View Full Version : 200 DeVille Blows hot air at low speed turns cold

01-24-08, 02:54 PM
I have a 2000 DeVille. I have had this car for 6 months now 85,000 miles on it. When I drive with the heat on everything is ok, then (not all of the time), at low speed or idle the warm air turns cool. Sometimes the warm air (at the same speeds) will go stone cold and the temp gage goes to overheated stage. If I jump on the expressway the temp goes down to normal. my mechanic checked and told me that my water flow is ok (this was done when the car was running ok). He told me to change the thermostat.
I called a Caddy repair center for a 2nd opinion and was told that it is the head gasket. I don't have coolant in my oil, no misfiring, just the hot air turning cold and overheating. Can someone help me with this before I start throwing money away on parts.


01-24-08, 03:22 PM
Is the system losing any coolant ? Is the coolant at, and staying at, the "full, hot" arrow or marker on the reservoir? The low-speed flow loss sounds like air in the system, low coolant, or a slipping w/p belt. Have you checked the 3-rib water pump belt and tensioner at the driver's end of the front cam cover? The tensioner should exert significant pressure on the belt and the pulley bearings should be quiet.

Has anything been done to the cooling system since you bought the car, or has the engine always done this overheat business? Thermostats on these engines are not a common failure item.

Last resort is to buy a coolant test kit from one of the large auto parts houses. Follow the directions and cross your fingers.

01-24-08, 03:30 PM
No coolant loss, no work done on cooling system yet, the belt seems ok, I can depress it though. One thing I almost forgot... you can hear a slight hissing sound from the engine, especially when I am at idle, when I turn the steering wheel slightly, the hissing goes away.

01-24-08, 03:35 PM
If you can wiggle the small 3-rib belt easily, it's too loose. The hissing sounds like a return or check valve in the p/s rack or pump area, and should come from the front (passenger) end of the engine, away from the w/p at the other end.

01-24-08, 03:38 PM
OK, I will tighten it and let you know the results.


01-24-08, 03:38 PM
Can you tell me anything about the hissing noise, that stops when you barely turn the steering wheel?

01-24-08, 03:41 PM
To make a head gasket diagnosis over the phone, without checking it is a bit of a stretch. Check the purge line to be sure it is clear an flows coolant. If it is clogged, air can get trapped in the system causing overheating and heat loss from the heater core.

01-24-08, 03:48 PM
There is no provision for tightening the belt. The tensioner uses a coil spring and swing arm with a pulley in order to lever against the belt and take up all slack.

If you remove the belt cover/guard you'll see either a 3/8" square drive hole or stud in the swing arm. Stick a socket breaker bar in the hole and carefully lever the swing arm toward the w/p pulley, releasing the tension from the inside of the belt. If the pulley bearings (on the arm pulley) are noisy, or if the spring does not seem tight and free, go to www.rockauto.com (http://www.rockauto.com) and drill down to the cooling system parts for your car. 85,000 is quite enough mileage for the w/p belt. Change it.

01-24-08, 04:11 PM
If you remove the belt cover/guard you'll see either a 3/8" square drive hole or stud in the swing arm.
Unless it has been changed, it is a 1/4" square hole for a 1/4" drive ratchet or breaker bar.

01-24-08, 04:18 PM
Is the coolant full. It sounds like it's low? Remember, these engines are notorious for unseen coolant loss so just because you don't see a leak, the level may very well be low.

Check again. Those symptoms are very synonymous with low coolant

01-24-08, 04:22 PM
Are you saying that, I can't tighten the belt, that it is a self adjusting system?

01-24-08, 04:35 PM

01-24-08, 08:32 PM
However the belt could very well be stretched / worn and slipping. Also pretty common over time.

01-25-08, 06:42 AM
You should have this checked by someone. It could be head gaskets, but as Ranger said, pretty risky diagnosis with just a few posts on the internet.

The main cause of this issue is "air" or some type of gas in the cooling system. The heater core is a perfect place for any air to rest at low rpms in a cooling system. What typically happens is the increased flow of high speed rotation of the water pump will "churn up" whatever is in there and keep coolant flowing through the system. When the rpms drop the air works its way to the heater core and your heat transfer drops off since there is insufficient coolant in the core.

There can be a couple causes here, one is simply low coolant. You could have small leaks that aren't showing up very well that have caused the level to drop over time.

Regarding the belts, you have 2. Most people don't know the second one exists so it gets ignored. It is on the left side (driver) of the engine on the front head driven by a pulley attached to the front bank intake camshaft.
If the belt is in really bad shape it could be slipping at idle and working okay under load.
The tension on the belt is controlled by a spring loaded tensioner. It is not adjustable, but the spring can break causing poor pressure and belt slippage at low rpms. The main accessory drive belt works the same way, but on a bigger scale.

Take the car out on the highway, usually blown head gaskets will cause the engine temperature to rise under highway cruise conditions. The increasing levels of combustion gas cause the thermostat to stop working properly and the temperature to rise, but when coming to a stop the vehicle will seem to cool back down to operating temperatures.
If yours acts like this, then as I said at first, have it professionally checked to verify the problem and go from there.

01-26-08, 01:47 PM
I called a Caddy repair center for a 2nd opinion and was told that it is the head gasket. I don't have coolant in my oil, no misfiring, just the hot air turning cold and overheating. Can someone help me with this before I start throwing money away on parts.


Those are the symptoms of an N* with HG's just starting to fail. The problem is it can be other things failing. I agree you do not want to throw money away. The N* does NOT put coolant in the oil or run rough with bad HG's.

Ewill gave some good advice. The coolant can be checked for signs of combustion byproducts. That's a pretty easy test. You cannot add fresh coolant just before doing the test though or you will get a false negative.

01-26-08, 09:02 PM
I changed the Thermostat, I also checked the old thermostat in boiling water (it didn't move a bit). The Caddy still runs hot when idling under 1k RPM's. On the expressway it DOES NOT run hot. Now it takes off like a bullit (I'm sure due to a bit better water flow). When it went 2 bars over normal range I pulled over and took the RPM's to 3k, the engine cooled down to normal. when I continued to drive it started to overheat again.
Three mechanics (that I know) who have seen my car are all saying that it is not the head gaskets. One mechanic said he worked on a car with these symptoms and the radiator was clogged. He suggest either taking the radiator to a shop that can unclog it, or get a new radiator.

P.S. I have a radiator for a 1994 STS that I had, it was never installed, does anyone know if this radiator is the same model that is in my 2000 Deville?