Long story (and low revs)

[JUV]

I've posted a few times here about the various problems we've had with our 02 plate, 1.2 AZQ-engined Polo we've had for the past few months. Unfortunately the problems keep coming, and we could really do with a bit of help from somewhere.

The history - we bought the car knowing that the engine jumped it's timing chain at around 50,000 miles, and had the work done to repair/replace the tensioner etc in 2006. We bought the car at 65,000m in September 08, and within a week had an emissions light which turned out to be a failed cat (not the usual probe failure, but the cat itself). That was replaced, and all was fine until car suddenly started revving strangely when started, and the emissions and later EPC light came on. The AA diagnosed a coked-up throttle body and cleaned it as best they could on the road, our local garage confirmed and suggested we see how it goes, it carried on fine for a couple more weeks and then the problem re-occured. We booked it into our local garage to have the throttle body off and cleaned, they did this but told us that the throttle body seemed to be broken and we'd probably need a new one. It was driving okay, but we booked it into our VW specialist place for them to take a look at.

The specialist garage agreed that the throttle body needed replacing, and would cost us about £350 - they said it was a very rare part to replace but on ours it appeared someone had tried to adjust it at some point and actually damaged it. Once that was on, they were still getting faults on their system so they went on to replace the thrust sensor for another £50, which they theorised might have been the original problem that the "adjuster" of the throttle body was trying to fix.

The car was returned to us, but the next time my wife tried to start it, it started to rev wildly and stalled itself a few times - basically an even worse version of the original fault! We took it back in, and eventually they surmised this was the EGR valve playing up. They replaced this and tried to give it back to us again, but this time - even after the increasingly desperate mechanic had spent the night road testing it - my wife started it at the garage and again found the engine was "hunting" for the right level of revs - although not as wildly and never stalling.

This time the mechanic, who's genuinely trying to help but admits he's running out of ideas, tried disconnecting the "cold start sensor". We've been testing it for the past few days with this disconnected, and although the emissions light came on almost immediately as the mechanic predicted, both starting and running have been absolutely fine. In fact the car generally seems to pull away much better in low revs than it did before all of this.

The mechanic's latest suggestion is that VW at some point released new firmware for the ECU to improve cold starts, quite possibly to solve this issue, and theorised that this car had missed the upgrade - but we've gotten onto our local dealership and they claim the ECU's been updated already according to their records. So we're left with a car which continues to start and drive fine without the cold start sensor, but no cold start sensor (whatever it is exactly that that does), and an emissions light stuck on.

Does any of this sound familiar to anyone?

[munsterpolo]

The Cold Start Sensor is basically the choke in an old car. But the ecu controls all this in your car. I'm kind of amazed that the ecu didn't pick up a problem with the cold start.
After cleaning the throttle body he must of tried to adjust the revs so it ticks over slightly higher, but this on the modern polo is a no no. Hopefully now the car will be alright for you.

[JUV]

Thanks for the reply, it's tricky to know what's going on really.

The "cold start sensor" they detached was the coolant temp sensor (my wife pointed it out to me), I suspect the mechanic called it that just to simplify things as it was specifically the cold start they were testing for - the theory being that the ECU was sending the wrong signals to the EGR valve when a cold start was detected, resulting in the erratic revving. They tried to reset something in the ECU to correct this (they're VW specialists with ex-dealer computer gear) but the ECU was coming back with an error - not a fault code, but an error to say this setting couldn't be changed (I know this all sounds a bit vague, but I wasn't there and I'm no expert on VW ECU's anyway). That's what lead the mechanic to believe the ECU needed some kind of software upgrade from the dealer, but the dealer's since confirmed it's already up to date (at least according to VW's records).

Anyway, to bring this story up to date, the last revving situation was at the garage last Saturday (10th Jan) which is when the temp sensor was disconnected. It ran fine without it until Tuesday (doing a couple of 50 mile round trips and at least a couple of cold starts) but with the emissions light on as expected, before we took it back to the garage and they reconnected the sensor and reset the light. It carried on starting and running fine until Wednesday light when (dissapointingly) the emissions light came on. We assumed at the time that the ECU was still playing up and not registering/incorrectly registering the EGR valve, leading to a fault, but carried on driving it (without any other issues) until this Saturday (17th) when we took it back into the garage.

This time they connected up the computer and found none of the previous faults, but a brand new one - "Flow to EGR too low" or words to that effect. The mechanic diagnosed a "gunked up" pipe feeding the EGR valve, took that off and cleaned quite a lot of gunk out of it, reconnected it and cleared the fault. We've since done a few more miles than it initially took for the emissions light to appear on Wednesday, and so far no real problems - although yesterday it apparently cold-started in higher than normal revs, circa 20-15 thou before settling down without issue.

No idea if anything is really "fixed" or there's some kind of intermittent ECU fault at play, or something else entirely.. but I'll post progress here just in case it helps anyone else searching for the same faults, and if anyone else can help explain any of this then feel free.

[munsterpolo]

In regards to the software update i had one done to my car about 4 to 5 months ago. Its sounds very strange like in regards to the gunked up pipes all i can recommend is a good long trip to try and clear the system.