Restoration of 1952 MG TD 2



Author: Bob McCluskey
First posted: 1 Sept 2000
Last amended: Dec 2015
Please email Bob McCluskey
Car No TD/11935
Engine No XPAG/TD2/12333
Body Type 22381
Body No 11301/78948






Overheating At first, after the resurrection, all went well. But after the car had done a few hundred miles, when I started to have more confidence and took it for longer runs, a problem developed with overheating . This car had always been prone to overheating, but it seemed to be more severe after the resurrection. This may be because previously it had usually been used only for relatively short trips - to work and back, or to the shops, and when it got there, it had a long wait during which it could cool down. In contrast, now it had frequent trips of an hour or so, followed often by a relatively short recovery time.

The first attempt at a fix, long ago, had been to remove the thermostat. Well, no thermostat at all must be better than a fully open thermostat, because it must have even less resistance to flow, right? I could never work out why it made the problem worse. Of course, it was due to the radiator bypass. As everyone else knew, the bypass was designed to allow the engine to warm up quickly, an absolute necessity in cold climates, but a bit of a luxury here in Sydney. Originally the thermostat had a sleeve to cover the bypass: when the thermostat was closed (ie when the engine was cold) the sleeve would leave the bypass open, so coolant would be pumped back around the block without going through the radiator; but when the engine was warm, the thermostat would open, and the sleeve was supposed to blank off the bypass, so coolant would be forced to circulate through the radiator. By simply removing the thermostat there was nothing to prevent coolant from simply recirculating through the block, avoiding the radiator altogether, even when hot. The fix was pretty simple: blank off the bypass circuit altogether, so the coolant had to be pumped through the radiator, giving both a chance to fulfil their destiny. Most people make a little blanking plate to blank off the bypass; I was not so professional, and - just as a trial, intending to do the professional fix later - I used a cork to block it off. Luckily we had a champagne cork, left over from celebrating the car's first run, which was just the right size, and which, regrettably, is still there...

Anyway, on a run up into the mountains, it ran well, and because there was a long lunch at the top before returning it had time to cool, and I thought the problem was fixed.

But it wasn't.

At the next club run there was only time for coffee at the rendezvous, before setting off. And here either the problem, or a new one, resurrected itself. The engine simply wouldn't start. The carburettors flooded, so it couldn't have been a fuel starvation problem. When it did finally start, I drove home, the car spluttering and stuttering all the way: as long as it was in first or second gear, and with no load, it ran OK, but as soon as I opened the throttle, or changed up a gear, it missed and threatened to stall. Well, I had seen that problem before in one of the mistakes I described earlier: while there was no load, so the cylinder pressure was low, the spark could fire the mixture OK, but when the load came on, and the cylinder pressure was higher, the spark failed. The problem was that the condenser (or capacitor) had failed, giving an open circuit, and was not fulfilling its critical tole in the ignition circuit. Steve Maas has shown that the capacitors usually fail internally due to poor construction techniques giving a poor internal contact, and it is easy to see from his results that the internal contact could be intermittent or fail completely when hot.

Solution: get and fit a new condenser. I couldn't find one to the original specification, but I bought one designed for high performance engines, and made it fit; and that particular problem hasn't reappeared (yet).

Next club run we had to take a ferry, and naturally had to turn off the engine while aboard. And the engine wouldn't start when it was time to disembark. There was a group of cyclists aboard, who helped push the car off the ferry, and enjoyed making clever jokes about the reliability of mechanical versus organic transport. Very embarrassing! The first thought was a vapour lock, which of course is known to be a problem with these engines. The fuel pump appeared to work properly: it was quiescent until I pressed the float chamber tickler, when it ticked away as expected. And when I slackened the carburettor union, it pumped petrol. So there didn't appear to be a vapour lock, and there wasn't a blockage in the fuel delivery line either, which I have seen. And after about 20 minutes, the car started normally and ran properly. And we passed the cyclists labouring up the hill on the other side of the river, but we didn't stop to discuss who was entitled to last laughs.

The problem - whatever it was - was clearly related both to overheating in the first place, and to getting a little rest after overheating. So I began a systematic effort to eliminate any possible cause of running hot.

Well, one known cause is lean running. I couldn't set the idle speed, and in the past I have found that this can be due to a leak in the inlet system somewhere, probably in the inlet manifold, which of course leads to lean running. Using a piece of plastic tubing as an ear trumpet, I couldn't hear any leak around the manifold itself, but there was a faint hiss around the carburettor linkage. So off to the carbie shop to get the butterflies renewed. And that fixed the idling problem, and must have helped with cooler running.

The next issue was timing. The spark was timed perfectly against the timing mark on the pulley. But the carburettor man suggested that the timing should be advanced until just before it started pinking under load. That made a huge difference to power and to petrol consumption: now I could drive up several hills in top, whereas previously on those hills I'd had to change down to third. And it's getting well over 30 mpg, even around town. So now less fuel is being burnt, and it's being used to make the car go, whereas before at least some of it was being used to heat the water jacket and exhaust manifold.

And the next to be addressed was the cooling system itself, comprising water jacket and radiator. The first element here was a two-part rust and scale remover from Liquid Intelligence. Judging by what came out, there had certainly been a lot of rust in the system, but I cannot be sure that it got it all. The second element was the waterless coolant, also from Liquid Intelligence. They won't tell me what's in it, but they claim it has better heat transfer because it is more thermally efficient. And they claim it has a corrosion inhibitor, apart from having no water to cause scale and rust. But most importantly, it has a high boiling point - 190 deg C. They claim that with normal coolants, as they approach boiling point, bubbles start to nucleate inside the block, and these bubbles impede the heat transfer, leading rapidly to a thermal runaway. The high boiling point of this coolant means that these bubbles never get to form and the heat transfer is not compromised. Well, I have no way of verifying all these claims. But subjectively, judging from the temperature in the cockpit, it does seem to run cooler.

But I still had the bloody problem.

Now I began to get desperate. I had previously rebuilt the petrol pump, with a new diaphragm, new valves and gasket, and of course new points. And even though I knew the problem lay somewhere else, I took it all apart again, and even reset the diaphragm return spring. And then I put it on the shelf, and bought a new pump, with electronic control. (It looks and sounds identical, and, although I haven't taken it apart, I'd be pretty sure the working parts - the diaphragm, the valves, the points, and possibly even the escapement - are the same, but with electronic contact breaker to avoid the old problem of burnt out points. It is out of warranty now, so I might take it apart to see).

And I bought a new coil. I found there was a lot of confusion over modern coils, which are intended for -ve earth systems and are marked simply + and - , and how they should be connected in a positive earth vehicle.

I resolved the issue to my satisfaction by connecting the + on the coil to the negative (live) battery terminal, via the ignition switch of course (see the discussion in the electrical section).

But it still did not fix the starting-when-hot problem, even though, shamefully (believing it to be wrong), I did try with the coil the other way.

Finally I thought to use a timing strobe to make sure the timing hadn't slipped, or worse. And there was no spark at all. And after about 20 minutes, as before, while I was still cranking the engine and trying to find the fault with the strobe, the spark suddenly reappeared and the engine ran as though there had never been a problem.

I am still a long way from resolving this problem. What could make the spark disappear and then magically reappear? I am currently leaning towards the contact breaker points as the problem: I think when it gets hot enough, the points jam open. In that case, of course, no current would be established in the primary, and therefore no spark. I believe I have measured an open circuit when the points should have been closed, but it is not easy because firstly, it only happens after the car has been running for about an hour, and secondly, merely putting the probes of a multimeter onto them is enough to move them and make contact again.

But I will get to the bottom of it!

More to come...




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