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I have a Volkswagen Polo 6N Petrol 1.4 8v 60cv from 1997 with 168 000KM.

It's my only car and I can't afford to get a new one. It's been looked after and has been in the family since new. I have been having some problems with it and I would like some help, if possible.

One problem is with the engine. Something odd starts to happen when driving on hot days. For some reason, when the car starts to reach the normal temperature it starts to lose power. This is more noticeable when accelerating. I can associate this loss of power with the engine reaching normal temperature for this reason: the car has no thermostat. In winter, when temperatures are near 0º C, the car runs constantly cold and drives smoothly and with all the power. The same thing happens during summer right after a cold start, it starts well but loses power when it gets to normal temperature.

Another problem I have is with the gearbox (I think?). Sometimes when I accelerate I can hear a buzz like a whistle. This happens more frequently in 1st and 5th gear. Once, on a long drive with four people, the buzz started to become louder. There were two times on that trip that when the fifth gear jumped to neutral. A mechanic changed the gearbox oil and said it had no metal pieces on the old oil.

Also, I obtained the following error code from the car:

00524 - Knock Sensor 1 (G61)

31-00 - Open or Short to Ground

Readiness: N/A

I’m looking for any advice you can give related to these two problems I’m facing.

Thank you in advance.

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Regarding the issues when the engine is warm:

Check your coolant temperature sensor and its wiring. If the engine control module (ECM) does not get a correct reading from the coolant temperature sensor, it assumes the engine is cold. On a cold engine the ECM changes the fuel mix to be more rich as this is required in cold conditions to properly start and run the engine. Cold as in "not on operating temperature". A too rich fuel mix limits the power output of an engine.

Another note: I don't know your engine, but you may have two independent coolant temperature sensors: One for the instrument cluster, the other for the ECM. So don't assume the ECM gets the right temperature, just because you see it on the dashboard.

Why this happens only on hot days

This problem might be noticable only on hot days as an engine without thermostat might be cooled "too much" during winter days.

Why your ECM assumes it is cold

This is the safest thing to assume as it'll help during engine start and a too rich fuel mix is considered to be less dangerous to an engine than a too lean fuel mix.

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