Overheating

Technical MGB discussion
Vic Butler
Posts: 514
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Overheating

Post by Vic Butler » Sun Jun 14, 2020 10:27 am

Yesterday I took the MG for its first run and like last year it overheated. I had initially put the cause down as retarded ignition, but having advanced the ignition to the point of kickback when starting that's obviously not the cause. It doesn't overheat when I leave it ticking over until the fan cuts in but when it's on the move so it's not the water pump or thermostat. This is accompanied by a lack of power.
There's no loss of water, water in the oil or vice versa so I'm wondering now whether the cylinder head has cracked. It looks like the only answer is to remove the head. If the head is cracked I'll have to get another one which I want to be Stage 2 but it will have to be outright.
Peter Burgess is the obvious choice so I hope he can supply me one outright. It's not a rubber bumper type head as it has a centre oil feed to the rocker shaft. The block does have the cut outs at the top of the bore.
The oil pressure remains normal.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Ian Fozzard
Posts: 391
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: Overheating

Post by Ian Fozzard » Sun Jun 14, 2020 11:22 am

Hi Vic. I think for a cracked cylinder head to be causing overheating you would be seeing some other obvious signs - over pressurised cooling system? Gas in cooling system? (test kits are available).
If you can rule out a cracked head then I would think that ignition timing and mixture strength are the two areas to check.

I recall you have an unusual set-up, so unless you have had it all tuned on a rolling road by someone who knows about these engines, it may not be set up optimally.

If you can I would try reverting back to standard as much as possible - carbs, distributor etc. and set ignition timing using a timing light and employ standard needles in the carbs. My hunch would be that this might be all you need to do.

Have you checked the plugs after a run at speed/under load? You need to switch off and coast into a lay-by, whip out a plug and see how it looks. This would be a rough check on mixture under load and ignition, if both are incorrect this can cause over heating.

You could also try pulling away in 4th gear from a low speed (say 20mph) and see how much pinking you get. If you are getting pre-ignition or detonation at high speed of course you would not be able to hear it, but your engine would certainly suffer!

Hope this may provide a few pointers😊

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

User avatar
Charles Farran
Posts: 210
Joined: Fri Oct 31, 2014 11:08 am
Forename: Charles
Surname: Farran
Location: Warwickshire

Re: Overheating

Post by Charles Farran » Sun Jun 14, 2020 12:03 pm

Hi Vic,
In your opening sentence you said "like last year it overheated" - what was the cause then - did it clear itself after a period?
When did you last change the coolant? Is the rad partially blocked? Is the thermostat gummed up?
(I changed to "4 Life" coolant & was impressed how clean it was when i drained down after it had been in for some years compared to previous times when a traditional mix of antifreeze & water was used).
When the engine was cold, was the expansion tank half full?
Cheers
Charles
1980 Roadster

Vic Butler
Posts: 514
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Overheating

Post by Vic Butler » Sun Jun 14, 2020 12:21 pm

Ian. This morning the ignition was overadvanced and I tried pulling away at about 20 mph in top, no pinking. Everything was OK until last year I fitted a distributor without vacuum advance and then the overheating started. I didn't use it afterwards but I replaced the distributor for the original which had a vacuum advance. I can't put it back to normal as I don't have any original parts left. If anything it's running on the rich side.
Charles, it doesn't overheat when I run it on tickover. The electric fan cuts in, cools it down and switches off which rules out thermostat and water pump failure. The problem is when it's under load. I checked the expansion tank and it's half full when cold. I don't know if running it without a vacuum advance has caused the problem or whether it manifested itself during that time. I think pulling the head off is all that's left to do. It's not a difficult job. I can remove the manifolds without taking the carburettors off thanks to the brass barrel nuts on the studs. I'll leave it until I get the Land Rover back this week.
Thanks both for the advice.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Ian Fozzard
Posts: 391
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: Overheating

Post by Ian Fozzard » Sun Jun 14, 2020 12:30 pm

Best of luck with it! Keep us up to date with developments.

You could run a sweepstake on what you find - might offset some of your costs?😁

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Vic Butler
Posts: 514
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Overheating

Post by Vic Butler » Sun Jun 14, 2020 2:38 pm

I forgot to mention that there was no misfiring. I wonder if exhaust gases are leaking into the cooling system somehow when under load.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Ian Fozzard
Posts: 391
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: Overheating

Post by Ian Fozzard » Sun Jun 14, 2020 4:14 pm

Vic,
exhaust/combustion gases leaking into the coolant will cause overheating, but should be obvious as they will cause over-pressuridation of the cooling system and gas bubbles. Very often an eruption of hot water also when you take off the pressure cap. There are testing kits available to check for combustion products in the coolant.
Failed head gasket or possibly a crack could cause this.
Have you carried out a compression test? A burnt/leaking exhaust valve could get very hot.

If compression tests check out OK and you think the radiator and water pump are OK, I would still put my money on ignition timing or lean mixture at high revs/load. Check those plugs!!

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Vic Butler
Posts: 514
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Overheating

Post by Vic Butler » Sun Jun 14, 2020 5:23 pm

Ian, I carried out a compression test last year when this started and they were all within the 10% of each other. I didn't use it from May 2019 until last Friday but I regularly ran it stationary and it showed no signs of overheating off load. It revs cleanly up to the red line very easily.
I've constantly adjusted the timing to advanced that the engine kicks back against the starter and it makes no difference. I checked the plugs and they showed it was running rich and I haven't altered the mixture. I replaced the plugs this year with NGK BP6ES, same as the old ones.
I can't take it for a run due to the overheating.
Having eliminated everything else I've no alternative but to pull the head. It won't be done just yet though, but fairly soon.
This morning with deliberately over advanced ignition which contributed to a loss in power, the temperature gauge was nearing the red after about 3 miles but I was able to drive home slowly round back roads and the temperature eased back a little. I've put the timing back to where it was now because it's not that.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Vic Butler
Posts: 514
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Overheating

Post by Vic Butler » Sun Jun 14, 2020 9:17 pm

I've analysed both journeys made in the MG. Yesterday after refuelling I drove it along a level dual carriageways at 70 in overdrive top and it performed reasonably and the temperature gauge needle was in the middle. After a couple of miles there was a dip in the road and accelerating out of it caused the temperature gauge to rise. I turned off the dual carriageway and returned home via back roads and the temperature gauge needle went back central. I advanced the ignition when I returned home and checked water and oil when it had cooled down, both OK
Today's journey was on a single carriageway road
The ignition was too advanced so power was down. There was a long incline on.this road and accelerating up it caused the temperature gauge needle to head towards the red. This incline was longer than the one on the dual carriageway. I turned off the main road and headed home round the back roads and the temperature gauge went down to near normal.
After it had cooled down I retarded the ignition and checked oil and water again. Both OK
So, on the level on a light throttle,no overheating but accelerating up inclines causes the temperature to rise.
Many decades ago I had a car that exhibited similar symptoms and it was a slightly blowing head gasket so that's what I hope it is.
I won't be driving it again until it's sorted.
A pressure test of the cooling system has been suggested but with the position of the expansion tank I can't see how it would be possible unless it could be done through the thermostat housing filler. Has anyone ever had a coolant pressure test done on a late rubber bumper B?
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

User avatar
Charles Farran
Posts: 210
Joined: Fri Oct 31, 2014 11:08 am
Forename: Charles
Surname: Farran
Location: Warwickshire

Re: Overheating

Post by Charles Farran » Sun Jun 14, 2020 11:03 pm

Vic,
A silly question.
I assume you haven't got lazy rear drum brakes that are sticking/ hanbrake as the car hasn't been on the road for a year?
Cheers
Charles
1980 Roadster

Post Reply