Ok, so I've been a reader of this site for years, but this time could'nt search out the exact problem.
I have a customer (boat trailer shop) with an 06 Mega, mild tuner, 6speed. He tells me his truck keeps nearly pegging the temp gauge with no load, so he pulls over and it retreats to 140 in a minute or less. Dealer changed thermostat twice already. He drives to my shop this week, jumps out and tells me "hey, its doing it right now!"
I open the hood, shoot the upper rad hose with temp gun, a chilly 56F. The head temp reads only a degree or two hotter. He says the temp goes to the top of 240, but no "CHECK GAUGES" light illumiates.
Do we have a known temp sender or wiring issue I haven't read about, or is this a cluster/gauge malfunction?

 

Need more data about truck, is it chipped, what mods are on it

 

Maybe its his instrument cluster,alot of 06s have had tach problems,mine included and at one time I had fuel gauge problems which have now gone away and of course could have been a sticky sender unit,So if you have changed the temp sender unit twice,it kind of leaves the inter connecting wiring and the cluster .How about measureing the temp of the block right at the sender unit,just to be a little closer to it.You could also try running the instrument cluster self test to see how the gauges perform,though a jumpy tach will not show up as a problem doing it.If you have a service mannual check out 8J-64, 8J-79,9-86, and 9-94.

 

Thanks kwking, I'll check that. But the sending unit hasn't been replaced twice, it was the thermostat. I did check the block temp, definitely ice cool, as this happens after driving only a mile or so.
Supergewel, I mentioned in the post that the truck has a mild tuner(edge on mild setting) with no other mods.

He is going to drop it off tonite, I'll keep anyone that's interested in the outcome posted.

 

Do you have a service mannual? I think I read that the ECM reads 2 air temps and compares them to the coolant temp,its in section 9.My missunderstanding on your sender unit.The mannual will tell you the expected voltages cold and hot for the sender unit.

 

My truck was doing that . . it was the sender.


CD

 

Also something to check you did not state the level of the coolant. Air pockets will super heat in cooling system and read hotter then the coolant itself. Be sure that you system is FULL, and not air pockets exsist. Found this out the hard way with the wife's car. Turns out that if you keep topping off the system while running heat in car it helps to purge out the air. Took me almost a week of chasing her around town because her car was "getting hot" Luckily I got her to believe that anything hotter then 200 degrees cause it normally stays around 190 is TOO HOT...LOL 2001 still going good...hehehehe, helps me out with my MODS...

 

Thanks guys, I'll test the sender unint tomorrow. When the truck came in tonite, I pulled codes, and got a coolant overheating code. Now I think the sender is the culprit, since the gauge wouldn't be setting the code.
Keep you posted.

 

checked sensor today, couldn't get coolant hot enough without driving it hard(temp here about 45.F .
While test driving it , the Edge sounded an alarm for engine temp at 219. I jumped out and temp gunned the head around the sender and came up with about 160. Temp would spike only during boost, sound the alarm, and then drop back down before I could even get out. Radiator is about 110 on the drivers side to 80 on the pass side. Sensor now leading culprit, edge and dash read same temp, so they likely use same sensor.
Any reason temp could spike and return that quickly without heatin up the coolant or block or head? Seems unlikely.

 

Just a follow up for those interested. Small puddle of coolant under truck next morning, small leak from hose behind turbo. Pulled the temp sensor, and found air pocket, with the coolant level below the sensor. Coolant resivoir still full, so I checked the overflow hose from the rad cap and found an air leak. New hose behind turbo, and clamp on the overflow hose, fixed.

 

Yeap, air pocket will get you every time. That is why it is good to check the radiator when cold and not just look at the coolant tank.

 

Glad to know that I have not been the only one driven nutz by air pockets!!!! LOL Your right they can be a complete pain in the ***. Also finding where the air is coming from can be almost as fun!!!!! Glad to here you have it all squared away. Sounds like you ended up pressure testing the system to find the leaks?