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MG MGB Technical - V8 overheating

I have taken the V8 out today for the first time in 28 years for a run. She does about 5 miles and the temperature gauge is up at max. I have run with the fans on but it still gets to max. The radiator has been checked during the engine rebuild and was found to be ok. The water pump, hoses, thermostat are all new, I am running it on OAT antifreeze/coolant.
I have tried to bleed out any air by leaving the expansion cap of bit it just spits water, the internal heater is blowing hot so I know there is water in there.
Does anybody have any tips for bleeding the system.
a goldup

Hi,
A radiator that has stood for 28 yrs may still have a partially blocked core which would stop sufficent coolant from circulating around the engine and also contribute to the rise in temperature, maybe you might consider getting the radistor recored or replaced.

Regards P.T.
P.T. Tighe

Is the gauge reading correctly?
Mike Howlett

It may simply be a defective thermostat. I've had them fail, brand new, right out of the box. Worth checking into, and not a lot of money or labor required. RAY
rjm RAY

The radiator was checked by a specialist radiator company so I don’t think this is the problem.
There is water coming out the overflow on the expansion bottle when the gauge goes to max.
Could try another thermostat, or take it out and put in a saucepan on the hob and check the water temperature with my special infrared gun.
a goldup

Sounds like thermostat to me Is the thermostat fitted around the right way with the bulb towards the engine, if it's round the wrong way it will do what your car is doing Willy

Also if you have had the fans out or disconnected check that they are connected up around the right way. If they are hooked up back to front the air blows out the front instead of through the radiator and it will overheat as soon as you get above crawling around speed
William Revit

I have had the thermostat out today, it is working and was in the correct way round.
I have not had the fans out but is something to check. I know they run clockwise as you stand at the front of the car.
The car overheats very quickly if you turn it off and remove the expansion cap she spurts like a killer whale but not in one go, squirt stop squirt stop sorry best way I can describe.
I have put a sniffer on the expansion bottle but no sign of a head gasket failure.
Took the rad out again so off the the rad man tomorrow to have it checked again.
a goldup

V8's do tend to run on the edge of over heating. If you are struggling to bleed the system, try either parking on a slope, nose down or jack up the rear.
I have a 78 thermostat with the ring drilled. I also have baffles to direct air through the rad. I assume you have given the car a service and the oil and filter are new. The lubrication system plays a big part in cooling, check that the oil cooler matrix is clear.
Other causes of over-heating are, 1. Slipping fan belt, 2. Slipping impellor on the water pump shaft, 3. Over advanced ignition, 4. Weak fuel mixture, (the brake servo could also be leaking air), 5. Binding brakes, 6. Blocked or restricted internal engine waterways, (flush the system through) 7. Ineffective or wrong pressure cap, or leaks depressurising the system.
Allan Reeling

I have a 78 thermostat with the ring drilled. My stat is a 92 with four 4mm holes in the outer ring. I assume you have given the car a service and the oil and filter are new. The lubrication system plays a big part in cooling, check that the oil cooler matrix is clear. The engine has just been rebuilt with new oil and filter oil pressure is at 20-25.
Other causes of over-heating are, 1. Slipping fan belt. Belt is new and correctly tensioned
2. Slipping impellor on the water pump shaft, Water pump is new 3. Over advanced ignition,Ignition timing is set to 8% 4. Weak fuel mixture,Plugs are all a nice colour (the brake servo could also be leaking air) Servo is New, 5. Binding brakes,overheats while standing still 6. Blocked or restricted internal engine waterways, engine was cleaned when rebuilt(flush the system through) 7. Ineffective or wrong pressure cap,cap new 15psi or leaks depressurising the system. No leaks.

Thanks for your suggestions keep them coming someone will find the answer.
a goldup

After all those years, the laws of physics tell us that the rad is shot.
R Walker

The radiator has been checked twice and they say it is fine. I would replace it if they had any doubts.
a goldup

After years of working on MG's, I've learned that new doesn't necessarily mean faultless, Slipping impellors in water pumps is not that uncommon! How long since the engine was re-built, in mileage terms? Is it a standard factory, 3.5 V8? and what work was done on it?
Allan Reeling

Engine had new camshaft and front bearing, polished crankshaft and new bearings. The block was washed by the engine specalists. New rings and heads skimmed. New oil pump gears,new rocker shafts. Car has only covered about 30 miles since the rebuild but has run in the drive many times. The car is a standard factory V8 chrome bumper.
a goldup

I take it, like a lot of V8's, the bores were hardly worn and new,standard, rings were fitted?
Does the engine turn freely by hand? How much was skimmed off the heads? Are you using composite head gaskets? How old is the water pump, there was a batch within the last couple of years, which had slack impellors, worth a look.
Also check the firing order against your plug leads..............not unknown!!, V8's are so tolerant!!! Also check that you have the correct mechanical advance springs in place, in the dizzy and the vacuum advance diaphram is working.
Allan Reeling

The bores where within tolerances so only new rings where required.
The engine turns freely by hand.
The heads where only cleaned up and refaced not actually skimmed.
Standard tin head gaskets are being used.
The water pump is new it came from MG Hive a few months ago when the engine was being rebuilt.
The engine runs sweet so I don't think the firing order is incorrect but will recheck tomorrow.
The dizzy is the original and the vacuum advance is new.
I will remove the water pump tomorrow to check the impeller.
a goldup

I took the water pump off today and the impeller is still attached to the spindle.
So another idea out of the window.
Anybody with anymore ideas.



a goldup

When the shop cleaned the block, did they knock out the freeze plugs (core plugs) to check/clean the water jacket?

It does sound a bit like air in the system. Any possibility that you could rig up some plumging and fill it from the bottom up?

Charley
C R Huff

The core plugs where replaced with new ones when the block was cleaned in the machine shop did the engine work. I have tried to fill the system from the top heater hose with the radiator filler removed and the expansion cap off.
I have today removed the thermostat and put it in hot water it opens at about 85c on the ir gauge and about 90c on the thermometer. It is a 82c stat.
a goldup

Firing order double checked and all correct.
Going to refit the water pump as soon as the gasket arrives and then run it up without a thermostat too see what happens
a goldup

Sometimes it can take a while for new piston rings and other new engine parts to wear in. Until then, they may produce more friction and heat than normal. If removing the thermostat will keep the engine cool enough, put a few hundred miles on it and see if it's any better. -G.
Glenn G

Don't run without a restriction in the thermostat housing, you will only make matters worse. The water will circulate too quickly, hence not spend long enough in the rad, exchanging heat. Take the centre out of a thermostat and just install the outer ring.
Allan Reeling

One more thing to check, even though you have replaced all the hoses, is to make certain that the lower radiator to water pump hose is not collapsing when the rpms are increased. This will cause cavitation at the water pump inlet and greatly reduce cooling. The better hoses on the market have a spiral spring inside the hose to prevent this from happening. Make sure that your hose has the one of these supporting springs. RAY
rjm RAY

Todays update on theV8.

I have put the water pump back on today and left out the thermostat, took the car for a run with the fans on manual override. Temperature went up to mid way between N and H. Switched them back to auto and the gauge went a little bit further. Took it for a blast down the motorway at 70 and the temperature came down to just over N. Then back around town with no traffic an the gauge was midway between N and H. Pulled up in the drive and the left it running for a while and the gauge was between N and H only moving slightly when the fans kicked in and out.

Not sure what that points to so still thinking about replacing the radiator and the otter switch.
a goldup

I would be very suspect about the old radiator. How did they check the radiator? If they only tested the flow that won't say it will cool perfectly (which it has to do with a V8).

If you have the radiator still out of the car, put it flat on the floor and fill it with undilluted vinegar (the kind sold for cleaning purposes, although the salad-type may work, too ;-)).
If the vinegar starts to bubble you know it's reacting with the chalky sediments that may not block the flow but will reduce the cooling capacity. Let this simmer for a few hours and flush afterwards; you may be surprised about the amount of crud released which improves the cooling enormously.

That said, having the radiator recored is the best insurance for the preservation of your reconditioned engine.

I went the vinegar-route with my YA and improved the cooling but I used it to prove that the radiator was the cause and had it recored after a few months for peace of mind.
Willem vd Veer

I have now put a Holts speed flush in the cooling system. I will let you know the results once I have drained it out.
a goldup

Took the car out today for a long run 43 miles. I watched the temperature gauge like a hawk all the time I was out on the run.
The gauge came up to just under the ‘N’ and stayed there for quite a few miles when I was on the open road travelling along at speeds between 30 -60. Then went into some country roads then the gauge started to clime just over the ‘N’ but never went much higher. Got about 30 miles into the run and at the end of the country lanes just getting to a 50 limit the gauge went back down to ‘N’.
At the end of the journey after a run on the motorway the gauge was still on the ‘N’.
The thermostat is still removed and the cooling system still has the Holts speed flush in, it has now covered the stated mileage so it can be drained. I am still not sure the radiator is providing the cooling required by the engine. The radiator as stated before has been checked for flow twice by a specialist radiator company.
I have been reading articles on the web and the common factor is that many people recommend taking the radiator off and filling it with undiluted vinegar. I have been looking around the usual supermarkets but can’t find undiluted vinegar. Has anybody tried this? Does anybody know where I can get some undiluted vinegar? It has to be worth a shot before buying a new radiator.
a goldup

Don't understand your problem. It seems that it ran hot with the thermostat, and when you took it out it is now OK. So, the rad works, and has been checked twice by "experts".
But this all depends on the temp gauge not lying. Is it an electric gauge? Have you checked gauge readings with something more accurate? System Voltage fluctuations can cause funny gauge readings, and since this car sat for so long I think the chance of bad connections in many places is very high.

FRM
FR Millmore

The gauge is the standard V8 type with a screw in bulb in the inlet manifold.
The car boils up with the thermostat in place, water is pushed out the overflow on the expansion bottle. I have also used an IR gun to confirm the temperature of various places on the engine and radiator.
The radiator has been checked for flow but how can you check the cooling function without it in the car.

The car should have the thermostat in and I need to be sure it will not overheat and ruin the engine when it is put back in.
a goldup

1. have you re torqued the heads down after build and first run in?

2. new engines always overheat for a little while because there is a lot more friction due to tighter tolerances due to rebuild.

3. If the engine runs ok without the t stat buy a blanking sleeve or knock out the centre of an old t stat and run it for a while until the motor settles.

Make sure you have burped out all the air and check your water and oil for any contamination between each other. Check your radiatior cap as well. If it doesnt hold 13lbs or whatever it is the temp will rise more rapidly. Get the rad shop to pressure test your systems as well they will know if it is leaking and causing you to over heat.

Undiluted vinegar is cheap vinegar from the supermarket ordinary white vinegar. Buy the cheapest bottle you can get and use it. Having said that you have had it checked and flushed so I dont think it will help much.
A J Ogilvie

Just had a thought --

Ignition timing - have you set the timing
- is the mechanical advance, advancing up as the revs increase
- is the vac advance working properly

Willy
William Revit

I have not yet re torqued the heads as it was suggested this needs to be done at 200-300 miles and it has only done 85 miles since the rebuild.
The ignition timing has all been re checked and is all working as it should, the vacuum advance is new and working correctly.
Radiator cap is new and holds 15lb of pressure as marked on the cap. I have just borrowed a pressure tester and will check the system later in the week.
There is no sign of water in the oil and vice versa.
a goldup

Hi, I would re torque the head now while its hot. Alloy heads in alloy blocks are notorious for losing torque. Check with RPI engineering to be sure.
A J Ogilvie

Them tin shim head gaskets- are they like the Durapro gaskets with little water holes at the front and larger to the rear - If so would there be a chance that one or both are fitted around the wrong way with the little holes at the rear
If so this lets the coolant circulate in the front of the engine and the rear gets HOT
On the Durapro there is a little tag poking out the side at the front cylinder on both sides so as to check if things are fitted correctly - do your gaskets have these tags ??
Cheers Willy
William Revit

The problem with re-torquing the head is that if head bolts were used, as opposed to ARP studs, they are put in with thread sealant. This is because the threaded holes in the block often penetrate waterways. Re-torquing will break the seal!
Allan Reeling

I have had the radiator of at the weekend and filled it with caustic soda left it for 24 hours. When I emptied it out the water came out green. Washed it through and put it back in the car. Took it for a run when you give it some beans the temperature gauge starts to rise quickly past the N mark.
I have today ordered a new core for my radiator so lets hope for the best.
a goldup

Retorquing isn't mentioned in the Workshop manual, I didn't do it, and have suffered no apparent problems. However I'm not sure the seal would suffer, as what the book *does* say is that on removal the bolts must brush-washed immediately with 3M solvent or immersed in petrol etc., as it is only on contact with air that the sealant fully hardens, making subsequent removal for reuse very difficult.

Andy, you didn't reply after I commented that the direction of cooling fan blades and/or rotation appeared to be incorrect.
PaulH Solihull

Paul I took the radiator out to double check the flow and had another look at the fans. They do actually run the correct way.
The new core for the rad has not turned up so I ordered a new rad from MGB Hive today. Once the rad turns up I will also put the thermostat back in and give it a run.
a goldup

New radiator arrived from Hive but it was for the later car so had to return it and get mine recored.
New core is a three row staggered design so better than the original.
Refitted rad and put the stat back in, took for a 12 mile run, no sign of the overheating and the fans did not kick in once on the run. Gauge still runs just over the N mark but all appears to be fine now with the new high flow core. Next step is to drain the water and put the antifreeze back in.
a goldup


Nice one - well done

Back to your headgaskets, if they are the steel shim ones as you mentioned, they have no give in them and don't need retensioning Cheers Willy
William Revit

I fitted an uprated rad to my V8 but was disappointed. By the time air has flowed past the original number of tubes any extra tubes are going to dissipate very little. As it has made such a difference, I can only conclude that the original was significantly obstructed. This can often be sorted by unsoldering the top or bottom tank and running rods or strips through the tubes to clear them. Obstructions like this can be caused by the use of leak repair solutions.
PaulH Solihull

Back in the Dark Ages, when you could still get a radiator repaired at your local shop, this was referred to as having your radiator rodded. It worked just about all of the time, but has become a lost art now in the States. RAY






rjm RAY

This thread was discussed between 13/04/2011 and 10/06/2011

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