Off Topic (automotive) heater issue

A call to the common sense here. The automotive forums seem filled with confusion.

I have a vehicle that displays a "common" deficiency of no heat at idle. (It's been fricken cold here lately)
Yet, the heat is just fine when the engine is turning over 3000 rpms. (sport mode driving! ;-)
Scouring the web, it appears this issue is familiar to many, with a frequent call to repalce the heater core. Some reports indicate 3 replacements in as many years, and some owners of similar cars get the heater cores flushed "professionally" every year. $$$

My question is, if the heat core can output good heat when the engine revs are high (read, coolant flow is sufficient) how can replacement of the heater core be the appropriate resolution? It seems to me that increasing coolant flow would be the correct response.

Now, I can agree that if the heater core were restricted, the resistance to flow would limit heat exchange. but doesn't the the high RPM condition prove that the flow through the core is possible? ( the math would support pressure/restriction = flow)

I've never seen such issues with "older cars", with simple coolant flow shut offs and fan speed to control cabin temps.
What might be a solution? An auxilliary pump to improve flow through the heater core?

ps. flushing the core shows free flow in both directions with very moderate pressures. to say the "core is plugged" would be in direct conflict with evidence.
Also, the trouble appears to be wide spread, if the internet postings are sampled, with many varied attempts at a cure.

So is it a new core? citric acid for 6 hours, or add a pump to boost flow through the core?

Also, Does anyone have a comment regarding "vacuum filling" of the coolant system? this vehicle is notorious for difficulty in bleeding the air when refilling the coolant system.

TIA

Cal

The most common cause for no heat at idle is low coolant. On a good many cars and trucks the core is one of the highest points in the cooling system. It doesn't take much and having a so-so level in the overflow tank often isn't enough. Vacuum filling is just that. Usually a tool fits the filler neck or reservoir and pulls a vacuum on a empty system. Mine has a valve on the side that will allow it to siphon or pull from the bottles or tank I'm filling from. It can also can be used as a diagnostic tool by pulling a vacuum on the system and seeing how long it will hold. Some vehicles will pull in air on cool down cycle after the vehicle has been shut off. I've seen it happen on GM intake gaskets and other vehicles with quick connectors on heater hoses that seal fine when hot but leak and or pull air in when cooling. If you give a specific model and make perhaps someone can give you a "pattern failure" diagnosis that is common to that model. HTH's Paul Watts

make and model might help some.

not having enough flow could cause an issue letting the coolant cool off in the core and not staying hot. is the air coming out cold/warm when it is below the 3k rpm. depending on how the fan is setup for the radiator, maybe it's possable to find a smaller water pump pulley so it turnes faster

Seems curious to me that your would write all that you did and not specify the make, model, engine option of the vehicle in question. Give us the info so we know what not to buy.

If replacing the heater core every year or annual flushing are required, I'd sure as hell get a different car.

Vacuum filling is a gimmick, not a requirement. Vacuum filling may result in a better fill, first time, without need to check/top off but, it will not remove all trapped air better than fill/run/check/top off.

I recently replaced the heater core on an F250 of mine. From looking at the unit, I'm sure that there's an air pocket, in the top tank of the heater core, that can't be filled with coolant unless I were to hang the truck from the front bumper. The heater work just fine with that air bubble in there.

Is it possible that your thermostat is closing during idle (low demand conditions) due to very low ambient temperature and a closed thermostat is affecting heater performance (coolant flow)? just a thought.

The orifice is to limit heater flow. Its commonly a washer that's fitted on 4 "legs" to be sort of loose, but "crushes" flat against a face as the dynamic pressure from increasing flow increases. In other words its a simple device to try to allow lots of flow at idle and restrict flow at high rpms. The basic reason is core sand acts as an abrasive ala extrudehone although slower, to wear out the heater core...and make it leak. The old school copper and brass cores were resistant, the advent of aluminum cores drove this device. Slow down the abrasive flow, make the heater core live longer.

On either inlet or outlet side thermostat (outlet side = think classic ironhead V6 or V8 with thermostat top front center) the heater core is a bypass around the thermostat and so naturally gets all of the flow when the 'stat is closed.

So you could be dealing with a thermostat which does not fully close, at higher rpms there is enough flow to produce heat.

Another issue could be a worn impeller on the water pump itself. I've seen that one also. Just wasn't pumping enough water.

what about the valve? it could be sticking and only open at higher pressure.

I disagree about vacuum filling being a gimmick. It is a required service tool for several car lines. I have had cars that took hours of fiddling after a repair to get the system full. Only minutes with an air lift/ vacuum fill tool. Without info on the details of the ride my GUESSES would be air in the system or a slipping impeller on the WP shaft.
Joe

Why would you not mention the make & model?

When Ive had these symptoms it was always the water pump. Worn out pump has low output at idle, but when you rev the engine flow is enough to provide heat.

Change (or at least test) the thermostat while you have the pump out.

My Volvo and other cars in the past is if the coolant is low the heat comes in waves....All I do is top off the radiator or the tank most cars now have. Another is could be a loose fan belt. Might have to relplace it or experiment and squirt some belt dressing on it when engine is shut off. The thermostat could be an issue or a leak in the vacum line going to the heat by pass valve and with low rpm's it doesn't get enough vacumn (my spelling sucks...lol) Oh and if you flush it, Put use a garden hose and BACK flush it not forward flush it. If your loosing coolant and it's not puddling under your car you may have a blown head gasket.
Rich

PS: sorry for the mispelled words...lol

I'm gonna guess it's a E46 BMW. Mine had the same symptoms. Slow leak in the system would allow a pocket to form in the heater core. That is, until the slow leak turned into a quick one quite abruptly.

Make sure your heat is on the highest setting and the key in the on position when filling/bleeding the system. My understanding is the heater core valve is electronic and high heat, high fan ensures its open. I'd guess this is likely applicable to many other makes and models as well.

While it does sound like you have a problem, some cars with small engines simply don't produce enough waste heat at idle to keep the passenger compartment warm.

On the low end of the tech scale, pressure bleeding the coolant system with a bleed screw cracked can eliminate air pockets that no combination of fill and drive seem to get out. On earlier (E30) BMWs, a couple of PSI applied to the overflow bottle while the bleeder screw above the thermostat is cracked will save you from trying to bleed it with the front of the car elevated 18", and a bunch of other time consuming 'tricks'. If you don't have the means to apply very low pressure compressed air, a crusty old mechanic once showed me that putting your lips to the overflow tank and blowing is a straightforward and quick way to get 'er done. There's been a few times at the race track where I've employed the trick, and it's never failed me. A brake pressure bleeder with a universal round adapter is a more 'tasteful' way to achieve the same results.

Tonytn36 said:

Another issue could be a worn impeller on the water pump itself. I've seen that one also. Just wasn't pumping enough water.

Click to expand...

I had that experience once. The cooling system would only work properly at high speed. After changing the thermostat and a couple of hoses I pulled off the water pump...no vanes left on the impeller.

John

what about the heater temp control? is it directly connected by a cable to a water valve or is it connected via a vacuum line? ala thermostatically adjusted by a wheel. possibly not opening enough at idle to let water flow into heater core.

The FSM for my Nissan v6 has a procedure for bleeding the cooling system that involves removing a plug on the intake manifold and the radiator cap and revving the engine to 3500 with fans set to full. This is a very messy affair as it burps coolant out the radiator and the fans blow it all over the engine bay.

Does this car have a electric fan? either way may be too much airflow over the engine while cold. very little pressure in a cooling system so any restrictions reduce flow very fast. A hotter thermostat may help.
Bil lD.

Not to be a pain in the ass but, have you considered taking the vehicle in for professional service? I'm all for do it yourself but your location says Vermont and this is February. Not a time to have defective heating and defrosting.

Now,

Air in system, possible leak

Defective thermostat

Clogged heater core

Engine management problem, including possibly turning fan on at wrong time

Defective sensors lying to engine management system

Water pump issue

Low coolant recovery tank, pulls air back in instead of coolant

Any combination of the above

I've said it before, and I'll say it again; without proper diagnostic tools chasing problems with modern vehicles is an exercise in frustration. At a minimum you should have a good multimeter with temperature measurement capabilities.

I was not trying to be deceptive or difficult by not including my specific model. (A 2003 Audi with the 2.7 liter Bi-Turbo)
As stated in the original post, this issue is not unique to this make and model, but it is common. The same trouble shows in many models both foriegn and domestic. (Search the net for an hour or two using terms like, "Heater Core flush") Granted also, this modle is a royal pain to bleed the air from the cooling system. The audi special tool is an expansion tank filler neck extension to get the filler to the highest point of the system. Other wise the high point IS the heater core. (dumb)

I am trying to form an educated assessment however. The "make and model" forums are filled with unrelateable conjecture (Things like "crank angle sensor" and "engine speed sensor".

For the suggestions of a deficient water pump, or thermostate, I can advise that the engine always runs a constant temp as monitored by both the instrument cluster gauge, and a PC based diagnostics tool through the OBD port. The Engine is making heat, and the temperature is being efectively controlled. The overall coolant temperature in the engine does not drop when the revs do. The idea that the thermostat does not CLOSE properly has interest, as there is a bypass that comes into play only when the thermostat is closed.

For the "air in the system" diagnosis, this I would like to better understand. Does flow diminish if there is an air bubble in the heater core? And if high flow conditions provide sufficient heat for a while, wouldn't the trapped air be moved out? I can't see an air bubble coming back again as soon as the revs drop (except through vacuum leaks as mentioned, I'll do a vacuum test as soon as I set up an adaptor.
I can see an air bubble reducing the internal active heat exchange capacity, I don't yet see how the air bubble problem can persist right after conditions of "good" heater performance.

Regarding the air flaps and flow valves. This vehicle has diagnostics for everything but driver. The entire HVAC controller has been probed for "codes" and basic settings restored. The air controls are stepper motors. Everything "appears good.

An added note, I personnaly flushed this core for over an hour using a garden hose and a catch bucket. Flow is free and clear with very moderate pressure. In fact just setting the bucket on the engine and alowing the water to siphon through showed full pipe flow from the outlet. That would be less than 1 foot of head (both ways too ;-) That does not seem like a blocked core to me.

In the present condition, heated output is running about 80 degrees from the center vents (Pocket thermometer stuck in the vent) I Couldn't see the thermometer dial last night when doing the HIGH RPM RUN ;-) but the thermometer needle was would around a bit more. maybe 115 F.

Now aren't you glad I didn't put this all in the first post! ;-)

We have a 93 jeep that has hoses above fill point and it is pain in butt to fill.

But there is a way.

Fill to almost top and start it.

Grab high rad hose and squeze it a few times as this will remove volume from there and replace it with water.

May need to do many times on warm engine.

You will see bubbles at fill point or level dropping.

Also be sure control valves working fully.

Measure temp of hoses with ir gun.

Excessive drop across core is low water flow and minimum drop is low air flow