Temp gauge showing hot but engine is running normally

[basspro1886]

I have an 99 Tahoe with the 5.7 engine. I just had the Tahoe worked on by a local mechanic and he had replaced and flushed the coolant and thermostat. He performed other work but nothing to do with the coolant. When I got the truck back, I noticed that the temp gauge was fluctuating more than normal. It would go up to 240 and then back down to 195. I returned the truck and told him what it was doing. He replaced the thermostat again with a 195 thermostat and then pressure checked the system. The system was solid and then installed his scanner and drove the vehicle. He stated that the engine is running at 195 to 200 degrees. With this said he told me that the sending unit is probably the issue. So over the next few days I had noticed that the gauge is starting to creep up to 260 and then it pegged all the way over past 260 and would not move. So this morning I replaced the sending unit. Refilled the coolant and drove. This time the gauge is still over to the 260 mark, but is not pegged. I checked the wires and the connection and everything seems to look okay. I went back to the mechanic and his reply was your cluster is most likely going bad. Is there any other troubleshooting ideas that can help before I go and get a new cluster. I have the manual but it is very limited on wiring.

 

[gpracer1]

I have an 99 Tahoe with the 5.7 engine. I just had the Tahoe worked on by a local mechanic and he had replaced and flushed the coolant and thermostat. He performed other work but nothing to do with the coolant. When I got the truck back, I noticed that the temp gauge was fluctuating more than normal. It would go up to 240 and then back down to 195. I returned the truck and told him what it was doing. He replaced the thermostat again with a 195 thermostat and then pressure checked the system. The system was solid and then installed his scanner and drove the vehicle. He stated that the engine is running at 195 to 200 degrees. With this said he told me that the sending unit is probably the issue. So over the next few days I had noticed that the gauge is starting to creep up to 260 and then it pegged all the way over past 260 and would not move. So this morning I replaced the sending unit. Refilled the coolant and drove. This time the gauge is still over to the 260 mark, but is not pegged. I checked the wires and the connection and everything seems to look okay. I went back to the mechanic and his reply was your cluster is most likely going bad. Is there any other troubleshooting ideas that can help before I go and get a new cluster. I have the manual but it is very limited on wiring.

Where was the location of the sending unit you replaced? Near the thermostat housing or on the drivers side head by the 1 and 3 spark plugs?

 

[basspro1886]

Yes on the driver side right between spark plug 1 and 3. You cant miss it.

 

[KPOD]

Are you losing any coolant at all from the reservoir? Changed the oil recently? Just because you don't see any external leaks doesn't mean you don't have one internally.

 

[basspro1886]

Yes oil was changed at the mechanic, and he checked the whole engine out. He replaced the transmission, the rear seal, all the rocker covers and something else that they found a leak with. The leak they fixed was a coolant leak. After wards he pressure checked it and it was rock solid. Compression check was all good. I checked the coolant and it is up to the neck. The Rad cap was replaced as well. The more I think about it, with the gauge ******* it is a short somewhere. Just have to find it.

 

[clownracer]

how fast does it hit that temp??? does the oil pressure drop when it peggs the thermostat???

 

[basspro1886]

It doesnt move. As soon as I start the truck it is already there. When I shut the truck off it does not go to 100. It will stay on 260. The oil pressure does not fluctuate when the temp is that high. Normal driving it sits in the middle. I am not sure what the pressure is. I think around 50 psi.

 

[retorq]

If it's not coming back down to 100 when the truck is first turned on there is an electrical issue between the temp sender and gauge. What does the ECM see??

I had a clutch changed on my 86 Blazer, they pinched the wiring for the distributor between the block and tranny when they put it back together. Probably something like that on yours ...

 

[basspro1886]

I am kinda thinking the same thing. I am going to follow the wires from the sending cannon plug back around to the firewall. They had so much apart around that area, I kinda of feel that its has to be around there. I havent scanned the ECM yet. I will tomorrow.

 

[basspro1886]

Okay here is an update on the Tahoe. I got in it this morning to leave to go hunting. When I started it I noticed the temp gauge was sitting at 100 degrees. As I drove down the highway I noticed it just moved all the way up to 260 and then stayed at 255 the rest of the time. I had the truck in 4 wheel drive and was running it pretty hard all day and it never came off 255 the whole time. So now I am wondering if the sending unit they gave me at Orielly auto parts was a defective one. i am going to go into there tomorrow and have them put there scanner on it and see if there is any codes. Any ideas of what this might be besides the sending unit.