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MG MGB Technical - '68 runs hot when idle

I have gotten tons of great help on my MGB and hope for some more tips... sils done, floor pans done, front end done, rear done, clutch done, etc.

My '68 MGB runs well, but had some temp issues in the summer heat while in traffic. First, does this sound common?
- maintained good engine pressure
- recently pulled and had the radiator repaired (it also has good pressure)
- engine still has its oil cooler (some people say take it off... I have not)
- I never had an issue when moving. In fact, temps went down as soon as I started moving after the traffic jam. I guess that makes sense).
I am sure I should also ask if people know why it got hot.

If this is somewhat standard, then what do people do? Are aftermarket fans added? Do people use a switch to turn them on or off or do people use the sensors provided?
SJH

Fan on backwards?
David DuBois

SJH,

If rad. is OK (clean)and tuning fine: petrol mixture and timing. If temp is not in H section, I noticed with my '68 in traffic jam a tendancy to be slightly on the hot side. It is normal...
You could also check if thermostat is in good shape or more easily replace it with a summer 160F thermostat like Stant #45356

Cheers,

JGC
Jean Guy Catford

A few things spring to mind. Idle overheating could be either;
1.fan not pulling enough air through rad....belt slipping etc.. the later engines had a larger dia crank pulley maybe to increase the fan speed.
2. Water pump not "paddling" enough water at idle. I have just re-built an engine and the discarded water pump vanes were really worn down, presumably affecting it's efficiency.
3. Over advanced ignition.
4. Weak mixture would tend to be more obvious at idle.................less cooling from air and water flow.
Allan Reeling

But is it really overheating? Does it boil off coolant? Or is just that the gauge goes up higher than usual? Unless it is losing coolant it isn't overheating. OK the gauge might read higher than usual but that doesn't matter, and that happens to all cars, ancient or modern. But on modern cars the clever electronics prevent the gauge from going above the central position even when the engine is hotter than normal. On a modern car you will only see the gauge rise when it is about to boil over.

On your MG the gauge isn't so cunning and shows you the real temperature, which, as you might expect, will go up and down depending on conditions. As for your oil cooler, I guess you have hot summers in Tennessee, so leave the oil cooler in place. Here in the cooler UK it is marginal whether the oil cooler is useful and some folk remove them.
Mike Howlett

They do gradually creep up the gauge if stuck idling in traffic in hot weather, this year mine got into the H zone without losing any coolant, came right back down when we eventually got going again.

Fan on backwards is not as bad as it might sound, air still flows in the same direction, although in theory it is not quite so efficient. Mine was like that for years before I discovered I had the short-nosed pump which needs a spacer for the fan or the fan clatters the block. Having corrected it (before this year) there has been no discernible difference.

FWIW cooling at idle would be *better* if the fan did blow backwards, as it would be blowing hot air out of the engine compartment instead of into it. Less so underway of course ... unless in reverse.
PaulH Solihull

I installed an aftermarket electric fan with manual switch. Kept things under control in the hot California summers when idling for longer periods. Once cruising no problem.

Even before the fan installation using 50/50 antifreeze it never popped the cap. But the idle suffered when the underhood temperature went up and the temperature gauge was in the upper reaches of what I wanted to see.

Here in New Hampshire I hardly ever use the fan. Less stop and go traffic and not as hot usually.

I still have the inefficient 3 bladed fan. I run an oil cooler but that really doesn't help when you're stopped.

Bob McCoy
65 Roadster
Robert McCoy

" I run an oil cooler but that really doesn't help when you're stopped."

Too right. In fact when stopped with no breeze a significant amount of hot air comes out of the bottom front of the engine compartment to be 'recycled' through the 'cooler' and rad. With a slight tail breeze it is even worse, and very noticeable on the temp gauge.
PaulH Solihull

I think a major contributor to overheating is poor hot air ventilation from the engine bay. When I lift my hood I don't get much heatup at all even without the electric fan.

I wondered if inner fender louvers would help extract some heat.

Robert McCoy

Hot air exit should only present a problem when moving, i.e., when there is a positive pressure under the car resisting air flow through the engine bay. There are enough gaps round the engine to evacuate hot air while stationary.
In hot climes by the time the air spill from the engine bay has got back in through the grill, it's probably no hotter than ambient. The metal fan is about the equivilant of 5-10mph forward movement at best. Fitting a better fan blade, shrouding it, fitting the bigger crank pulley (and timing cover)or going electric will all help idle/stationary cooling.
Reducing the anti-freeze proportion will also help, anti-freeze is not as good at transferring heat as water!
Allan Reeling

To me it's the opposite, Allan. Stationary only the fan is moving air out of the engine bay, and apart from a little that goes along the tunnel which is mostly filled with gearbox, it can only come out from below the inner wings at the side and around the same level front and rear. Inner wing slots or holes, even when partially filled with RV8 type exhausts, do help get hot air out of the engine bay, but there is still some restriction from the wheel in the arch. What does exit is nowhere near ambient but barely less than what has been blown out of the radiator, where would it lose its heat to? It was when I was doing temperature tests on my V8 after working on the cooling system I first noticed this effect. If the temperature coming out were the same as that going in, in any climate, the engine wouldn't be cooled and it would boil in minutes. As soon as you are moving the ram air effect is going to be much more effective at getting heat out of the engine bay than the mechanical fan (and the MGB electric fans aren't much better), the front shroud creating a low pressure area behind it, as well as ensuring only ambient air is passed through the rad anyway, both effects are precisely why the temperature goes up when you are stuck in traffic and comes back down when you start moving again.
PaulH Solihull

This has been a great thread. Thanks for all the thoughts. I have a nice list of things to check.

The immediate check is the fan itself. I'll make sure it is working as I thought it was...

I'll check the radiator fluid again and monitor it. It is on my current check list, but I will be more specific with it.

I plan to pull the carb in January, which should provide the chance to check timing.

I'm not 100% sure how to check the guage itself, but I'll figure that out... you've given me enough to work with. The internet is a wonderful thing!

The water pump is interesting thought on "paddling". I haven't messed with that, but I'll put it on the list to look into.

SJH

"It's the opposite?"???? Don't understand!! The mechanical fan pulls air through the rad and into the engine bay, that warmed air has to escape at the same rate it is coming in otherwise the engine bay becomes a positive pressure area and flow through the rad reduces. BUT I can't see that at idle a fan can move air faster than it can escape! Neither can I see hot air falling down and forward to be re-ingested through the grill, as you suggest, it is more likely to spill rearwards, because the fan has imparted it with that directional energy. What is required is to pull more air through the rad and maybe move the coolant faster. Temp rises in traffic because these two things are compromised as well as the ambient air being warmed by the traffic itself. As I said a positive pressure in the bay is more likely at speed when under the car is a positive pressure area and hence a resistance to escaping air. This is when wheel arch vents are an advantage for V8's..the wheel arch is a negative pressure area and will thus pull air out of the bay. Bonnet louvres, which tend to be put near the back of the bonnet,for aesthetic reasons, do not vent much air at speed as this area is a positive pressure area, that's why the air intake for the heater is there.
Allan Reeling

Left to its own devices air heated above ambient will rise of course, which is why Robert says "I don't get much heatup at all even without the electric fan" with the bonnet raised which is to be expected. However with the bonnet closed, the closed sides of the engine compartment, and air being sucked/blown in the front through the rad and heated, it has to escape somewhere or it wouldn't go through the rad and cool the engine. It can only exit from the back of the engine compartment if there is space for it to do so. But not only does the tunnel taper to a relatively small cross-section, but it is largely filled with gearbox, so not much can go that way. Heated air is not 'falling' out of the engine compartment, i.e. opposite to natural convection, it's escaping downwards as that is the only place it can go with more air being sucked/blown in through the front and the rad. Blow hot air in through the side of an upturned bucket and it can only exit the bucket by going downwards. Stand by a V8 after a good run on a warm day with the fans going and you will feel hot air billowing out of the arches at least. After working on the cooling system of my V8 I shut it in the garage (exhaust piped outside) on a near 30 degree day, and not only did I observe air flowing forwards from the bottom of the engine compartment to be sucked back into the intake by the fans, clearly visible as a heat-haze, but I measured it at over 40 degrees.

This effect was also obvious at one of the Bath to Bournemouth classic runs where the entrants are interviewed at the finish. There was a slight tail breeze and not only did I notice a faster than normal increase in the temp gauge, but several other drivers were panicking and they had to cut the interviews short so cars weren't stationary in the queue for so long.

I would expect anyone to be able to detect this with the car standing in still air, but I guess we will have to differ.
PaulH Solihull

I have to say I'm almost convinced, if we can assume that the fans can create a low pressure in front of a stationary car. BUT!!! your argument does support the oft recommended, (by myself and many others), trick of removing the centre section at the rear of the bonnet seal to allow the escape of rising hot air!!
Allan Reeling

>BUT!!! your argument does support the oft recommended, (by myself and many others), trick of removing the centre section at the rear of the bonnet seal to allow the escape of rising hot air!!

Keeping in mind, of course, that whatever is coming up & out of the engine bay will then be drawn into the fresh air vent and blown into the occupants' faces!
Rob Edwards

"removing the centre section at the rear of the bonnet seal to allow the escape of rising hot air!!"

At a standstill it *will* aid the escape of hot air from the engine compartment, just as having the bonnet up does but to a lesser extent.

But by your own arguments it cannot do that when travelling as there is a high-pressure area at the base of the screen which will be pushing air down that gap. However by blowing cooler air into the engine compartment, it may well be pushing more hot air out of the bottom of the engine compartment than ordinarily. However again by your own argument, if there *is* a high pressure area under the engine compartment restricting how much hot air can escape that way (which I don't agree with anyway) then pushing cool air into the engine compartment from elsewhere is going to make it even harder for air to flow through the radiator and cool the engine. But then again, if the air pressure under the bonnet is higher than at the base of the screen, then hot air *will* be pushed out, and go straight into the heater intake as Rob says. Do you end up like this after a run :o) http://s2.hubimg.com/u/1238229_f260.jpg

PaulH Solihull

But in all this Paul you've lost sight of the fact that SJH didn't have a cooling problem when the car was moving AND it isn't a V8!
Allan Reeling

In fact, did we ever determine if SJH has a problem at all?

SJH, when it's in the hotter-than-normal situation about which you're inquiring, are there any other symptoms? Or is it just showing on the gauge? If it's just showing on the gauge and no other symptoms, then in answer to you last question in your initial post:
>If this is somewhat standard, then what do people do?

... the best answer would be, "Ignore it!" ;-)
Rob Edwards

" Paul you've lost sight of the fact that SJH didn't have a cooling problem "

I haven't, my comments stem purely from your statement of 30th Dec and not what SJH might or might not be experiencing.

As far as that goes it's much the same as Rob - "If it isn't steaming or losing coolant, it isn't overheating". Both my roadster and V8 have been right up in the H zone at various times and didn't. You shouldn't need to remove the centre section of the bonnet seal, particularly in the UK.
PaulH Solihull

Rob-- you asked for other symptoms after hot. It does not idle well once the gauge shows it's temp is rising. I will be pulling the carb this February. It does ok idling normally, but it sometimes diesels when I turn it off. The timing screw is almost all the way out so it needs attention. I am hoping this helps solve the problem.

The radiator fluid was not low and there is not an overflow system. I have been looking at them, but not ordered.

Thanks again
SJH

This thread was discussed between 27/12/2011 and 07/01/2012

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