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MG MGB Technical - It's a hottie

Thanks to this B/B and an under$tanding wife, the '73B is doing pretty good. The new, .o98 head gasket is holding and life is good. The, hopefully, last issue is that the car runs quite hot, almost to the "H" mark! It has an electric fan, freshly rodded radiater, new oil cooler and Evans coolant. The pistons are .20 over and there are about 800 miles on the engine rebuild.

Is this a normal condition for a new rebuild that more miles will fix? If not, where should I start looking? Oh yeah, no pinging under load. Thanks, Tom
Tom

Tom,

Glad you got it back on the road.

Have you replaced the thermostat? Have you checked to see if the gauge is telling the truth?

Charley
C R Huff

Hey Charley, thanks, in part, to you and FRM I'm on the road again. I will research the thermostat issue, but the one in the car is new. I saw a picture somewhere that showed the "coil" facing down so that's how I installed it.

The temp sender issue has been mentioned before but are the hot/cold ohm readings available for the old & new units?

Do you know of the significance of a thermostat sleeve?

Regards, Tom


Tom

Tom, There are a few different temp. sending units.(you may have the wrong one). Put a thermometer in the radiator to check coolant temperature. Instead of changing the sender, I installed a resistor (Radio shack) in the circuit to obtain the proper reading on the gage.
Len Fanelli

Hey Len, good to "hear" your voice. I will check that out. Do you know if the new IR units are accurate enough? I see them starting to show uo at various parts places, often at a reasonable price. Regards, Tom
Tom

If the coolant really is getting that hot then take out the thermostat altogether, for testing purposes only, it's not good to run them like that. You can test you stat in a pan of water being brought towards the boil, stat and thermometer should be suspended off the base of the pan.

Thermostat sleeves were recommended with the old MGA type 'bellows' stats as the heads had a large bypass port which resulted in a slow warm-up, the opposite problem of course.

Without a stat you will get a similar slow warm-up, but if then still goes up towards the H you know either the engine is generating much more heat than it should or the radiator isn't getting rid of it as it should.

Freshly rebuilt engines can run hotter when things are still a bit tight, but not to that extent unless there have been mistakes made in the rebuild. Check the timing and mixture carefully, retarded timing results in hotter running, as can overadvanced but that should cause other problems like stalling the starter or pinking uder load. It would have to be very retarded and underpowered to make that much difference though.

An IR thermometer is useful for looking at the temperature gradients of a radiator, but it needs to be as close as possible or it starts averaging over a wider area, even with a 10:1 aspect ratio, and some are only 1:1 aspect ratio. Alternatively just feel across the surface with your hand for cool areas.
Paul Hunt

Hey Folks, I went and got an infrared thermometer to help with this problem. I took some measurment while the gauge was at 3/4, and read 180 on the "neck" above the cars thermostat and 231 on the senser, just below the thermostat.The back of the head read 245!

It is my understanding that the hot head coolant passes through the thermostat on its way to the top of the radiater. Should the two measurments not be near the same temp?

Also, the senser reads 720 ohms to earth when the engine was cold, (about 78 degrees F), and about 40 ohms @ 3/4 gauge, (about 200 degerees F).Is this correct for a 73? Thanks as ever, Tom the nephyte

Tom

As the 'neck' (connection to top hose?) is further away from the head and the head was even higher than the sensor, then I would say that is just a 'normal' temperature gradient and the top hose connection is showing the actual coolant temperature more accurately than the others.

What does the radiator right by the top hose show? I can't recall having measured things like the block and head with mine, just parts associated with the coolant circuit. 245 for the head does seem high though, is that parts of it or all along? It could be there are blockages in the head and/or block passages so not all cylinders are getting a flow of coolant, resulting in some parts of the block/head being very hot but the coolant being relatively normal at 180F (in fact 185F *is* N on the gauge). However the sensor needs to be 'seeing' only the part you are measuring, and not surrounding components or air as that will give an average temoerature. Ignore the laser spot, that isn't measuring the temperature at that point as some think but is just an aiming pointer, for small parts like the thermostat outlet the sensor needs to be almost touching the part being measured.

OTOH gauge sensors only work if they are in contact with coolant, once the coolant level drops away from them they tend not to indicate any increase in physical engine temperature - BT, DT. So if your gauge is reading H then either the coolant level is correct and actually that temperature, or the gauge or sensor is wrong.

From work Ralph in Germany has done the resistances are as follows:

C F Ohms
0 32 2,307
20 68 885 'C' (Cold)
40 104 384
45 113 317
60 140 155
75 167 112
85 185 82 'N' (Normal), about the temperature of a typical thermostat
90 194 71
95 203 62 About the point at which the later electric fans cut in
100 212 54
105 221 47
110 230 41

i.e. very close to your figures. If the instrument voltage stabiliser is faulty it will result in gauge readings noticeably higher than normal, but will also be affecting the fuel gauge. The light-green/green wire feeding the 'stabilised' voltage to the gauges should be showing system voltage i.e. 12v to 14v switching on and off about once per second (after showing system voltage for 2 or 3 secs after first switching on the ignition) for a factory stabiliser, or about 10v steady for an after-market electronic unit.
Paul Hunt

Hey Paul Hunt, THANK YOU very much! :) That is the data I was looking for. Also, thanks to Ralph in Germany for sharing. I hope his readings are for the older sending unit.

I measured the highest temperature at the rear side of the block, by the spark plugs, as close as I could get. I was concerned with the thermomitor seeing too large a "window".

As Len suggests, a 100 ohm resistor in series with the sender would probobly bring the gauge down to "N" but my primary concern is the proper operating temperature range for the engine.

As an aside, FWIW, I purchased this car entirly rebuilt but disasembled completly and with no interier. It has proved to be a 3 year project and I hope this is the last issue.

Thanks and regards, Tom
Tom

Hey Folks,

I finally got some time to further the search concerning the hot '73. I drained the cooling system and removed the themostat. (Paul Hunts information kind of eliminated a defective sending unit).

I testrd the thermostat on our kitchen stove and found it opened fine around 180 degrees F. I next disassembled the unit and put just the "washer" portion back in. I have read about the lack of a restricter preventing the radiater from cooling the fast flowing fluid.

After filling the system back up I took the car for a test drive. It was 90 degrees F. and very humid. The gauge needle never got above 3/4, about a 15 to 20 degree improvement. I will keep the car like this until I see what happens on a cooler day.

My question is this; why do some (knoledgable) MG folks advise against leaving the thermostat out completly?

As ever, thanks for sharing your opinions and knowledge.

Tom
Tom

Tom, The prevailing theory is that the velocity of the water leaving the cylinder head and flowing through the radiator is too great for it to be exposed to the radiator heat dispapating fins long enough for the coolant to drop in temperature. The thermostat, or blanking sleeve, slows the coolant velocity down allowing for it to remain in the radiator long enough for the temperature to drop a reasonable amount. RAY
RAY

Ray, close, but "no cigar". The issue is cavitation of the water pump impellers, due to no restriction because of the thermostat being removed. (Also called head pressure.) Also a veh. with no thermostat will take too long to warm up,choke is on too long, & condensation does not evaporate from crankcase, leaving "mayonaise" in the rocker, (valve) cover, & fill cap. Tom what is the temperature of the coolant, cap removed, with a cheap thermometer, imersed in the coolant, at a gauge reading of
"N", (in the middle of the range?)
Len Fanelli

Hey Len, I drove for over an hour today and it never went past 3/4. I will go to a dollar store or such and get a themometer tomorrow. None I have go that high except the infrared one. Tom
Tom

It's flowing through the engine as fast as it is flowing through the radiator, so if not being in the rad long enough to give up its heat were the problem it wouldn't be in the engine long enough to pick it up either. It's not cavitation either, which is more a function of the speed of rotation of the pump blades, which isn't different to before. It *is* related to speed of coolant flow though, but too fast a flow causing local turbulence and recirculation in the engine which causes hot spots by not passing the hottest coolant through the rad. In temps of 90F choke use isn't the problem that it would be in cooler conditions, and removing the innards but keeping the plate is basically just what restrictor plates to replace the thermostat are. I wouldn't go as far as that, just remove the spring and leave the pellet in place, which will open and stay open. 3/4 the way to H still sounds way too high to me, we get that in the UK occasionally and mine has never been anywhere near that. Having said that it is only slightly higher than where electric fans are designed to cut-in, so isn't going to cause a problem to the engine. But there is still something else wrong that opening up the stat is only working around.
Paul Hunt

Tom - You stated that you had the radiator rodded out. I would ask if you had the block hot tanked and all of the oil and water passages cleaned out? A free flowing radiator is great, but you also need a free flowing block and if the water passages are blocked with years of rust and sediment inside it is at least as much of an impediment to proper cooling as a blocked radiator. Also, do you know if the radiator is an original unit or perhaps one of the cheap after market units? If the latter, the rodding of it may have been a wast of time. Cheers - Dave
David DuBois

Len, Not to belabor the point but cavitation is caused by a system that is run without a radiator pressure cap which causes as much as 20% reduction in the volume of water passing through the pump. RAY
RAY

Hey folks, sorry for the information omission. The engine was completly rebuilt by the former owner to be more powerful. ( The shop that has done 2-XPAG's for me did the work and it is first class). I know that the "B" has the Moss, (read Crane), upgraded cam and .020 over pistons, a shaved head and hardened valve seats. I don't know how to tell if the radiater is original. I do know it has no drain cock!

I just ordered a blanking sleeve from Moss. I will put the sleeve and a working thermostat back in the car as soon as time permits. I will keep this thread informed.

As ever, a thankful Tom
Tom

Hey Folks, here is the latest on the running hot issue. I checked the sending unit with Paul Hunts information and everything was reasonably close. I next ran the car without a thermostat and it still ran hot. I proceeded to install a themostat with the "guts" removed, to use it as a restricter. Better, but not good enough.

Lastly, I installed a Moss sleeve and a new thermostat. That seems to do the trick. While the weather has not been very hot, I have only been able to raise the gauge needle a little bit on a long, slow hill at about 75 degrees F. air teperature. If we get some really hot weather I will re-test it then.

Thanks to all, Regards, Tom
Thomas McNamara

Interesting, I wonder if it has an old head. I've seen sleeves recommended for MGAs, as they reduce the amount of recirculation which bypasses the radiator. A large amount of recirculation is good for a quick warm-up, but not for hot conditions. The original stats for those cars had a cylindrical sleeve on the stat (a 'bellows' stat) which blocked off the bypass port when the stat opened, but the more modern stats don't have that. See http://www.mgb-stuff.org.uk/wn_coolingframe.htm and click on 'Thermostats ...'.
Paul Hunt

Hey Paul, (and others), as far as I can decern, the head is original to the block. It definatly runs cooler so far. regards, Tom
Thomas McNamara

All the heads have the same bypass port. The main reason to use a thermostat is to prevent engine wear in the front cylinders of an engine. Any mechanic thats been around awhile will tell you that if you pull any engine down that has been run a long time without a thermostat that the front cylinder or in the case of a V8 the front two cyls will have a lot more lip in the bore (some times two or three times) than the other cyls.
The other points mentioned are relative also, particularly the pressure cap serviceability to prevent "local" boiling. The water flow speed is a point that even the experts cant agree on, but I must say I like the restriction of a thermostat and have actually seen water pumps with the vanes eaten off with cavitation. The best way to get by with a us thermostat is to turn the two opposite sides up. I use the sleeve and a thermostat on my supercharged B and I did not notice any difference but I was not having any trouble before hand. One thing that must be mentioned is that if you use a thermostat and a sleeve you must drill a hole in the thermostat or if it has one enlarge the hole with the little toggle in it to about 3/16"-1/4" to let the air bleed out and make filling the system easier.

Denis
Denis4

This thread was discussed between 08/08/2009 and 08/09/2009

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