Volkswagen Polo III (1994 - 1999) - 98 1.6 Lumpy acceleration when cold and jerky over

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Mon 15 Feb 2010 12:13
98 1.6 Lumpy acceleration when cold and jerky over - Experienced Insight
This Polo is in immaculate condition, has a very low mileage and is regularly serviced by a VW dealership. It is mainly used for long runs rather than local trips. Problem is that when cold it has recently developed a persistent tendency to flat spot when accelerating at low to medium throttle settings - when warm or hot it runs perfectly. The engine also jerks on the over-run at all temps as if the fuel cut off is being overly enthusiastic. Starting is always fine and fuel consumption normal. Interestingly, when it came back from its annual service very recently these faults had seemingly disappeared only to return a few weeks later. The car is run on top quality juice only. Any ideas?
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Mon 15 Feb 2010 13:53
98 1.6 Lumpy acceleration when cold and jerky over - CraigP
Hi,

I have a similar (same?) engine and had these problems. It's very common, and this is from my experience -->

First up, most common and cheapest is the Coolant Temperature Sender - £28 from VW for a revised unit, very easy to change -- briefly, pull out the tab sitting below it in the pipe, then CTS lifts straight out after unclipping the connector. Be fast with the changeover to minimise coolant loss. Top up reservoir with coolant, take it for a run. Reservoir is highest point in the system so no need for faffing about bleeding the system. Reservoir should be at max when cold.

Next up is MAF meter -- can buy a recon Bosch unit very easily on exchange basis from eurocarparts at £60. Have read a lot of people strongly advising to buy only new MAFs (£110) not recon. You pays your money...

Again pretty easy DIY changeover, this time around you will need 2 tools -- water pump pliers to compress the wee spring clips VW use and a smallish flatblade screwdriver to make popping off the wee spring retainer on the connector plug easy.

Disclaimer -- i am not a mechanic, just keen DIY & this is what worked for me.

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Wed 17 Feb 2010 12:14
98 1.6 Lumpy acceleration when cold and jerky over - Experienced Insight
Thanks for this. Will fit a new sensor for starters. Must say that as a keen home DIY-er am suprised that these are commonly encountered faults. Leaves at best an open question about the quality of the original components ! Strongly suspect that it is the sensor because the garage may have disconnected it for the purposes of the MOT emissions test,then reconnected it only for it to fail again after successive warm ups.
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