a/c
Postmaster
Joined: Mar 2001
Posts: 4,968
Likes: 3
From: So Cal
a/c
Maybe a couple things. When AC is on, it dumps heat in front of the radiator which of course picks the heat up and raises water temp, normally not noticable. The AC also puts a bit of load on the engine which also would create heat, but normally not noticable.
What may be happening with you is that the radiator or engine cooling passages are starting to clog and the cooling capacity is marginal. A good chemical flush might fix the problem.
The other thing is maybe your T'stat is margina and only opens up all the way when the heat load goes over the setting of the T'Stat.
Another possiblity is that the fan thermostatic control may be getting flakey and the fan doesn't kick in until the heat load is higher than desired.
Also what do you mean flies up? If it moves up say maybe to the next tick mark on the gauge and then settles back down I might not worry too much. If it kicks from normal to very high then I would start looking for the problem.
I assume you have plenty of 50/50 mix coolant.
Good Luck,
Jim Henderson
What may be happening with you is that the radiator or engine cooling passages are starting to clog and the cooling capacity is marginal. A good chemical flush might fix the problem.
The other thing is maybe your T'stat is margina and only opens up all the way when the heat load goes over the setting of the T'Stat.
Another possiblity is that the fan thermostatic control may be getting flakey and the fan doesn't kick in until the heat load is higher than desired.
Also what do you mean flies up? If it moves up say maybe to the next tick mark on the gauge and then settles back down I might not worry too much. If it kicks from normal to very high then I would start looking for the problem.
I assume you have plenty of 50/50 mix coolant.
Good Luck,
Jim Henderson
Senior User
Joined: Jan 2003
Posts: 383
Likes: 1
From: Newport News USA
a/c
When Jim says that the a/c is dumping hot air in front of your radiator, he means that any of the cool air that would normally pass THROUGH the a/c condenser is now getting heated before it hits the radiator, therefor not doing a whole lot of cooling. I would check both the condenser and the radiator fins to ensure that they are nice and clean and free of any and all debris. It's not unheard of to get a bunch of crap lodged between the condenser and the radiator.
That's just one possiblity. It can very well be anything else to include what Jim described.
Luke
That's just one possiblity. It can very well be anything else to include what Jim described.
Luke
New User
Joined: Oct 2002
Posts: 20
Likes: 0
From: Texas
a/c
Mine did the same thing (91 F150 5.0)....significant temp rise when I turned on the a/c. It was the fan clutch. 
An easy fix, and you'll be back cool again. Of course, I was ready to check the radiator for clogging, thanks to the heads-up from the FTE gang.
Thanks!
An easy fix, and you'll be back cool again. Of course, I was ready to check the radiator for clogging, thanks to the heads-up from the FTE gang.
Thanks!
