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joel, GM Dealer Technician
Category: Buick
Satisfied Customers: 1603
Experience:  18 years plus at same GM Dealer
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buick rondezvous: I have a 2005 Buick Rendezvous that is getting

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I have a 2005 Buick Rendezvous that is getting a coolant temp message, i changed the temp coolant sensor on the engine block. The car runs rough at start up, and when i put the heat on i am getting cld air...then slightly warm air even though it looks to be up to temp? Any ideas?

Hello and welcome.

Sounds like you have the common problem of a leaking head gasket. The rear is most common. When they fail, combustion is pumped into the cooling system., this is the reason you dont have heat and the engine is overheating. Usually it is no heat at idle, then gets warmer as the engine speed increases.

The engine misfire is probably caused by a rear spark plug that is burned away because that cylinder has been burning coolant., or a cylinder has coolant in it after sitting overnight.

Customer: replied 3 years ago.

Wouldn't I have a milky oil substance or signs of lost water/antifreeze if the head gasket was blown?

That is the rule of thumb, but in the case of these failures, compression is being pumped into the cooling system, coolant is not going onto the oil.

If coolant is getting into a cylinder, the amount of coolant loss is so slow that is can be hard to determine that there is a loss. It takes quite some time for the level to get low.

And with the combustion in the cooling system it causes air pockets, air displaces coolant.

Customer: replied 3 years ago.
With that issue shouldnt I have some diagnostic code when plugging the machine in....also, what is the fix a new head gasket?

You may have a misfire code stored in the computer.

Yes, once confirmed, the headgasket would need to be replaced.

joel and 3 other Buick Specialists are ready to help you
Customer: replied 3 years ago.

Actually i figured it out. There was a relay for the fan control that needed to be replaced. Cost a total of 12.99. Trial and error is the best . Doom and gloom is the worst.

So your saying that the coolant fan was not working, the engine was overheating, but the heater does not blow hot at idle? That makes no sense. Also, that has nothing to do with the runs rough at start up. HMMM ?
Customer: replied 3 years ago.

no the deceiving thing is the fans.were still running. tbere are 3 relays.1 for ea h fan and another that conrols how fast the go based on the info sent from the engine coolant tem sensor. the car ran rough when it overheated....once i got the new relay and let the car ran for a while it smoothed out

Strange. Hope the overheating did not cause any damage.

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