How long would it take for an NA to overheat with an air bubble?

[davekmoore]

As title.

The car went into my local garage with the specific instruction to make the heater hot. They told me they'd succeeded with this by merely bleeding the cooling system.

The next day the car very happily did 2.5 hours at 75mph on the motorway, with the temp gauge at 11.30, albeit still with only lukewarm air from the heater. Then it overheated. This time I spotted it immediately, not from staring continuously at the "gauge", but by the heater suddenly giving me a one second blast of hot air. There was a smell of coolant during that one second and, of course, the gauge was off the scale. The difference with this one was, unlike BARMY's famous overheat, the motor was still running until I switched off after another second, and the fluids were still in the engine, and the steering rack wasn't locked by engine debris, and I wasn't doing a triple spin with flames coming out of front and back.

I let it cool. No coolant under the bonnet or under the car. Radiator full. Googled the nearest garage, which by pure chance was 2 minutes away (at normal speeds), and was an MX5 specialist. Drove the car to them in 6 slow short steps, taking 30 minutes, not 2, because of allowing the engine to cool down.

The dealer who sold me the car 3 weeks earlier is trying to blame my garage for leaving an air bubble in the cooling system when they failed to fix the heater. But surely an air bubble would have caused an overheat well before 2.5 hours? And might have caused a slower overheat that I could maybe have caught before the head gasket went?

He's also trying to tell me I did the head gasket by driving the car to the garage. But surely any NA that's gone off the top of the gauge has already done the head gasket? And in fact, by turning the motor off so quickly, I've saved the seller having to find and fit a complete, proven low mileage motor?

The MX5 garage diagnosed a faulty water pump, based largely on the heater not blowing hot air, and there being NO pressure in the cooling system. As you can see from this pic, they were right:

water pump very small.jpg

There are NO vanes on the impeller AT ALL, so the cooling system had not been under pressure for years, maybe decades. No wonder the heater wasn't getting hot.

This proves a couple of things:
1. My local (now ex) garage lied about getting the heater hot, as that would be impossible with the water pump in that state.
2. The seller was incorrect in describing it as an excellent, well maintained car.

I'll be going after both of them to recover my costs. If I get more than my costs from the two of them, I'll donate the extra to meningitis research, as I'm largely as survivor/recoverer, from 40 years ago.

[93_Clubman]

But surely an air bubble would have caused an overheat well before 2.5 hours?
And might have caused a slower overheat that I could maybe have caught before the head gasket went?
But surely any NA that's gone off the top of the gauge has already done the head gasket?
And in fact, by turning the motor off so quickly, I've saved the seller having to find and fit a complete, proven low mileage motor?

Congrats on the NA purchase Dave!
Heard of some that overheated without apparently blowing the head gasket, but not all are that lucky.
Ensure the MX5 specialist that's fixing the car checks or has the head checked for hardness, as the metal can get soft from overheating. By the look of the water pump either its never been changed or it's been run on tap water for a long time, neither of which is consistent with the advice from the seller. Noting you've only very recently purchased it, my greater concern is that it's not the first time that it's overheated given the condition of that water pump.

[greenMachine]

But surely an air bubble would have caused an overheat well before 2.5 hours?
And might have caused a slower overheat that I could maybe have caught before the head gasket went?
But surely any NA that's gone off the top of the gauge has already done the head gasket?
And in fact, by turning the motor off so quickly, I've saved the seller having to find and fit a complete, proven low mileage motor?

Congrats on the NA purchase Dave!
Heard of some that overheated without apparently blowing the head gasket, but not all are that lucky.
Ensure the MX5 specialist that's fixing the car checks or has the head checked for hardness, as the metal can get soft from overheating. By the look of the water pump either its never been changed or it's been run on tap water for a long time, neither of which is consistent with the advice from the seller. Noting you've only very recently purchased it, my greater concern is that it's not the first time that it's overheated given the condition of that water pump.

This (bolded). Some further investigation would be good, preferably before the HG is done - though I suspect it might be too late for that. Certainly the head should be checked for hardness, I'd be a little concerned about the rings too.

[davekmoore]

The seller is making noises about refunding me for the purchase, and I'm making noises about the costs of the breakdown. So, for the moment, the only work that has been done is the WP and TB, plus a compressions test showing 170/150/150/170, which I suppose shows the HG gone between cylinders 2 and 3, which should come back to 170 to match or near match the rest. As far as I can tell, 170 isn't the best, but not the worst for a 157k kms car, but does suggest that the rings are good for a while yet.

Thanks for the thought of checking hardness. If the head is to be done by the garage where it's broken down, I'll ask them to have their machinist check the hardness too.