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I recently bought a 2012 1.2 polo match 85k with an obvious engine swap at some point (date stamp 2008 on engine). As well as it running like a bag of spanner’s, rough idle, cuts out, hunting revs, smoking etc and also oil leaking from rocker cover, main seal. P.s fault codes were PO171 00.

I rebuild a 70k CGPB engine and had it sat there ready to go in (head, chain, rings) Whipped the old engine out, put the rebuilt one in. However, it still runs like a bag of spanner’s minus the oil leak. Pulsing/hunting revs, rough idle, cutting out and same fault code.

Replaced throttle body, maf, inlet manifold with new seals, cat, lambda, plugs, coils, no vacume/air leak, ECU, key, clocks, bcm and realigned via autel ds808.

Still the same.

Absolutely baffled..any ideas?

Throttle control?
Fuel pump?

I’m open to anything.

Injectors and fueling, maybe?

I thought the same, however I replaced injectors, rail, loom, throttlebody, ECU, bcm, clocks, whole engine, coolant temp sensor, air filter obv. Only thing left is fuel pump/filter.

Very weird that 2 engines do the same thing like that.
Maybe it is a clogged fuel filter. I would think the pump would either be running or dead rather than reducing flow.

P0171 is O2 sensor. I wonder if disconnecting it would let the engine run in open loop. Maybe that'll identify it as O2 or something else.