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MG MGB Technical - overheating

Ok I live in Florida so it is hot, but the 78 B is overheating on a 18 mile run, fans are working , flushed the radiator, thermostat tested and opens at temperature. Boiling water when I get home, needle not quite in the red.
I run a C on alternate days to work and this has no heating issues.
Would appreciate any suggestions.
Dave
DJ white

If the fans have been replaced make sure they are blowing the right way. I guess unlike UK spec yours came with two fans. But if it did not, there has been a problem with overheating before, for it to have two.
c cummins

It could be the cooling system, e.g, no pressure in system (leaks or rad cap), blocked cores, etc.,............does the heater run hot? But there are other causes of overheating. 1. over advanced ignition. 2. Weak mixture. 3. binding brakes 4. slipping fan belt 5.oil pump problems. But look at the simple and easy to check/fix first. Rad cap, mixture, timing, brakes.
Allan Reeling

Agree with rad cap first, should be 15lb. I've seen my V8 on the verge of the red and it wasn't losing coolant, but a pals 4-cylinder was spitting it out with the temp gauge only half way there from the N. Tested his cap at about 3lb!

As the C is OK in the same conditions it must be a defect. Could be something causing the engine to generate more heat than it should e.g. 1 to 5 as above or could be partial blockage of the water passages in the block, or something causing the radiator not to get rid of it. Try an infra-red thermometer on the rad looking for cool spots (low dissipation), and measure the inlet and outlet temps. Hold it as close to the thing being measured as practicable, although it may have a laser pointer the area it is sensing will be much bigger than that. If measuring a small object from far away it will be averaging it with the surroundings to give (probably) a lower temperature. If the two are very similar it indicates decent flow but either too much heat being generated, or not using the whole rad. If there is a reasonable difference between the two (you will need to compare it with something else) it implies low flow, which could be pump problems. Also measure along the head to see if some parts are hotter than others, which could be blocked water passages. Some thermometers give erratic readings if too close to the HT, so switch off and measure.
PaulH Solihull

Thanks all.
Rad cap is only 12 months old, I have a '78 so the expansion tank has the cap.Nothing changed on my tach/mph ratio and I have an overdive switch which I use at about 45 MPH, belt is not slipping. I flushed the rad through the thermstat housing and got a very good flow through.

The infra red light thingy is a good idea. But I have a feeling its the pump, how can I check this?
Dave
DJ white

As it's the pump that generates the flow then as above I'd expect low flow to show up as a significant difference in temperature of the inlet to the outlet i.e. the water is spending too long in the radiator and getting cooled OK, but it isn't getting it out of the engine fast enough. Compare it to your C.

I did mean to say that whilst a bad cap can cause coolant loss with normal or only slightly higher temps, it can't in itself cause those higher temps. Remember new parts can be faulty. I knocked up a pressure testing point out of an old inner tube Schrader valve and housing soldered to two stubs of copper pipe of the same bore as the ports on the rad and expansion tank, inserted in that hose and an extra bit of hose, then pumped it up with a tyre inflater.
PaulH Solihull

You could try it without the stat for a short time preferable with a blanking sleeve. And check what temp stat it is, there are winter ones which open at 190 F, and summer at 165 F, but these are for early cars.
c cummins

DJ an easy test, remove all four spark plugs put a pressure pump on the radiator bring it up to 14 psi and let it sit while you watch for pressure loss after 5 min. have your helper turn the key to spin the motor while you watch the plug holes for coolant spray.Any spray indicates a bad head gasket/cracked head.Keep us informed. RIC
RIC LLOYD

I have been away for 10 days.
Took the stat out and have run the car for 3 trips to work and back 16 miles each way.
I have noticed the temp gauge is not as near to the red zone as previously( previously just touching red zone).
Boiled the stat in a pan of water its a 165 F unit ( Florida)and opened slowly , but of course I could not accurately judge the temp's

Any ideas would be welcome for next steps as obviously still an issue.
regards

Dave

DJ white

Not good to run it without a stat or blanking sleeve for any length of time, it can give rise to turbulence in the water jacket and localised recirculation and overheating.

Without a stat you should find the temp gauge takes quite a long time to rise, indeed may never reach 'normal' when driving it (OK, maybe not in Florida). If the thermostat were restricting flow this should be noticed as a very hot inlet to the rad but a cool outlet. You really need to monitor the temperature of your pan of water, raising it from cool, with both stat and thermometer suspended in the water and not resting on the bottom. Note the temperature at which it just starts to open, and when it is fully open. You could just replace it, but unfortunately these days you will have no better idea if the new one is working properly as the old!
PaulH Solihull

Paul.
Thanks again, I have a blanking plate somewhere, need to find it. With regard to the water flow, I know this is not very scientific but shouldn't I be able to feel the water flow in the top hose while idling .Also if the water pump is not working- no circulation- I should still be overheating, or perhaps it takes longer to overheat because the stat is out.

Dave
DJ white

Cars didnt always have water pumps, it was called thermosyphon and worked about as well as you would expect. So Florida would be a problem if the water pump has failed and it is just relying on thermal expansion circulation, and yes in this case no thermostat would help. The water pump isnt hard to remove and check, I have read threads on here where the plastic vaned ones have suffered erosion of tthe impellor, to the point where they dont work.
Stan Best

DJ and Stan, how many waterpumps have you run into where the impeler has shown wear without a serious mainshaft seal leak and minshaft bearing wobble?I'm not sure I have ever seen one. RIC
RIC LLOYD

I don't think you would feel normal water flow in either hose, I certainly never have noticed it. As I've suggested before if there is very little flow then whilst the top hose would be very hot the radiator would soon get cool radiating down from there, and the return would be pretty-much cold. That surely has to be the best way of getting an idea of flow rate?
PaulH Solihull

This thread was discussed between 28/08/2010 and 20/09/2010

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