Twingo 1.2L (Renault R4 engine) 1993
The car has done only 40,000 miles. The engine works perfectly well, but recently acidy fumes started to come from around the air filter, but not from the exhaust! This happens when the engine has warmed up, but it does not seem to affect the working of the motor.
What can cause this?
Many thnaks for your help.

i do not know much about this car but... if the engine otherwise starts and runs well the vapour you describe is unlikely to be excessive piston ring ****by. if no obvious cause is present, such as a minor coolant leak or minor oil leak a common cause is an engine breather fault; such as a perished, blocked, kinked or displaced hose or blocked pcv hole into the inlet manifold or vacuum system. of course if the engine does not run well!!! most of these things are caused by a lack of careful maintenance and can be cured by same.

Thank you for your suggestions. In the meanwhile I discovered a considerable amount of coolant loss which seems to indicate that the head gasket is the culprit. We will now check this and if neccessary, replace the gasket.
Regards
Petervm

Well, if that's the case, then good luck. Please post about the result. If a new gasket does not solve coolant leak, then the mating surfaces could have warped. Has you car ever experienced severe overheating?

it would be unusual for a ****n head gasket to cause smoke from the dipstick, etc. could be there is more than one problem here and a good idea to gather as much information about what is going on as possible before the strip down and then to proceed steadily; looking for trouble as you go with a view to doing the job thoroughly first time rather than just trying things.
i suggest if the gasket is faulty the reason for this is investigated as part of the job. replacing a head gasket should not be undertaken lightly, especially on modern engines, and doing it by the book in the smallest detail is very important to get a long lasting result and value for money. due to corrosion more than warping most cylinder heads need machining these days and the possibility of a crack after any overheating cannot be ruled out and the valves must not be forgotten; inlet valves especially must seat perfectly these days.
while the head is off the condition of the top piston rings can be felt with a feeler gauge and a visual check of the bores and piston tops carried out. filling the bores with parafin and timing its descent past the rings will highlight a faulty assembly.
a point to remember; when a head gasket leaks water into a cylinder the result is superheated steam which removes all carbon from the valves and surfaces in that cylinder, including that which has been taking up wear around the piston rings. a noisy engine is often the result.
i hope this all helps without being too daunting. best of luck!