Another update: the 1992 polo is now working and sounds good. Does anyone know what coolant goes in the polos of this age. I've always used blue for the current one but websites are saying maybe red?

The coolant temp sensor will make a difference, but not a huge difference. It's a thermo-resistor known as a Negative Temperature Coefficient (NTC) which measures around 3k ohms when the engine is cold, to about 300 ohms when it's hot. A failed sensor will be telling the ECM the engine it's hotter or colder than it actually is, which will affect engine starting and running but not hugely. Based on your description of the seriousness of the effect of the fault I'd say you're barking up the wrong tree.

Coolant temp sensor changed and it didn't change anything. I didn't really think it would but it was only a few quid and it's done now.

Once again check for a blocked exhaust. The symptoms sound like a perfect example of it. If you have a vacuum gauge connect it to the inlet manifold. Look on YouTube how to use it and what to look for. There are plenty of tutorials. A vacuum gauge will tell you a lot about the mechanical state of the engine.

Couple of items you may want to check . Do you have the facility to check fuel pressure? Should be around 1 bar. Easy way to check for blocked exhaust, is to remove the lambda sensor . It will make a load of noise , but will prove a point .

I'm sorry but I don't have the facility to check fuel pressure, neither do I have a vacuum gauge.

Is the removal of the lambda sensor a guaranteed way of seeing if the exhaust is blocked. Is it possible it could be blocked before the sensor?

Unlikely, but a blocked catalyst or rear silencer is very likely. Drop the front pipe and run the engine. Be aware it will be loud, so you may annoy your neighbours. But if the engine runs properly you'll know you've found the problem. If it makes no difference then the fault is elsewhere, but at least you'll know.

Air filter is dirty but dry. Although the air box and filter isn't on the car at the moment.

Took the lambda sensor off, reconnected it (wiring) and it started and ran okay.

So the exhaust is blocked. Now you just need to identify exactly where, but it's more likely than not it's the catalyst that will need replacing. Could be the rear silencer too though.

Any recommendations on how to clean up the lambda sensor while it's off the vehicle?

I wouldn't even bother. It's highly recommended that if you have to replace the catalyst you should also replace the lambda sensor with it.

Nope scratch that, still doing the same. It's running, idling fine but it seems to lose power when heavy throttling.

Cleared out the centre pipe and backbox and refitted them and car seems to be working okay. Drove it to work this morning (about 17 miles) and didn't seem to have any issues.

The only thing is the idle revs seem to be quite high once the engine gets warm / up to running temp.

It'll take a bit of time for the ECM to re-learn new values after it' been running with a blocked exhaust for however long. Assuming you have no vacuum leaks or anything else wrong I'd drive it for a couple of hundred miles and see it if settles down, which it may well do. If after that time it still idles higher than it should then remove the idle speed stepper motor and clean it.

That's what I've heard.

I reconnected the battery after leaving it disconnected for a little bit as I here that will reset the ECM back to default values and it'll relearn things in due time.

Still need to replace a couple of breather pipes (thick one coming out the back of the block and skinny one that comes off the top.

Weirdly on the way to work this morning my wipers stopped working. Checked the fuse and it's broken. Replaced with another fuse and yet the wipers still don't work. If you wiggle the stalk between off and on (speed 1) they try to move a little but not quite.

On another note: THANK YOU ALL FOR YOUR HELP, ESPECIALLY RAILROAD!

These use an external electronic idle controller that incorporates an idle switch. This unit may be faulty. If I recall, the idle speed can be adjusted electronically, but you need diagnostic kit to do it.

Long time, no speak.

Car has been working fine... until today when I got the same problem as stated above.

Loss of power and rough idle. Took the sensor off and car was working better but not perfect.

Now the car won't start. Sensor still connected (wiring) but not in the exhaust.

What?

Took the lambda sensor off (still connected) and the car won't even start. It turns over but nothing else.

Tried the lambda sensor off this one on my other one and that starts up and runs.

When I had this prob before, removing the sensor would at least get things moving (although roughly) but this time nothing.

Will try the other forementioned checks when I can.

Update:

Car is running again. The rotor arm had snapped off and smashed to pieces inside the dizzy cap.

Oh the joys of old cars. Thank you for your time and help.

Took the spark plugs out and the 2nd from the belt end is covered in oil. Cleaned up and all sparking fine but as soon as you run it again the oil returns.

Long story short: HEAD GASKET had gone. Took head off, got it cleaned up, skimmed and refitted with new head gasket kit (including valve stem seals) and new head bolts.

Seems to be working fine now. Will try to let you know more as I drive.

Here's some pics:

MK2F Polo

imgur.com/gallery/NzK6CF2