Question: My Volkswagen Polo Classic 1.6 (year 2000) only performs reasonably well at light throttle openings. As soon as the revs rise, it begins to exhibit “flat spots”, and struggles to go up any incline. It feels like it’s being “choked off”. It backfires through the air intake chamber when under load, and spits through the exhaust when coasting downhill against compression. Although it starts easily in the morning and idles fine while cold, the performance and idling deteriorate as soon as it warms up. Occasionally it suddenly starts revving, and the car leaps forward so rapidly that we run out of road. I should add that the car has stood for three years. The fuel pump has been removed twice for inspection. Nothing visibly untoward was found, but its casing was cleaned. I ran injector cleaner (mixed with a little paraffin) through the fuel system. The fuel filter has recently been replaced and the air cleaner is not blocked. How can I cure my deadbeat Polo?
Answer: Derek, the symptoms suggest multiple problems. I can only recommend that you systematically investigate each of the most likely sources, namely:
-The fuel pump: the seals could be shot because the car has stood for three years. Have a complete pressure test done on it.
-The throttle body: it may have to be recalibrated if the battery ran down while the car was standing.
-The injectors: the injector cleaner (and the paraffin) may have loosened deposits in the fuel system which are now blocking the fine-mesh strainers in the injectors. Have them professionally cleaned and tested.
-The temperature sensor: Some of the symptoms you describe are consistent with a faulty temperature sensor. With the right equipment the sensor can easily be checked by monitoring its signals as the engine warms up.
-The fuel pressure regulator: fluctuations in fuel pressure can obviously cause weird behaviour. Have the regulator — which sits on the end of the fuel rail — tested by a workshop with proper equipment. One should not overlook the possibility that the culprit may be something as mundane as defective plug leads. Remember: any troubleshooting exercise should start by eliminating the simplest possible causes first. You can take heart from the fact that the electronic control unit on that VW engine has an excellent reputation for reliability. I am sure that with a systematic approach, always testing exhaustively before replacing any component, you will soon flush out the gremlin(s).
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