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MG MGB Technical - Cooling system MGB

HI All
i noticed recently my car is overheating. Iam trying to clean up the cooling system on the block, i removed the drain plug and nothing happens its completly blocked, is there a tehcnic or rust removing fluid to help loosen the rust? i had the engine out and forgot to do that then... silly me.
a rodrigues

The drain hole is very small and prone to clogging. It won't affect the cooling of your engine. When you had the engine out, did you remove the head? The area behind #4 cylinder tends to collect sediment and makes that cylinder run hot. RAY
rjm RAY

i resleeve the block new pistons and rings the lot, new water pump, clean the radiator also clean the engine maybe not properly...but now i have had wrong the radiator cap on,( the one used on latest engines) also fill in to much the radiator, i think i was missled by a faluty temp gauge..
also concidering the piston and rings being new needs to settle on the sleeves...or not...
took to a mechanic did pressure test is ok, using another temp gauge it reads 70 to 80 when reving...
this week end i will take for nice ride see what happen..

a rodrigues

Overheating? Or just running above N on the gauge? There is a big difference.

It's only overheating if it is steaming or otherwise losing coolant. Otherwise, particularly now and in southern Europe, one can expect it run above N on the gauge under certain conditions. I've seen both mine on the edge of the H zone in stuck in traffic in hot weather in the UK, and no running problems. MGBs are nothing like modern cars that seem to be able to indicate the same temperature on the gauge even in extremes - probably because the gauges are frigged to only indicate otherwise if the ECU detects something wrong, and that probably to prevent owners worrying unnecessarily.

The gauge could also read high, but that is less likely with the dual capillary gauges than the single electric as used on North American spec. If another instrument only read 70 to 80 it sounds fine.

I don't think I have ever heard of anyone getting coolant out of the block drain, short of chiseling the crud out, there can be casting sand left in there right from manufacture.
Paul Hunt

With a newly rebuilt engine, you can expect it to run a bit hot until all of the new parts have had time to break in. This process can take as long as 1,000 miles, but is usually complete after about 500 miles. RAY
rjm RAY

Hi All
I run the can now for a few kms and seems to come to normal temp. one question i set the carbs a bit rich for now and noticed that the 2 middle spark plugs are much grey than the the others...
tomorow will take for a long run and see what happen. thanks
a rodrigues

"the 2 middle spark plugs are much grey than the the others..."

Could be a vacuum leak near the middle of the manifold.
Paul Hunt

The 70C-80C temps sound good to me. I run an 88C thermostat and unless in stop/go traffic temperature is around 94C. (Mechanical temp/oil Smith's gauge that is accurate.)

Never boiled over with 0.48 bar (7 psi) radiator cap. I supplement 3 blade original fan with pusher electric for traffic conditions.

My block drain is clogged and I expect 95% are.
Robert McCoy

Ah yes, meant to comment on the rad cap. The higher cap on an earlier engine won't hurt, but the lower on a later engine could burp coolant out on switch-off. 15psi (1 bar) should be the max though, I've know 20psi burst a cheap hose.
Paul Hunt

Using a higher rated pressure cap, on older cars, should be avoided. For many years an older car ran with a low pressure cap. Switching to a higher pressure cap can find the weak link in your cooling system, quite often your heater core. RAY
rjm RAY

Only if the pressure in the system reaches the level of the cap, which it shouldn't do under normal circumstances. I've done pressure tests on both my cars at various times while diagnosing cooling system funnies, and to prove they were fixed. The V8 with 15lb cap only got up to 11lb when the fans cut in and the temp gauge was nearly mid-way between N and H. 6psi when the fans cut out the the temp gauge just above N, and 3.5psi in motorway running with the temp gauge just below N.
Paul Hunt

Paul, I was working on a '99 Land Rover last week that had been brought in for overheating problems. There was a receipt, in the glovebox, showing that a new water pump, automatic transmission and transfer case had been installed. The car had then been given back to the customer along with a $2,000 bill. The transmission and transfer case were used items, not rebuilt. I pressure tested the cooling system only to find a leak in the heater core. After bypassing the heater core, so that I could test the rest of the cooling system, I realized that the thermostat was frozen in the open position. After starting the engine, I noticed bubbles in the cooling system reservoir. A lot of bubbles! I left the cap off and the engine proceeded to dump over a gallon of coolant out the overflow hose in a matter of a few minutes. Being that the engine was aluminum, I feared that a blown head gasket, cracked head or heads and possibly a cracked block were a strong possibility. My belief was that the vehicle had been used for long hard towing and this was the probable cause of the transmission and transfer case failing. At this point, I called the owner and reported my findings, along with the probable cost of repairing the Rover. He picked up the vehicle and decided to wait until he had time to digest all that I had told him. RAY
rjm RAY

Have the same thing going on with my new engine. Never seen the temp gauge go above about 190 F (87.8c). Even with my trip across the US in July, I only saw it get up to about 195 (90.6) once.

With the new motor, it goes to about 198 when climbing hills, but drops to normal quickly when over the hill. I am being told that this is actually normal and actually the car runs better between 190 (87.8c) and 200 (93,4 c).

Couple differences. Was living in a part of the country where hot was 90f. Now live where 100+ is the norm.

I did note that there is a tear in the gasket in the rad cap, so that is being replaced.

On the topic of pressure. Moss says that the 7 pound cap is correct for a 67, but others are saying that I should run 10 for the 67. Thoughts?
Bruce TD4139 Cunha

I posted this photo on the V8 Register Bulletin Board. The yellow line shows where my V8's temperature gauge got to while stuck in traffic in France recently in 35 degree Celsius temperatures. It didn't boil, nor did it lose any coolant. With a 15 psi cap (normal on a Range Rover engine) and 4life coolant it shouldn't boil until way over 300 F.

The passengers (my wife and I) definitely did overheat!

Mike Howlett

I've seen my temp gauge go to there as well, but that was when the steam pipe was blocked.

The annoying thing with temp gauges is that they only show the actual engine temperature while you have sufficient coolant. If you lose coolant and the engine starts getting hotter the temp gauge stays exactly where it is! Because of that I fitted a coolant level alarm while I was losing coolant, and even though I resolved that problem many years ago I've never taken the alarm off.

Clausager says a 10lb cap was fitted originally, changing in 1976 to 13lb, and implying it changed to 15lb for 77-on.

However the Parts Catalogue shows it as 7lb for Mk1, 10lb for Mk2 up to the 75 model year, 13lb for 1976, and 15lb for 77 on. Neither does it diffentiate between markets.

7lb to 10lb is pretty minimal, if you don't lose coolant with a 7lb cap then there is no point going to a 10lb.

If you do lose frequently lose coolant with a 7lb under extreme conditions then you can try a 10lb, but if you are bothered that might be too high and cause damage elsewhere then you could always take the expensive option of Evans Waterless which is said to develop only 25% of the pressure of conventional coolant at the same temperature.

But if you are losing coolant under 'normal' conditions then you need to investigate why, not simply stick a bigger cap on or chuck expensive coolant in.
Paul Hunt

Bruce, a 7lb. cap is what the owners manual called for on a '67 B. I've never had a problem with it and as long as the seal is intact, it should work just fine. Going to a higher pressure cap, after four + decades, is asking for trouble. All of your cooling components have been under 7lbs. pressure since new. Increasing the pressure now will aggravate any weak link, such as the heater core, and could result in expensive and time consuming repairs with little gain in cooling performance. RAY
rjm RAY

Just to clarify why I have a 15 lb rad cap, my B's engine is a virtually standard 1991 Range Rover 3.9 litre complete with the hot-wire injection, and 15 lb is the recommended cap.
Mike Howlett

"my B's engine is a virtually standard 1991 Range Rover 3.9 litre "

Factory V8s always had a 15lb cap. I did burst a bottom hose during heat-soak using a 20lb :o)
Paul Hunt

My 7 psi cap has never popped. I run about 50/50 old school green coolant/water mix.

I've considered a 10 psi cap for added margin but the electric fan keeps things under control in stop and go so I'm leaving as is.
Robert McCoy

Regarding the drain plug hole blockage, you can usually open it up by inserting a wire and moving it around.
Ted Mack

You can try, but many have found it does not work. I got a bit of dampness on the end but despite persevering it didn't get any better than that.
Paul Hunt

rjm RAY

You can also take out the drain plug and run the engine. The pressure will blow it out when the engine warms up.
Ted Mack

This thread was discussed between 25/06/2015 and 12/07/2015

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