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MG MGB Technical - Running hot

I moved over to the technical side of things rather than the general board.

My newly finished car is running very hot it seems. Was getting to almost 120 C (nearly 250F) when stopped in traffic and generally running at over 100C (212F) when moving. 120C is maximum on the gauge. 80 is the mid point on the gauge.

It's a 78 American B but it's been 'fixed'. It is a newly built engine. Nothing fancy, just 20 thou over from memory and with HC pistons instead of the LC ones. No emissions gear at all. HIF carbs with K&N filters instead of the standard cans. I have a nice, big shrouded fan pulling air though the radiator from behind (and it is blowing the right way!). Radiator was re-cored.

Tonight I warmed the engine and measured the top hose temperature on the outside with a thermocouple. I think the thermostat starts opening around 65-70C (according to the temp gauge in the car) since that's when I start seeing the top hose temp start climbing rapidly.

I also adjusted the idle speed down to 800RPM from about 1100RPM which was too fast obviously. The timing I set to about 14 degrees. That was off little too at around 17 before I tweaked it. I have also enriched the mixture from my initial setting by half a turn now. After my adjustments I went for a drive. The temperature seemed better but still a bit hot I think. I pulled the plugs (NGK BP6ES) and I have attached the pic below. This shows plugs 2, 3 and 4 in order. 3 looks a little sooty (no sign of sweep). Plugs 1 and 2 were pretty much the same and are right I think.

What else can I try before I give up and get Garth to look at it for me! With a new engine I guess you'd expect it to run warmer as it's broken in. How long should that take? I have just tonight done the first 160km (100 miles).

Simon



Simon Jansen

Simon, have you used your thermocouple to check the readings provided by the gauge as I am guessing your in traffic readings came from the gauge?

David
David Witham

I need to put the thermocouple inside the hose for that I think to get a true reading. But reading on the outside of the hose wasn't far off and went up at the same rate as the gauge. The other, similar thing I thought to do is check the gauge sender by putting that in boiling water with the thermocouple and making sure the temperature reading is accurate.
Simon Jansen

You don't say what the thermocouple on the top hose said, on the header tank would be more accurate especially with some insulation medium on top. Stats are normally 82C, or 74C for hot climates and 88C for cold. Electric gauge I assume, these will read high if the voltage stabiliser is stuck fully on, easily diagnosed by seeing if the temp drops almost immediately with the engine stopped but the ignition still on. There can also be a mis-match between senders and gauges.

Other than that, and in general terms, either the engine is producing more heat than it should, or the cooling system can't get rid of it as it should.

The former can be things like tightness, retarded or over-advanced ignition, weak mixture. The latter blocked core, thermostat not opening fully, or incorrect or corroded pump not circulating coolant properly. A laser-pointer thermometer can check for cool spots on the core easily enough, and checking by the outlet should reveal if there is insufficient circulation by the outlet being a lot lower than the inlet. Conversely if it is hardly cooler than the inlet that would indicate circulation is OK, so unless the core is blocked (either coolant side or air side) it would seem the engine is generating too much heat.
Paul Hunt 2010

Paul, I stopped paying attention after I worked out the thermostat seemed to be opening. But it was generally about 10C less than what the gauge read.

These are all new, aftermarket gauges and matching senders. No voltage stabiliser needed. I will pull the sender when I get the chance and make sure the gauge reads a true temp.

The block was all cleaned out before assembly and the water pump is new.

One thing worth mentioning is I don't have an oil cooler.

Silly question but is it possible to put the thermostat in upside down?
Simon Jansen

It certainly is. Someone on another forum bought a late model B and when he removed the thermostat housing the thermostat was installed upside down. It even had Top printed on the surface that was facing downward. A newly rebuilt engine will run hotter for about the first 500 miles as the new parts bed in. Did you flush the radiator out before you installed the engine? It could have deposits that built up over the last 32 years. RAY
rjm RAY

I don't think I put the stat in upside down but I do seem to remember puzzling over it a bit and I have done stupider things in my time! The engine was assembled in 2004!

Radiator was rebuilt/recored by a radiator company so it's basically new. Seemed silly to have a rebuilt engine and an old radiator so I made sure everything was new.

I just need to open the fill plug and I can see if the stat is right. Only I won't be home till tomorrow night to check. Not knowing is killing me!

Simon Jansen

Simon-
The fact that the temperature rises so radically once the thermostat opens obviously means that something is making the coolant very, very hot. Let the engine run with the thermostat open and the radiator cap off. look into the filler neck and see if there's any bubbles. If you see any, then there's a very good chance that combustion gases are leaking into the cooling passages of the head. That would explain the high rate temperature change when the thermostat opens, along with the hot running condition.
Stephen Strange

Simon, I had a over heating problem with my 77 after rebuilding the motor. It is possible that the cooling system is not full and has air in it. When cool check the expansion tank. If empty, fill to the 1/2 full level and replace the cap. Next turn the heater to hot, remove the filler plug and top off to the bottom of the threads. Start and run the motor until the top hose gets warm. Turn off the motor and when it cools check and top off the expansion tank to 1/2 full,remove the filler plug and top off to the bottom of the threads.
Hartley Mayers

"The fact that the temperature rises so radically once the thermostat opens obviously means that something is making the coolant very, very hot"

Surely that is normal? To a large extent anyway. The header tank will remain cool until the stat opens during warm-up, typically at about 82C, then gets very hot very quickly, too hot to keep your (my) hand on.

If the coolant is getting up to 120C, or even 100C once the stat has opened that is the problem, not that the radiator suddenly get very hot during warm-up.

What cap and coolant do you have, i.e. plain water or anti-freee and if the latter what solution? Plain water needs a pressure of 16psi before it will get up to 252F without boiling, 12psi with a 33% solution of Ethylene Glycol.
Paul Hunt 2010

OK, worked it out. Stupid user error. I put the thermostat in upside down! I reversed it and refilled with coolant (I am using 50/50% coolant to anti freeze I think from memory) and went for a 6km or so drive. Temperature now doesn't go above 100C which is 3/4 up the gauge.
Simon Jansen

You havent by any chance got a water pump with a severly coroded impeller have you or alternatively is the impeller slack on the pump shaft as I had recently. I wouldn't have thought that the thermostat being in upsides down would have made that much difference once it had opened in the first place.
Iain MacKintosh

Was a brand new pump so the impeller isn't corroded. The old one definitely was. How would you tell if it wasn't spinning? The radiator definitely gets warm so hot water is flowing into it somehow at least.
Simon Jansen

Simon,

100C still seems a bit high to me. My 70 BGT is running at 160-170F (about 70-75C) in normal traffic and/or highway speeds with a standard cooling system and just the engine driven fan...and I'm in Brisbane with it's higher ambient. Highest coolant temp I've seen is in the depths of summer (38C ambient) in stop/start traffic at 200F (about 95C).

Have you done a pressure test to ensure no nasty leaks?

John
J Tait

The inlet will get hot even if the flow is inadequate for any reason, like pump, or blocked rad. The important thing is the outlet temp. If that is much lower than the inlet then there is probably inadequate flow. If only slightly cooler than the inlet then circulation would appear to be OK, either a large part of the core is blocked (scan it with a laser-pointer thermometer, close up) or the engine is generating too much heat.

There are two pumps for MGB engines, and I'm not sure of the details but in once case at least both fit but one gives inadequate circulation.

Disregard logic and thinking "it can't be that because ...", go for the simple things first and check them regardless, see 'No Oil Pressure'.
Paul Hunt 2010

This thread was discussed between 26/04/2010 and 29/04/2010

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