HEATER - Set me straight

This has been bugging me for awhile, and since I have the same symptoms on both my '75 MGB & '78 Mini, it's time for me to seek the expertise of all of you.

Temperature gauge working fine, and running close to the "N" range. When I turn on the heater "fan", it starts with warm air, and then rapidly (30-60 seconds) becomes more cool, as the temp gauge hand drops about 1/4 of the dial range. When I shut off the fan motor, the gauge goes right back up, just as rapidly.

Thermostat is new as is the anti-freeze. Ideas? Cure? Brrrrrr...

Steve.in.Roanoke

In theory your temperature should drop off a little when you turn on the heater. That is a common way to help dissipate heat when your engine is over heating. In essence, you are running your coolant through a second radiator, albeit a much smaller one, with a fan to draw the heat into the cabin. As to the range that the gauge is dropping, are you running the correct range of thermostat, and have you checked your coolant temperature using an actual cooking or infrared gauge to confirm all is working as reported and it's not a wiring or gauge/sending unit issue.

When you turn on the heater for the first time during a drive, you're opening a valve and letting a bunch of cold coolant into the engine. The temperature should drop slightly and then come back up within a minute or so.

It's actually happening on two different vehicles! Same symptoms.

I had a poorly functioning 160* thermostat that I replaced, but with that, the difference was just as dramatic, just in a lower range on the gauge.

Also the air from the heater that starts as warm, rapidly becomes cool ???

Thanks for all insights.


Steve.in.Roanoke

sounds like your heater valve is mostly plugged (or not opening). When the water slowly seeps into the heater it will warm the heater water, but when you turn on the fan it cools the water in the heater matrix and since no water is flowing in fast enough, never gets warm.

balloonfoot Wrote:

Lee and Steve S. are probably closer to right about what is happening. There will be a short duration dip in coolant temp from opening the heater valve and getting the coolant to move. As stated, what has been sitting in the heater core will be colder in comparison to what is already flowing through the rest of the system, and will have a cooling effect on the mainstream coolant ~ to some extent. To what extent is dependent a bit on 1) how much cold coolant is being released from the heater core into how much heated coolant and 2) the relative temperatures of each.

However, none of this should be very dramatic. Neither the time it takes to heat or recover, or the temperature differential at the start of the process, certainly not a 1/4 drop in the range on the gauge. I live in a climate that is probably at least as cold as yours, perhaps colder, in winter and I do not have this same problem. Therefore, I would have to wonder if it is something that both cars share in common. There isn't much, I would assume, other than the coolant used (if the same) or the T-stat.

I would guess that you are using an ethylene glycol mix with water (50-50) and nothing more added to it??? I would suppose it is possible that it could be your choice of a 160F T-stat. A relatively easy and not too expensive way to find out would be to replace it with one that opens at a higher temperature, a 180 or 195F T-stat. The higher temp T-stat will not destroy your engine at this time of year (I run my car with one all year) and could hold the coolant in your engine until it reaches a high enough temp not to be "overwhelmed" by the cooling of the cold air through the radiator. At the higher operating temp, your heater would benefit from both a wider differential between the cold air passing through the heater core and the temp of the coolant, and also because the higher temp coolant would have more BTUs available to overcome the chilling of the cold air as it passes thru the heater core. I think it is worth trying a different T-stat just to see if it does make a difference. I think it could.

Also....
- Grab the supply and return heater core hoses and see if the temps are
a) the same, b) hot or cold... This with some deductive reasoning should
assist in the diagnosis of the valve.

- Grab the lower and upper heater hoses. Same as above.
- Open the cap and check for flow in the tank.

It could be that your theremostat is either too cool or not not performing
correctly. The heater hoses should be pretty hot both in and out. The engine
guage should be N or higher.

On my 75, if it isn't N or higher, the heater output isn't all that great. I run 190 most of the time, and a 180 only in the peak of the summer.

Bob - I believe you picked up on all I was saying, and thank you. I already have the higher temp thermostat in, with the same symptoms. Yes the typical anti-freeze with a 50-50 mix.

Ernie - This is a great starting point for me. It's one of those, gee, why didn't I think of that.

In the back of my mind I'm thinking I need to back flush the system. At least that's what we used to do. Any thoughts?

If I pull the heat valve, then it's likely the restriction at the engine and need a new gasket after cleaning out the valve. Guess I'm just being lazy, looking for the easiest "down & dirty" way to get the job done.

Lloyd - Does a '74 C3 L82 count?

The thing that is truly odd to me, Steve, is that you have the same symptoms in both cars. Obviously, they both (apparently) share the same type of coolant, the same ambient weather conditions, are probably using the same fuel (BTU content delivered into the engine and coolant), and both systems overall seem to be working as they should (water pumps moving coolant, radiators flowing well, hoses not collapsing, pressure caps functioning properly, etc, etc.)

If you want some notion of what to do to clean out your heater valve, go here: http://www.cibolas7.net/12245.html If you do clean it out, you may find your problem for both vehicles.

Steve.in.Roanoke Wrote:

To tell the truth Bob, on the Mini, I'm not sure of the engine temperature fluctuation as that gauge doesn't work. But, after a couple of minutes of the fan running on it, it too starts blowing cold air. The link and info is great. Thank you.

While low flow through the heater core could explain the lack of heat in the cabin, it makes absolutely no progress towards explaining the MGB temperature gauge dropping and staying down. That symptom would indicate that the heater core is taking a substantial part of the heat away from the engine. That's at odds with low flow through the core. And that heat would have to go somewhere!

Maybe the gauge is actually indicating a drop in the voltage applied to it due to the current drain from the heater fan? The voltage stabilizer is supposed to adjust for varying supply voltage, but maybe yours is not functioning?

As Lloyd and Ernie said, do the hose thing first. Even in summer, the heater core will get somewhat warm simply by virtue of heat conducted via the coolant in the hose and up to the heater valve, albeit, not nearly as much or with any capacity to affect the air passing through to the cabin as long as the valve is shut. So if the core is bad and the valve open, you could be getting essentially the same affect in the winter from reduced flow through the core. And it may simply be the same problem in both cars, something I don't usually think of as being the usual course of luck for myself or most people.

Are you sure you have a sufficient quantity of coolant in the car and is the thermostat (new or not) is actually working?
The heating systems in B's are not the most efficient. Any blockage in the core or heater valve makes a huge difference in heat output.

balloonfoot Wrote:

Well, here's where we're at.

Drove the car a few miles this am & pulled off the road, propped the hood, & started feeling hoses. every one I grabbed was good & warm, not scalding but hot.

The heat valve has been "jerry-rigged" by the previous owner & has wire wrapped around it, so I'm going to replace it regardless.

Drove on & fiddled with the blower fan switch, off - on - off - on, etc. Well my observation is that Mr. Lucas has been playing with my heater. As the temp gauge goes down, so too does the fuel gauge. Fan off and they both go back up.

I'd say I've got two parts to my dilemma, the electrical stabilizer, and the heat valve. Does that sound right?

Appreciate every ones expertise in guiding me to a solution...

your problem is not water circulating 'cause both hoses are hot...it is electrical. Probably is not voltage stabilizer....might be bad heater motor drawing too many amps. Turn on headlights...do they go dim when you turn on the heater motor?

Lloyd - You've been nursing me through this dilemma, and i really appreciate your help.

The headlights are not affected at all when I turn on the fan blower switch.

I replaced the alternator a couple of months ago, but this problem pre dates the alternator dying.

I figured the electrical stabilizer because the fan being turned on also lowers the fuel gauge level.

With all that said and done, the heat goes from warm to cooler after a couple of minutes, so maybe even though there's circulation, it may not be adequate.

Thoughts?

Steve, run that car over to Webb Motors on Williamson Road out towards Hollins and let Byron Webb get you straight.

I've already ventured too many opinions, but if the HEAT and not the Temp on the Temp gauge drops off, then it is a coolant circulation problem. I am guessing that you are judging your heater's output more by how hot it feels regardless of what anomalies are taking place at the Temp Gauge. If the gauge is also dipping, it might be reflecting coolant temp, but it may not. The two problems may be only incidentally related, but if you are judging the performance of your heater by heat output, regardless of the temp to your hand of the hoses, then it says to me that not enough of your heater core is doing its job of transfering heat to the air coming in, even if enough hot coolant gets through to heat both hoses.