Engines run by combusting fuel with air, the air being a readily available source of oxygen. They suck in air in an amount that varies with the position of the accelerator pedal, and a properly corresponding amount of fuel is added by the fuel injection system with the aid of the onboard computer (or on much older cars, the fuel is added by the carburetor).
In its attempt to suck in air, the engine produces vacuum. Some cars tap into this vacuum and use it to operate cruise control and the devices that open and close the flaps (doors) inside the heat-vent-A/C ductwork. A leak in the vacuum system that operates the HVAC doors can affect the temperature output and/or mode (i.e., defrost, dash vents, heater ducts, etc.). This won’t cause a problem with the blower fan itself but may make it sound funny if its air intake duct is affected.
Some vacuum leaks can be heard with the engine running. Often leaks can be found by applying some fuel to the suspect area(s). This fuel can be propane as mentioned above (and no, it won’t burst into flame or explode), but for convenience and pinpoint application I usually use spray carb cleaner or brake cleaner. The reason this works is that the air being sucked in by the vacuum leak isn’t being measured by the fuel injection system and isn’t getting its corresponding proportion of gasoline. When some kind of fuel is added to this air, the air/fuel ratio improves and the engine revs up a bit.
In a nutshell, there are doors in your vent system that direct the air to either come out of the defroster, dash-vents or floor. These are often controlled by vacuum. If that vacuum line breaks or develops a leak, the doors will all close, and either no air will come out, or the air will come out of a default location (in my experience, that would be the defroster but YMMV). It can seem like the fan has shut off, since you don’t have air blowing on your face anymore, but the air is actually being directed somewhere you don’t expect it to be going.
If the line is completely broken, this will be a permanent condition. If it just has a slight leak, you’ll notice the problem at idle, but when you step on the gas, you’ll feel the air again.
Stating it that way may be misleading. Engine vacuum is lowest when accelerating, moderately high at idle, and highest at a steady cruise (~2500 rpm). The relative numbers (in inches of mercury) might be 0-5, 16-19, & 22-25 respectively.
I wanted to update this thread…I noticed the same issue on colder days this fall, so I took it in. There was some PCM reprogramming regarding hot water diversion, etc, they did, butotherwise they just checked for leaks (found none) and added coolant back. The guy said “some evaporation is normal” but I don’t exaclty buy it on a 100,000 mile coolant system. Drive plenty of vehicles where this was not an issue. But, it’s under warranty and will be their problem in 5 months, give or take, so whatever.
I don’t buy it either. Unless there’s a leak, your car’s cooling system is not open to the outside world, and evaporation is pretty close to impossible. There may be seepage on the rough order of a drop or two per hundreds of miles from the water pump shaft, but otherwise, it’s a sealed system and not at all like an open pan of water that can freely evaporate into the air.
Yeah, if it were a car I was going to own, I’d take it somewhere else and have it documented out the wazoo before the warranty expired. As it is a lease, I figure I have done my due diligence. Not sure what else to do. Any system that only leaks out 1.5 to 2 quarts over 12 months is going to be tough to pin down a leak.
One of two things is happening. Either you are low on coolant, or your coolant system has an air bubble and needs “burped”. No biggie. But make sure you get it taken care of ASAP.
I’m not sure if you noticed, but I started that thread in 2008. I gave that car back to the dealership, drove another one for a year, sold it, bought another car and drove it for 3 years, and now own yet another car.
ASAP was a looooong time ago,
Incidentally, my 2010 Mercury Milan Hybrid is a very nice car and gets great mileage.
Hi…I know I’m late, but I just found you guys! So I have the same problem of no heat when idling, but in addition to that, I have no AC either. I honestly have not checked fluids since reading this post, but I will by the time someone responds. I do check regularly, so I would be concerned if fluids are low (indicating a possible leak). Btw…what’s often?
