It is necessary on cold, especial extremely cold days. REASON BEING: Is that it takes the oil to reach it's proper viscosity longer time than on warm days. When you do start car, don't race it or you'll be doing more damage. Let it stay at idle for about 3 - 5 minuets until you see the temperature gauge start to get to move to a hotter position, or better yet, until you have heat. Reason is that thick oil cannot easily flow though tiny holes in crankshaft, push rods not getting oil into small spacers in cylinder block, valve guides, rocker arms and oil control rings leaving a dry cylinder wall, which causing wear to cylinder bore. Sometimes in really cold places, they suggest using a lighter gauge oil.

I will give my answer which is in opposition the the above answer somewhat. In years past when cars had carburetors it was a necessity to warm the engine up before driving off because the engine simply would not run correctly until the carb got warm. Today it is a bad idea to let the car idle for more than a couple of minutes max before driving off. Reason being is that on todays modern engines with computer controlled fuel management systems it is not necessary and is in fact a waste of fuel and pollutes the atmosphere plus harms the converter. Modern fuel injection systems do not need to warm up to work properly as an old carburetor did. They work instantly and the ECU adjusts the fuel/air mixture to compensate. Modern oils of today are also a far cry from what they were decades ago. Modern Oil sticks to all parts better today than in the past. This is not the same formulation that we had decades ago. There is a thin layer of oil on all parts when you start the engine due to modern refinery processes, so you are not causing wear to that engine if you drive slowly until it reaches full operating temperatures. Modern vehicles will warm up very quickly if driven but not so fast if sitting at idle. You are also doing your catalytic converter no good by letting this very rich mixture of fuel/air enter a cold converter that does not work well when cold. Just drive away slowly, avoiding full throttle until the engine is warmed up. This is 2010 not 1955. Things have changed.I can see the above point of view. But there is still a disagreement on oil. yes there will be a very, very thin layer of oil on the cylinder walls, but even sythetic oil will settle downwards ,and on a 20 degree day will be sticky and wipe off in one piston stroke. it still ,in my opinion, takes a little bit of time for oil to thin out as it gets warm. Has enyone ever went to change oil on a 10 to 20 degree day. It barley comes out of the drain hole, unless you let it run for even a minute, can you picture this oil flowing through tiny openings. however, point is well made that idleing and driven slow for the same amount of time will do the same exact thing. Main point is ; don't reve the engine right away, or drive fast for a few miniutes. Note: in places like Alaska, they use a heating source overnight because the car may not even turn over or do it slowly, because the oil is so thick.

Ans 3 -The second answer is the correct one. Letting a modern engine idle for long periods is not good with modern oils being as 'clingy' as they are.

It happened to my Mazda/98, After i changed the engine mount, No more shaking.

I had bad shaking when idling in park or at a stop light. I replaced the timing belt and got the cams retimed. After that it was good.

Each of these things can be checked independently of the others without spending any money on parts.

1-Low coolant level.

2-Restricted airflow to the radiator as from leaves or plastic bags in fron of it, or the plastic under-skirting being broken away (as from running up onto curb-stops.)

3-Restricted flow of within the radiator or hoses. Restricted flow in hoses is rarer, but rev the engine to 2000 to 2500 RPM for several seconds after the engine is fully warmed and look for either radiator hose to collapse. To check for restricted radiator, you can take the temperature of the radiator with a non-contact thermometer in several places or you can open the cap (if so equipped) and look for white deposits. White deposits mean you need a radiator. If your radiator has no cap, remove the upper hose and look in through that opening.

4-Poor heat transfer. Poor heat transfer can be radiator to air (look for missing or greenish radiator fins) or internal due to a high ratio of anti-freeze to water (use a hydrometer) or internal due to scale deposits (with an iron engine your coolant will be brownish, aluminum engines can be checked by removing the thermostat or water pump and looking inside for whitish deposits).

5-Insufficient coolant pumping. Metal water pump impellers can erode over time, especially with excessively old coolant. Plastic water pump impellers can break randomly, and are prone to break if the engine is a Ford Duratec or Volkswagen or if the water pump hasn't been replaced after an overheat. Cheap substandard $5 replacement water pumps have often have a poorly designed impeller that can become dislodged. To check, remove the thermostat(s) and fill the system with coolant or water. Remove the radiator cap (if so equipped, otherwise surge tank cap) and observe the coolant level while rapidly snapping the throttle from closed to wide open. The coolant level should change drastically (more than 1/4 inch) if the pump is good.

6-Exhaust gases leaking into radiator. This is usually caused by a leaky head gasket, but can also be caused by a cracked or eroded engine. Note that GM Quad-4 engines are particularly prone to cracking, and four cylinder Chrysler engines and cars owned by youthful males or Jewish women are prone to head gasket leakage. An electroneic gas analyzer can be used to check this condition, but for the do-it-yourselfer, check with the local parts store or tool dealer for a disposable chemical based detection kit.

7-Boiling. Boiling can be caused by too much water in the coolant, insufficient pressure due to a leak or a bad cap, Partially restricted water jackets, or overheating due to another cause. Presumably, any other overheating has already been ruled out so you might as well check for leaks, then replace the coolant with a correct mix and put a new cap on it. If you have a restricted water jacket, you're going to need the engine rebuilt.

***Fan related problems will _not_ cause overheating at highway speed.***

A pressure test of the cooling system usually will show a leak.

Especially if you notice fluid under the car after driving.

There are several things that can cause this problem with Jeep Grand Cherokees. The actuator (door motor) can cause this, the door could also be broken. There is an good article that goes into more detail - the link is at the bottom of this page. Often, the actuator motors are too strong for the plastic doors, and they break over time. The typical fix involves removing the entire dash and replacing the plenum assembly. There is a DIY kit by heatertreater that provides instructions on how to perform the repair without removing the dash.

I just started experiencing this same exact issue with my 2002 Jeep Grand Cherokee Limited on a road trip yesterday! It works as it should on the driver's side, but only blows cold air on the passenger side regardless of what temperature you set it on! Also, even if I completely close the passenger vents, some cold air still comes out from them and also from the floor area!

I had the same problem! Here is my cheap quick fix. Remove the glove box lid, there are 2 rubber retainers that hold it in place, just pull them out they are easy to put back, the lid will now drop down to reveal the the door motor housing, disconnect the electrical lead(Make sure you remove the neg lead on the battery first)Undo the 2 screws and then pull out the motor. A new motor cost me approx 100 pounds. If you do take my approach make sure you test the motor before you buy new as it could be some other problem. Once you have removed the motor it's easy to open and close the door by it's spigot, just to make sure it's not stuck in any way. My final test was to check for power to the electrical lead. Reconnect the battery. While turning the temp conrol check with a meter to see if there is power to the lead.

It sounds like you have aq problem with the right temperature door assembly. I have a similar problem with my 1999 Grand Cherokee Limited, but, in my case, the unit just blows cold air. The question is: How do you access these door assemblies to change them? Help! I am a do-it-yourself person and want to know where I can locate information about how to access the AZC system door assemblies to change them? I understand it involves removing the entire dash board in order to get access to the passenger side one that is broken. To do so I need some type of repair manual.

All I know is I have the EXACT same problem with by 99 jeep grand Cherokee. It started with one side blowing cold and the other blowing hot. Now, both blow only cold air. I took it to the dealer and they wanted to charge me $800 to fix it stating it was some switch that has been going out in all jeeps this model with dual heating system yet they would not consider it under warranty as a issue. Hence, I still have no heat.

I just had the problem on my 01' jeep grand the dealer told me it was a bad bun door and actuator on the passenger side at a charge of $1200.00. If your good with cars you can do the job yourself and I mean good this job was a pain in the butt the dealer est. 12hrs (plus u need to disc. the heater core and evap. for the ac) of their time it took me exactly that. Once I got the HVAC system out of the truck I noticed since the car is temp. control zoned the passenger Sid bun door which controls the flow of heat to the passenger vents was broke. The dealer was aware of the problem picked up a new part for $200 and slapped it back together worked fine( the actuator was not bad it just doesnt respond to a broken door so don't let them sell you one, you should only need a new bun door)

The problem is a wear out mechanism with the blend doors. The doors break and are stuck in the AC position...NO HEAT. There is a new product that solves the problem. Search on Google for "heatertreater" for a complete explanation of the problem and a solution.

The problem with the system is the design of the actuators. The failure of this so called" bun" door is due to the poor design of these actuators. Over torque and excessive wear leads to failure....The only way to resolve this problem is by replacing both actuators and the temperature blend door assembly. Do not just replace the door!!!! ( note: fresh air door has same problem) I average one grand Cherokee a week in my Cooling systems repair shop. 675.00 is my going rate. It pays 6.1 hours and nothing more...don't be robbed by the dealership and stay away from driveway mechanics. Beware that your entire instrument panel must be removed along with the evacuation of the refridgerant. Not a "do-it-yourself" deal! Good luck and never by a Jeep again!

Your failure description is the classic fail description for broken blend doors, and this is a common problem on the GC. There is an inexpensive DIY kit on Ebay from heater treater that will fix the problems without having to remove the dash panel.

The HVAC system is computer controlled and when the doors break, the computer senses that the movement of the doors is out of spec and shuts the motor down. Actually it is rare for the actuator motor itself to go bad. Usually it's a broken door combined with computer control. You can check the motor with a 9V battery and a snap connector. Just touch the wires to the motor terminals and check that it turns.

I just thought that I'd add my experience: I have a 1999 Jeep Grand Cherokee Limited 6-Cyl. About a year ago heat went out on passenger side (cold air came out on passenger side when heat was on and coming out on driver's side). Spring and Summer came, so I forgot about it. This Fall (maybe 1-2 months ago) I was reminded of the problem when the cold air came back. Now, about 1-2 weeks ago, all blower functionality is gone ... a real problem if you need to defrost the windows! I'm going to try the AZC test and look into the HeaterTreater. I'd hate to put any more big money into this vehicle as it has been a true money pit. I'll never buy another Jeep. It's a classy ride, but I've put more money into repairs for this vehicle than any other two vehicles I've owned.

Easy fix is available

There is a video available that shows the complete repair process and goes into an explanation of the root cause of the problem and the solution. You can also find the video by searching on YouTube. This is a very common problem on the Grand Cherokee and the kit gives a simple effective fix for the problem and is much much cheaper than taking it to the dealer.

Assuming you are going to replace the 99/04 Jeep Grand Cherokee blend doors yourself.

You would start by disconnecting the negative battery cable.

If you are going to remove the dashboard, etc to replace the blend doors.

some websites give you complete instructions with plenty of pictures.

It will take the average person with little or no auto repair skills approximately 12 to 16 hrs.

The vehicle will have to be taken to a shop that can recover the refrigerant before you start removing the dashboard and HVAC box..

If you are going to do the type of repair where you cut an access hole in the side of the HVAC box.

A short version of how the access hole repair is done.

Disconnect the Negative battery cable.

Lower the glove box

Remove the actuator motor - You will need a Phillips screw driver

Cut an access hole in the plastic HVAC box - Use a multi-purpose cutting bit, you will need a rotary type of cutting tool, like a Dremel, RotoZip, etc. Or you could do the cutting by hand with any type of small cutting tool, it will just take longer doing it by hand. Or some people have used a narrow pointed soldering iron!

Remove the broken blend doors

Install the new Blen Dor dual control blend doors.

Replace the actuator motor.

Close the access hole - Replace the wall piece of plastic you cutout -Use foam tape or an aluminum tape to cover the groove made by cutting the wall.

Close the glove box.

It is almost as easy as what you just read, the hardest and the most time consuming thing to do is cut the access hole.

The job should take between 30 minutes to 1 ½ hours. Depending on how long it takes you to gather your tools, etc.

Before you do anything major find you hvac fuse, pull it, then put it back. It might be in the fuse box under the hood or the one inside. All mine needs is to cut the power to the havc unit then it resets.

• The duct fan is bad. Replace it. Beware though: it's a pain the butt.

• After you replace the blower motor and it still doesn't work, take it to a shop for testing if you can't do it. Other possiblities are a switch, relay, resistor, fuse, wiring. You can eliminate the fuse by swapping it with another of the same. You can try the "tap test" by rapping on the dash sometimes over the glove or on the blower motor itself, somteimes a weak motor may temporarily start working. Oh yeah, all blower motors aren't bad to change.

• If it's blowing air, but only COLD air, check that your engine coolant isn't very low.

• Sounds electrical, check the blower motor relay, although it may be switch or motor.

• This happened in my car! After replacing the blower motor (it wasn't broken afterall), found that it wasn't getting power due to a faulty ground. This was right behind the a/c heater radio console.

• With the fan only blowing at the 4 setting indicates the resistor need to be replaced. The resistor controls the flow of electricity to the motor, w/ the 4 setting not limiting the current at all, which is why you the fan works at that speed. I'm in the middle of trying to fix my blower too, but so far no luck. I've replaced the motor, but nothing's changed. The volt tester shows power to and from the motor, but no movement. I'm not sure what else I can check at this point.

• Is the fan actually running or not? Can you hear it...but no air seams to be moving? If yes, then one of the duct-work doors may be stuck. (There are little electric or sometimes vacuum operated door inside your duct work) A wire or tiny hose to one of these may be loose or fallen off. If you cannot hear the fan running at all, even on it's highest setting...then consider looking at fuses, loose electical connectors (sometimes they get kicked off by a passenger). It's actually rare to have a fan motor die...although it can happen.

• This is a really easy fix, it's likely that your thermostat is stuck. This has happened to me on numerous occasions when I had rusty water in my radiator. The thermostat was stuck, so I replaced it and tada! You've got heat. Usually less than 10 bucks to fix.

what is the make and model and can you hear the blower motor running

You didnt say if the water pump was changed. No hot air means no circulation.

Anyways, Coolant is a fluid that keeps a car from getting too hot among other things. If there is low coolant the car will overheat. When the coolant ciculates through the engine it will also go through a thing in the dash that is like a small radiator called a heater core. A fan or blower move air through this part and pushes the hot air from the heater core into the passenger compartment. This heater core is usually the highest point of the coolant flow. Since fluid seeks it's lowest level, the heater core is the first to lose the hot coolant. Therefore your blower blows accross a cold heater core and gives you cold air.

If you don't have a leak, then when the thermostat was replaced, the coolant may have not been bled of the air in the system.

I would first fill the coolant and check for any leaks first. There is a common problem with the Buick centuries if you have a 3.1 motor. The intake manifold gasket is common for leaking.

In any event, don't run the engine if it goes into the red. Overheating can cause headgasket problems. See othe posts regarding this issue.

Let me know how you make out.

The heater core does not need to be removed to be checked. If the hose going into the heater core is hot and the one coming out is cold, the tells you it is retricted. Another way to verify is to remove the hoses going to the core, hook up one without installing into the car and run it. It should get hot-very hot, and then see how hot the hose is coming out of the core is like now.