Running hot after a new thermostat

Hi Landy fans. I'd appreciate any pointers on what to try next with my series 3, 6 cylinder petrol which I have been sorting over the last couple of years. It always ran with temp gauge nicely mid range but last month suddenly the needle went to the red zone after warming. This was after a session of tinkering under bonnet so thought an electrical issue could be the cause but see no loose wires. The fuel gauge read normal so I thought not a voltage stabiliser issue so installed a new temp sensor. This remedied things to a v small extent; the needle hovered 90% of the way up to red zone. Tried a second temp gauge and it too showed high temp. As an experiment, I removed the thermostat and this fixed the issue with the gage reading in the mid range (as had usually need the case). But I did not want to run without a thermostat indefinitely so bought a new, high flow unit. As soon as I fitted this new thermostat, back to the old problem with needle close to red. Did I simply buy a dud thermostat? I should have tested it before fitting as removing the housing involves multiple fiddly bolts since the PO fitted the alternator mount to one of the housing bolts. Any thoughts? Out with the spanners, remove and test the new thermostat or am I overlooking another possible cause?

First check that it's actually getting hot and welcome

Could be a dud, but could also be an airlock

Very many thanks kermitt rr. Interesting idea. Isn't one of the jobs of the jiggle valve/pin in the thermostat to avoid that? Then again, maybe the air lock is elsewhere in system. I think one forum post mentioned parking the car on a slope to have radiator as high as possible and then refill cooling system. I'll have a go at that in my next tinker.

landowner said:

First check that it's actually getting hot and welcome

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Thanks Landowner. The system seems to be getting hot. About 70-90 oC (depending on what device I use to measure, where I put the thermometer probes, how hard I work the engine) but no sign of overheating.

What should it be, sounds about right isn't it?

I'd try a new sender unit old one may be earthing out

Anaconda said:

What should it be, sounds about right isn't it?

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Thanks Anaconda. Yes, the measured temp seems great but the gauge on the dash reads only just under the red section and it's bad for my blood pressure. Running without the thermostat fitted, the gauge reads normal. That's the mystery.

landowner said:

I'd try a new sender unit old one may be earthing out

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Thanks landowner. Makes sense, but that was the first thing I tried. The new sender seemed to make only a v slight difference to the dashboard gauge reading. That's why I thought of the thermostat next up. Running without it - gauge reads normal. New thermostat in, back to the same problem as with the original thermostat. New one must be a dud?

Get a capillary gauge, they are excellent. We had one on the S2a, you could see when the thermostat opened on the gauge.

Burp it

Fill radiator, leave fill plug off.
Fill coolant tank, leave cap off. Stick a burping funnel in it if you’ve got one
Start engine, turn heat to HOT both sides.
Run engine idle until thermostat is open and you have heat coming out of the heater if you’ve got one…
Once air stops coming out of the radiator fill plug close it back up
Once the coolant level in the coolant tank stays constant replace cap.
Shut off engine.
Drive it for a day, let cool overnight, recheck coolant level, add if needed.
You can get burping funnels. I’ve had to do this procedure on both my Landy and classic mustang. It’s air in the system. This’ll rule that out.

Link to the burping funnel I use, works a great, used it for getting air oot after rad replacement, water pump replacements etc Amazon etc have got similar items

Fatrover said:

Start engine, turn heat to HOT both sides.

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Both sides? On a series!?

kermit_rr said:

Both sides? On a series!?

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Aye sorry. Was writing the same post for a guy who’s got the problem wi a mustang. I’ve not even got a heater in my series so I just left the rad cap off an stuck the burping funnel in and topped it up with coolant as the air came out. ( on the ‘stang I had to jack it up on one side where the rad top is and put both heater sides on……)…….feck me I’ve made it sound hard

If burping doesn’t solve it I would remove thermostat and both sender units and test them in pan of water observing and measuring resistance v temp

There are different Thermos for different engines, check you have the correct one

Lucknow Landy said:

Thanks Anaconda. Yes, the measured temp seems great but the gauge on the dash reads only just under the red section and it's bad for my blood pressure. Running without the thermostat fitted, the gauge reads normal. That's the mystery.

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I don't think it sounds like you have a engine cooling problem but a gauge/sender/wiring problem.
Son's Ninety is exactly the same with mismatching gauge and sender.
Like yours the gauge starting sitting almost in the red after thermostat change, previous the gauge sat a little cold. After faffing around I have come to the conclusion that it was running too cold before but now running about right, although gauge says otherwise. Too many senders and thread types, also worried about stripping thread in thermostat housing means we will put a new sender and matching gauge in the top hose, when we get round to it.
Never tested, or even looked, it but I think the old thermostat failed open.

Thanks everyone for the kind suggestions. This site is a great way to get ideas for troubleshooting! During a tinker today:
1) I tried burping the system and got virtually nothing out.
2) Fitted temp probes to the engine block and the top hose to monitor temps during engine warmed up to test the thermostat. Engine reached mid to high 70s oC and hose remained around 20 oC initially; then hose climbed to be about same as the engine so clearly the thermostat is behaving.
3) The fact that the engine never exceeded 80 oC but the temp gauge needle was close to the red zone suggested a false reading.
4) When I turned on the heater & fan (during the burping attempt) the temp gauge dropped BUT, not gradually as expected from the extra cooling effect, immediately! Then the penny dropped. The more switches I turned on (wipers, lights, hazards etc etc) the more the temp gauge needle dropped. Clearly an electrics issue. This kind of makes sense because the whole issue started just after I fixed the alternator. Has the extra output blown the original voltage regulator?
Any further thoughts appreciated!

I think voltage out of regulator is 10v so that’s a check and the senders can differ giving a different resistance at same temp giving a shift in temperature dial reading
Good news no overheating problems though

Dint think you need the regulator with an alternator ?

All a bit modern for me, but isn't the regulator on the signal, to buffer it?