it). My temp gauge now comes up to temp, and stays rock solid.

I would say there is a small leak somewhere that's still letting a touch of air back in the system when the truck cools down? I don't know I'm just guessing, I stopped troubleshooting once I got the temp stable.

in what area of the t-stat did you drill the hole? I'll give it a shot before I put it back in the shop.

I dropped it off at the shop yesterday while I went to pick up a new Hemi Charger I gotta equip, and this morning my mechanic calls me and says he started it up and sat there with an laser thermometer and shot it at the temp sensor while it warmed up.

He sais the head is ACTUALLY getting to 260 so its not a mis-reading. He said it stayed up at about 260-275 for a minute or so then the t-stat opened and it went to 205 or so then back up to hot and back to normal.

Could the guy that fixed it in the first place have installed the intake or head gaskets improperly and blocked off a coolant port?

wow. i hate when problems should be easy like 'replace the t-stat' but then nothing helps.
yes, it is possible to block coolant flow running around. i mean, really anything is possible in the realm of how dumb people can be sometimes. he could have left a banana or sandwich in the head for all we know. i've heard of stranger things happening.
sorry, this isn't helping.

yes, it is possible to block the flow of coolant through the block.
it is completely within the realm of possibility that such a thing has occured, though nothing else has fixed it so i don't know.
i'd take it back and raise hell until they redid the work, or figure out what was done to it.
:2cents:

It is very possible he installed the intake manifold gaskets backwards. The port at one end of the gasket is partially blocked off. If he put the partially blocked off ports at the front of the manifold it could cause the temporary overheating. I say this because I know a guy that did just that, installed the gaskets wrong and had some temp. issues. that stumped him for a few months, you should be able to look through the TStat hole in the manifold and see if one or both ports into the manifold are blocked. Good Luck.

you need to find a new mechanic. Your temp should not exceed 230 at any point in time. I too don't follow why head gaskets changed. You don't just change head caskets for fun. You should have a 195 thremostat and if you are getting above 230 you've got problems. Personally anything over 210 is a concern.

After doing some testing yesterday, we found out that ONLY the head that jackass worked on, the drivers side where the temp sensor is, is getting up to 240+ degrees. The one he left alone didn't get above 195. So its gotta be something either in the intake gasket or the head gasket.