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MG MGB Technical - Running Cold

We're having false spring here in South Carolina so I took the '74 mgb roadster out for a 30 mile spin. Although the ambient was about 70F, I started out with the heater on expecting to have to shut it off after the day warmed up further. Because the car hadn't been driven for about two months I checked all the fluids before starting. The coolant level was right up in the neck of the radiator where it's supposed to be. I checked it after 15 miles and the level hadn't changed.

This car usually runs just above the "N" on the gauge but it's never overheated. Well, this time the problem has just the opposite anyway. After the engine warmed up to operating temperature I started noticing the gauge dropping toward "C." By the time I had gone 15 miles the reading was just barely above "C" although heat was still coming out of the heater (the air may not have been as hot as it should be but I really couldn't tell.

I stopped to do some shopping and then had some lunch. When I restarted the car and drove another 15 miles home, the gauge never rose more than a smidge above "C."

My suspicions are:

1. The temperature probe has failed;
2. The gauge has failed; or
3. The thermostat has failed in the open position.

I rank these 3, 1, 2 in order of greatest likelihood but I'm no mechanic I'd appreciate your suggestions as to what is probably going on and, more importantly, what I should start checking first. I'm inclined to check the thermostat first because (a) it looks fairly easy yo get to and (b) I think I know how to test it but I'd really like your input.

Thanks.

Jud
J. K. Chapin

if 3, it would not go from N to C, maybe N to 7pm or 7.30

sounds like failed capillary. is your gauge mechanical rather than electric?

G
Graham Moore

Graham, the gauge is electric. I've attached a picture of the sending unit (sorry about it being sideways) which I assume is a thermocouple probe. I guess I could test the probe/gauge combination by removing the probe from the motor, clipping a ground wire to the probe housing and putting the probe into a pot of water that I'd then bring to a boil. If the gauge reads the temperature going up to 212F, the combo is working.

I think your statement about where the gauge needle would be is correct. I'd estimate the needle stayed about 7 (maybe an eighth of an inch to the right of "C."

I'll see if I can perform my experiment tomorrow or Wednesday and I'll post the results. Do you think the experiment will be valid?

Thanks.

Jud



J. K. Chapin

I tend to think no3
If the thermostat is stuck open a tiddle the engine will warm up ok and dodging around town it will stay warm but as soon as air speed through the radiator increases by going faster the temp drops off
Modern thermostats seem to fail this way opposed to getting stuck shut in the past causing overheating
It's a problem that a lot of later cars have and goes unnoticed till it gets to the stage that the temp gauge(if it has one) doesn't move at all
The problem with modern fuel injected cars is that the ecu detects engine temp for fuelling purpouses and there are 'lots'of cars running around running rich because there temp isn't up to scratch
Usuallt(not always) is you have a later car with fi and its using too much fuel a thermostat is a good starting point
willy
William Revit

For what it's worth, here's a picture of the gauge.

Jud



J. K. Chapin

Infra red thermometers are quite cheap these days. It will indicate block temp and confirm whether gauge sender or thermostat malfunction.
Allan Reeling

Just disgusting! This morning my friend Peter and I pulled out the sending unit, clipped a grounding wire to it, put the probe into an electric kettle, switched on the MGB and let the water in the pot go from room temp to boiling. The gauge tracked the rising temp of the water perfectly. Next we opened the thermostat housing. The stat was, as it should be for a cold engine, fully closed. I’d expected it to be stuck in the open condition. Next, we closed up the housing, started the car and let it run for about 45 minutes. After about 5 minutes at fast idle, the temp gauge began to rise. We monitored the temp of the block, the thermostat housing and the radiator with an IR thermometer and everything worked as it is supposed to. Initially the block warmed up and the stat housing and radiator stayed cold. Once the block temp reached about 160F, the stat opened and water began to circulate through the radiator. We ran the engine for about 30 more minutes and the temp remained stable at about 170F. I just hate a problem that is not repeatable. At this moment everything seems to be working properly. Next nice day I’ll take it out for another drive and see if it behaves itself. Hopefully I’ll put the new sending unit, stat and gaskets on the shelf for future use.

Thanks for all of your helpful input. Sorry I can’t report on what actually caused the strange behavior but hopefully it was just some transient event.

Jud
J. K. Chapin

Jud,

Just wondering,

Accepting a small difference in coolant/block readings at any actual coolant temp, the stat appearing to open at around 160F (in a stationary engine) suggests a low stat opening temp - eg from memory, in my '69 the stat is rated at 81C, (approx. 178F) and the needle ranges from two-needle widths left of the N to the needle edge just touching the left edge of the N. That does not seem to change much between ambient temps from -3/4C to around 35/40C

Also, is it possible that the removal of the sending unit for your test may have overcome interference in the signal between unit and the spade connection, or improved a compromised wire to spade connection?

Full marks for testing the issue and finding cause for feeling optimistic about the integrity of the system components.

Regards

Roger
R Taylor

Roger,
the stat could be a 'hot climate' 165f/74c (the usual one is 180f/82c and 'cold climate' 1905f/88c).

The stats, even those of the same nominal temperature, potentially have a varying and wide range of operation though usually they seem to be within a tighter tolerance than the range below.

The rating on a ‘water’ thermostat is a nominal temperature at which it starts to open (not when fully open).

As an example with info from just one manufacturer – a 165f water stat may have a +/- 3f tolerance so starts to open at between 162f-168f. The stat is fully open about 15-20 degrees above its rated temperature, so fully open at 177f-188f.

http://www.stant.com/index.php/english/products/consumer-products/thermostats/abcs-thermostats/


Jud,
I can't remember the exact details but you have to take infra-red thermostat readings as a guide because of technical and physical variation. If the gauge is working reasonably then that is near enough, you only need investigate, as you have, when it varies its reading widely from the usual. The 'C'/'N'/'H' gauges are better I think as you don't worry about the needle being on a particular number.



Nigel Atkins

Roger,
just as a comparison the 82c stat.

Figures rounded to nearest 0.5c

. starts to open - 80.5c–84c

. fully open - 89c–95c

Remember these figures are to full potential manufacturing variance and most stats seem to be well within tolerance but you could get two stats of the same nominal rating that vary in operation.
Nigel Atkins

G'day Nigel,

Yes, I'm all good with your comments. Thank you too for the data based nature of your addition - nice to have in the archives for future access. My gauge is the C/N/H type.

What I was suggesting in my first part to Jud (probably just mind wondering around his testing description), was that given my temp gauge reads a little low also, not withstanding the stat operating around 178F, that, if he was operating a lower rated stat, a step up might bring his needle nearer to the range of N.

It sounds like Jud may have overcome the problem though - which will be great news if that holds with the longer run he is yet to get a chance to do. Would be nice to know the outcome of that run.

Regards
Roger
R Taylor

I'm still thinking Jud's thermostat is starting to fail, either sticking open occasionally or opening too early sometimes which is the direction they seem to fail nowadays
I've only ever run 72-74deg thermostats in B's- They don't need to run up higher, but then I guess it's a bit warmer out here
William Revit

Hi Roger,
yes it'd be good to know if the outcome differs from Jud's last post.

I see what you mean about changing the stat to be nearer the 'N', indeed changing the stat even with the same rating might give a different reading if the new stat is within a different tolerance to the existing one - and of course if the present one is faulty.

I can't remember the details for Bs but they may well have run at different points on the gauges over the years and I'm not certain that BMC/BL would have a dedicated gauge just for one model, then of course there's the extent of the calibration of the gauge. Personally, if all else is good I wouldn't worry too much about the needle being very close to the 'N' (they're not on 1275 Spridgets that I've experienced).

There are so many variances to the model over the years and for different locations.

It certainly sounds like Jud and his B might be more used to warmer weather as 70f (21c) here would be very pleasantly warm and you'd be more likely thinking of opening the cabin fresh air vent and dropping the top than having the heather on.

If willy is right, very often is, then at least the stat is failing in the right direction and the stats are a very inexpensive part.
Nigel Atkins

The new stat and sending unit arrived yesterday and I'll test them today before putting them on the shelf for (hopefully) future use. I do have a small suspicion regarding the wire and spade clip attachment to the existing sending unit and will reexamine (and maybe resolder) it carefully.

Nigel, all of my cars (TD, MGA and MGB) have full weather equipment but I haven't raised any of the hoods in the past three years of all-year driving. Our group completed our fifth annual "Polar Bear" run last weekend. Thirty-eight cars (all [about 30] but the coupes and sedans had their hoods down) drove from Gowensville, SC, through Saluda, NC, and on to Brevard, NC with a mid way stop in Flat Rock. Beautiful twisty, steep Blue Ridge Mountain roads. We came back across Caesar's Head and down to Pumpkintown. The only issue was that the fuel pump on my TD quit while going up the grade to Caesar's Head. A few sharp raps with a stick and it resumed ticking and delivering fuel and I had no problems for the remaining 30 miles to home.

The point of that tale (sorry, I got carried away with memories of a great and fun drive) is that I had both the hood down and the heater on when I was having the temperature issue with the MGB.

The forecast is that we are through with 60-70F days for a week or so but I hope to get the B out for a trial run soon anyway and I'll report the results.

Thanks for all the comments.

Jud
J. K. Chapin

By the way, I plan on testing the new sending unit using an electric pot and my ohm meter. I think the unit is based on a temperature variable resister (I originally thought is was a thermocouple but have been told otherwise)

1. Is the unit a thermocouple, a variable resister or something else?

2. If a variable resister, does anyone know the ratios of resistance to temperature so I'll know what I'm looking for?

Thanks.

Jud
J. K. Chapin

Jud, according to Paul Hunt's website the electric sender is a thermistor-negative temperature coefficient resistor. There are three types of sender and Paul has listed their resistance / temperature characteristics as follows:

°C °F Resistance 'C-N-H' indication
Red White Black
0 32 1605 1848 2283
10 50 997 1055 1420
20 68 640 650 928
30 86 440 410 594 'C' (Cold)
40 104 303 264 392
50 113 208 174 263
60 140 150 122 184
70 158 108 82.5 129
80 176 87.0 68.0 100
85 185 70.0 54.0 82.0 'N' (Normal), about the temperature of a typical thermostat
90 194 58.0 43.8 69.0
100+ 212+ 47.0 35.1 58.3 'H' (Hot)

See Electric Temperature Gauge section on this weblink

http://www.mgb-stuff.org.uk/electricstext1.htm#tempgauge

Andy
Andy Robinson

Jud,
I thought there was a table somewhere showing resistance but I might be thinking of fuel sender, I also thought there was a Moss video for this but don't rely on my memory for either.

If you have a bit of spare length on the wire I'd cut off the present spade and crimp a new spade to new stripped wire end as soldiering can be a bit brittle and damp can creep under the wire insulation so cutting the wire back will check this too and get a cleaner connection. I like to use Contralube 770 on the connections too.

Good on you all for having the roofs down, my comment was only a comparison of different perceptions, whilst you obviously don't mind the cold you had the heater on for 70f as you're used to higher temperatures whereas generally we're not over here so would find 70f to be warmer than you and not put the heater on.

Different people different ideas, I'd never put up with a ticking fuel pump, after the first time I had to knock it back into life it'd be swapped and for a non-SU labelled pump (they're overpriced to me), I've got a Hardi (was Q&H labeled when I fitted) electronic pump. as always each to their own.
Nigel Atkins

Nigel, heartily agree. Cold never bothered me until my uncle (Sam) sent me to Thule Greenland 48 years ago. Had all the cold I'd ever need but I do drive topless as long as the temp is above 35F (1.6C) and it's not raining. I just had the heat on because I'd forgotten to close the valve I'd installed under the bonnet and was too lazy to stop in route and attend to it.


Andy, thanks for the table. I tested the new sending unit this morning and got pretty much the same readings.

Here are the details:
Temperature (⁰F) Ohms

60 783
70 662
80 542
90 430
100 345
110 275
120 220
130 186
140 152
150 128
160 107
170 89
180 76
190 63
200 55
210 45

I'm at about 900 feet abole MSL so I can't heat unpressurized water above about 210. Test rig shown below.

Thanks all.

Jud



J. K. Chapin

Hi Jud,

I had a similar problem several years ago, and recommend you give the Stant Superstat a go. Different valving than the standard to assist faster engine warm ups (their words). Worked for me.

Regards,

Lazza


LC

The easiest way to check the electrical temp gauge circuit, when it has dropped below what you think it should be, is to connect an earth to the connection at the sender. That should drive the gauge smartly to above H, but don't leave the earth on any longer than that.

If it does so then the sender is not exhibiting a low enough resistance, either because it is faulty or the coolant is too cold.

If it makes no difference or only rises part way, then the circuit back through the gauge is at fault.

If the fuel gauge is similarly affected then the stabiliser circuit is at fault.
paulh4

This thread was discussed between 04/02/2019 and 09/02/2019

MG MGB Technical index

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