No heat until up to highway speed

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  #1  
Old 12-17-08, 10:26 PM
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No heat until up to highway speed

I have been having problems getting any heat out of our 1993 Mercury Villager. Living in Calgary, we get some pretty cold weather. When the cold hits, our van will put out little to no heat regardless of operating temperature of the vehicle, until we get out on the highway. Driving around on residential streets, there is plenty of air flow coming through the vents and the water temperature is within normal range but the air coming from the vents is cold. As soon as we get out onto a highway and get up to speed (100 km/h), the air coming from the vents heats up. This seems to be opposite of any other heat problem that I have ever had with any other older vehicle (I've owned my share). Usually with a plugged heater core, or bad thermostat, you can get by at slower speeds, but at highway speeds the vehicle loses heat faster than it can be replaced, or if the thermostat is stuck open, the coolant temperature won't stay up high enough to get the heat needed.

I was planning on replacing the heater core before I started to realize that there is plenty of heat at highway speed, but none at slower speeds. Now I am wondering if there is something else going on. Does anyone have any suggestions as to why I might be getting lots of heat at highway speed, but little to none at slower speeds? Would I be wasting time/money replacing the heater core?

Other details:

1993 Mercury Villager
V6 3.0L
AC
Rear passenger compartment heat with separate controls
Recently replaced thermostat and water pump
 
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  #2  
Old 12-17-08, 10:38 PM
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The same thing happened to me not to long ago and it turned out I was just low on coolant. Since the hoses running to the heater core were at a higher level than the rest of the system it was the first to lose flow. It wasn't until the fluid became hot enough at higher speeds and it expanded, becoming enough volume to begin flowing through heater core again, that I would get heat again from the heater.
 
  #3  
Old 12-17-08, 10:39 PM
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Purely a Guess...

Do you live in an wooded or suburban area?

I had a similar issue in one of my old cars where mice and squirrels actually got into the ducting, built nests, then left.

They were kind enough however to leave thier homes behind which blocked the vents at low speeds.
Highway speeds "Forced" some of the air through, so I then felt it.

I hope this is not your issue, because it is a pain to clean out.

Just a thought.

Anyone else have a thought?

Aces.
 
  #4  
Old 12-17-08, 10:48 PM
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Blocked ducts probable, low coolant not

AcesDwn - Blockage in the ducts is definately a possiblity. A number of times in the summer we have smelled a burning smell. Not electrical, oil or fuel burning, but wood burning like a campfire. I have often wondered if there are leaves/twigs in the ducts. I never considered nests, but I do believe that the van sat for quite some time before I purchased it.

aslave4him - the water level is okay.

I'll look into the duct blockage question, but still would appreciate any other ideas/suggestions.
 
  #5  
Old 12-17-08, 11:32 PM
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I hope I'm Wrong

This is one time I'd HATE to be right.

It really is a pain to clean out nests of any sort (due to the health issues.)
I had to pull the dash, then pull all ducting, cleaned out with disinfenctants, and then had to reassemble.

I'm glad you seem to be pointed in the right direction, but geez, I hope I'm wrong... lol

Aces.
 
  #6  
Old 12-18-08, 06:06 AM
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no heat

You could have a heater core partialy plugged, and when you increase the speed of the water puump it forces the coolant thru. Feel the hose and see if they are both hot. If there is a lot of difference this may be the problem. RW
 
  #7  
Old 12-18-08, 07:10 AM
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Did the problem come before or after the water pump and thermostat change.
I have run into cases like this where there is air in the heater hose after the system was opened for repair.
 
  #8  
Old 12-18-08, 04:53 PM
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I'm pretty sure that we first noticed this after the water pump and thermostat were changed. I've had difficulties before with what I thought was air in the cooling system. How does one get the air out? This may also explain why we have periodically experienced "near-overheating". A couple of times last week when the temperatures outside were -30 C, the temp guage got really close to the max line even when cruising down the highway in extreme cold. It only happened a couple of times and seemed a bit random. I kind of thought that maybe my antifreeze mix wasn't right and that I was having a bit of freeze up.
 
  #9  
Old 12-18-08, 06:21 PM
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I just did a bit of testing with some of the ideas so far.

It is definately related to engine speed. Driving around residential streets RPMs range from 1000-1500. On the highway it runs at about a steady 2400. Sitting still at 800 RPM the air is quite cool. If I rev the engine to 2500 RPM it gets hot very quickly. Pretty much instantaneously.

So I am 100% sure it has nothing to do with Ram air and everything to do with the speed that the water pump is turning at. Does this mean that the heater core is for sure the culprit? Do I just need to replace the core? Or are there other things I should consider/investigate first?

Thanks for everyone's feedback.
 
  #10  
Old 12-18-08, 06:29 PM
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I'm guessing the blend door actuator is vacuum controlled and has a leaky vacuum hose behind the dash somewhere. With higher RPM, it creates more vacuum and enough to open the blend door. When RPM's drop, so does vacuum.

Also, I believe there is a heater control valve. Should be attached to a coolant line going into the firewall.
 
  #11  
Old 12-18-08, 07:51 PM
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If it is getting hot also then I would say it is not the heater core and air is a very good possibility especially if signs started after cooling system work.
I would put one of those radiator flush hose adaptors to the highest point on the highest heater core hose and top up there.
I had a hard problem with a Suburban once for day and the temp gauge was all over and no heat from the heater core.
I finally put the rad flush adaptor on it and forced the water in there. Worked like a charm after that. Don't know why that certain vehicle could not self purge the air out by itself or why I could not get the air out the usual ways but after I force bled it everything was fine.
 
  #12  
Old 12-18-08, 08:36 PM
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I think that there may be some real substance to the vacumn leak theory. In the summer time we have had issues with what was probably the blend door. From time to time in really hot weather it is like the door gets stuck shut and we get no AC. No air coming through the vents at all. If we shut the vehicle off for 10-15 mins or so, all is back to normal. Only ever happened on long trips after 1-2 hours of driving. I always suspected a vacumn leak to be causing that problem.

Frankiee and others that have suggested air in the system also make good points as over the past couple of weeks I have noticed the temperature guage all over the place. Sometimes it seems to take forever to get up to temp, then other times it gets really hot for a short period of time and then drops back down to normal.

How do I purge the air in the system?
 
  #13  
Old 12-18-08, 09:05 PM
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Originally Posted by flamesfan167 View Post
How do I purge the air in the system?
Easiest way is to have a shop do a power flush. It can be a real pain to purge the air from a cooling system but there are ways to do it manually but the specifics vary with car/engine type and hose configuration.
 
  #14  
Old 12-19-08, 05:15 AM
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Flushing Your Cooling System - Popular Mechanics
Scroll down to "let it bleed"

HowStuffWorks "How Car Cooling Systems Work"
Not to scale animation. Some heater cores and hoses are quite high. Take a good look at the height of yours


Just a couple links to look over that pertain to the cooling/heating system
 
  #15  
Old 12-26-08, 11:50 PM
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Glad to be wrong

I apologize as I have no real substance to add at this point, but I'm glad it was not nests.

I hope everything turns out alright..

Thanks,

Aces.
 
  #16  
Old 12-27-08, 04:39 PM
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Get your heater core flushed & back flushed, (with CLR if at all possible to do yourself) When done, add coolant, and run engine with rad cap off, and heater on, long enough for your rad fan to come on 3 times. Rev engine slightly every once in a while, to increase flow & speed up air bleed through system. When done, check level in rad, & bottle & put cap back on & try it.

P.S. I noticed someone say that higher RPM's produce more vacuum, lower RPM's, No or low vacuum. That is incorrect ! At idle you have high vacuum. At a steady highway speed (cruising) your vacuum will be as idle or close to it. When you accelerate, your vacuum will drop, as quick as you push that pedal. Going up a hill, vacuum will drop,as you push pedal further down, when you release pedal at top of hill, vacuum in engine will increase.
 
  #17  
Old 12-28-08, 10:45 AM
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Thanks for the reply Mike B. Can you explain a bit more about why vacuum will not increase with RPM? Your explanation seems contrary to my own experience. I had a vacuum leak with a Honda Prelude that I owned a long time ago and while troubleshooting and fixing it, I discovered that there was a direct relationship between RPM and vacuum pressure. The higher the RPM, the more vacuum pressure I would get.
 
  #18  
Old 12-28-08, 11:08 AM
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Well, there are 2 types of vacuum. I can't remember the exact terms, but there is manifold vacuum, which will be as Mike B explained it. There is also port vacuum (?) that is based on airflow through an orifice. It is sorta the opposite of manifold vacuum in that higher airflow (such as high RPM) will produce a higher vacuum on a port to the side of the airflow. Its based on the venturi principle.

All depends on where you tap into the carb or throttle body.
 
  #19  
Old 05-29-09, 12:06 PM
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Did you find the heater core was the problem? Did you flush the heater core system as suggested by Mike B? I am having, what seems like, the same problem.

The 2002 villager temp guage was going to hot and the fan kicking on. After a cold start (driving for only a couple miles).

AND.... No heat until at higher RMP (highway speeds)

I brought it into the shop. They found no leaks (coolant level was fine). The only thing they could think of was that the thermostat was bad.

They replaced the thermostat and did a coolant flush

Just got the vehicle back. Still having the problems!!!! Frustrating. Just curious if the heater core was the culpret in your case?
 
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