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MG MGB GT V8 Factory Originals Technical - Heating mystery. Can someone explain.

Hot weather's back, and as you'd expect the old V8's running a little balmy again. Car didn't boil over, but the temp gauge went all the way to the right in traffic today. (Confession: I was on the cell phone and I didn't notice; could have clicked on the aux fans. Yes, I'm a loathsome being.)

Here's what I don't get: when that has happened, it's almost impossible to get the motor to stay back down at N. But if you catch it beforehand, and turn on the auxiliary fans before there's a problem, it's easy to keep it in line. Why is this? Does this have anything to do with what I've heard called "heat soak"? I've heard many other V8ers describe similar phenomena, and I never really understood what was going on. It's almost like you then need to reboot by letting the car fully cool off and the thermostat close.
Bob

I had a similar kind of problem last year. I got the impression that once the pressure has been released from the cap that it is difficult to get the system re-pressurised. Firstly I discovered that my pressure cap was only rated at 7lbs and I replaced it for a new 15lbs. I read someonwhere that pressure caps should be replaced fairly regularly anyway. I really have no idea of the circumstances which produce these kinds of problems but I have realised that it is possible for the pressure cap to vent off without producing steam. Very puzzling but don't always expect steam. I've also recently realised that that there can be some kind of steam trap within the system which will produce high temperature readings. Oonce again I don't really undertstand why this can happen but I have discovered that if my heater is switched on then there is a circulation of water which keeps my temperature rock steady at a nice 85c whereas if the heater is switched off then there is a lot of fluctuation. I associate this with some kind of steam trap. Frank who has contributed on another thread has had the identical problem. He rigged a bypass for his heater so that there is circulation when the heater is switched off and I am about to do the same.
Finally, I have seen Paul Hunt's V8 and also a waterflow diagram which shows that there should be a pipe leading into the thermostat housing. I do not have this and nor is there an inlet on the housing. I have been told that this would probably solve the steam trap problem instead of bypassing the heater.
I imagine someone who reads this will be able to give a proper explanation of what is going on. HTH.
Marc

I am not specifically familiar with the Rover/Olds/Buick engine design, but I get the feeling that a lot of these engines being used are a mixture of parts.

One thing that every engine needs is a water bypass. It can be either internal to the engine or external in the form of a hose/tubing arrangement. There should be coolant flow from the area right before the engine side of the thermostat to the inlet or suction side of the water pump. This prevents localized hot spots inside the running engine. Seems in your cases, the heater is acting like the bypass, which is fine as long as the heater valve is open.

I'm speculating that the (late model) Rovers have a constanting circulating heater system with no control valve, where the temperature in the passenger compartment is maintained using a mixture of appropriate portions of fresh air and heated air.

Wayne
Wayne Pearson

Wayne, maybe your comment was directed at Marc but in my case it has nothing to do with the heater per se (I should have wrote "cooling" rather than "heating" in the thread question). I think I have a water bypass hose, too. It's just unlike any other car motor I've ever experienced, in that once it gets saturated with heat, it's almost impossible to control cooling thereafter. In this case, quite literally, an ounce of prevention is worth a pound of cure.
Bob

Wayne, I'm extremely pleased to receive a confirmation of this water bypass hose that you refer to. This explains a small waterhose on my engine which has been blocked off as it has nowhere to go. This must mean that there is a different kind of water pump with another inlet nozzle on it. My present one has nowhere for the bypass to go to.
Would you happen to have a part number for the correct waterpump?
I would rather do a proper job and replace the water pump rather than rig the heater plumbing.
On the subject of water pumps I understand that there are some which are made so that the impeller does a far more powerful job as it is specially designed and others where the impeller veins are a sort of universal part and don't work as well as they could. Have you heard of this?
Marc

Marc, sorry, but as I posted, I'm just not familiar enough with these engines. Perhaps, now that the problem is identified, someone else has part numbers.

Bob, you're right--I was keying in on the odd heat problem from this and other threads. Question, though: Does your engine come back down to normal when you get back up to speed? I had a '79 Mustang that I put a 351 Windsor V in (headers, aluminum single plane intake, Carter 625 cfm carb, forged 9:1 TRW .030 over pistons, 3-row core radiator and two pusher electric fans. In the fall/winter/spring it was great--but get in a little summer traffic, the temp kept rising--sounds just like your car. Evidently, the electric fans just didn't move enough air through the radiator. The only thing that fixed the problem was an engine-driven flex fan and shroud. I was disappointed as I am a big advocate of electric fans (mainly, I guess because I like the concept). Later, I happended to read in a (hot rod type) magazine, the only way to cool a hot engine is to get rid of the electric fans and go with an engine-driven fan. The thermostatic-hub-type are the least HP costly.

I later sold my Mustang to buy my '73 B.

Wayne
Wayne Pearson

I've had temp readings of about 40psi on the (dual) oil gauge before now on my factory V8. This has the 'steam pipe' between thermostat housing an inlet manifold and it was blocked. What I have not been able to understand is why, when there is sufficient heat to drive the temp gauge off the scale, the fans did not cut in automatically when the two sensors are only a couple of inches apart. I also don't understand why switching on the fans manually would stop it happening if you are driving along.

Heat soak, as I understand it, occurs after switching off the engine and is where the mechanical gauge can rise sometimes into the red. When running there is a huge temperature gradient across the engine from 80 something F in the rad to many hundreds (thousands?)inside the cylinder as the mixture burns. Combustion temps bleed through the cylinder walls into the water jacket, where it is whisked away and dissipated in the rad. Switch off and the water largely just lies there, getting hotter and hotter as the residual heat that was in the cylinder and head walls transfers into the water, and the relatively small amount of convection allows the hot water to get to the temp gauge sensor.

Wayne's comment about water continually circulating through the heater core on 'modern' vehicles, temp control being done by blending varying amounts of cold air with hot, seems to hit the nail on the head as far as Marc and Frank's experiences go. But conversions are so relatively common I'm amazed this hasn't cropped up before. Since the factory V8 only has an internal diameter of 1/4" or so, I would imagine that this is all you would need as a 'heater tap and core' bypass.
Paul Hunt

Thanks for the info guys. One thing I noticed today is that the catch tank next to the radiator (the OE MGB unit) had a small crack on its back side. It must be that the system wasn't pressurizing fully, since it was leaking out coolant ... there were dried-up streams of coolant running down the inner wing. I'm going to get it fixed, refill the system, and report back. Hopefully it will deal with heat a little more happily then.
Bob

The Rover & hence the orig MGB GTV8 inlet manifolds have provision for a bypass hose to the water pump.The Offy inlet man. doesn't. I don't know about the Edelbrook or BOP ones. My GT has a modified Rover man. with the bypass hose & takes ages to warm up. My roadster with an Offy & no bypass warms up right away, but neither car overheats or has erratic temp readings. The GT has an electric guage & the rdstr a mech. one.As far as I know ,all Rover water pumps have an outlet for a bypass hose. I had to plug it off for the Offy man.
The mystery deepens. Barrie E
Barrie Egerton

This thread was discussed between 15/04/2002 and 17/04/2002

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