Hi,
I have a 2007 1.2 petrol Polo (9n3), manual, 89k miles. 
Recently she has been intermittently revving on gear changes - I'll drop the clutch and the revs will hang at whatever they were when the car was in gear, and sometimes even climb higher. It goes back down by itself with the clutch down, but very slowly. When I come to a stop, sometimes she idles high (~1500 rpm), and sometimes she'll oscillate between the ~850 rpm she should idle at and ~400 rpm where the engine almost stalls. This seems to be temperature dependent: she behaves at the start of a journey but starts acting up once the engine is hot.

 

Without knowing exactly what is wrong, I've just been throwing spaghetti at the wall and hoping that something I try will stick. Initially, I cleaned the throttle body and the valve (in place, I didn't take it out of the engine bay) with carb cleaner and a rag, hoping it was just gunked up and the valve was sticking. It was a little dirty, but cleaning it didn't make any difference. Then I tried changing the clutch switch as I'd read on an Audi forum that this had fixed a similar issue - no change. I've checked vacuum lines and the airbox but can't find any cracks or splits, there's no hissing/sucking noise that most sites describe for vacuum leaks, and no issues with temperature. I can't see any damaged or exposed wiring anywhere and have no warning/check engine lights on that would suggest faulty sensors. I ran a diagnostic check using an OBDII reader, but no codes came up. 

 

Has anyone else had this problem, or have any idea what it might be? Feel like I am very quickly heading towards an expensive diagnosis at a dealership...

TIA

Double-check the brake servo vacuum hose at all the junctions of the hard plastic sections. The non-return valve connections seem to be a particularly common point for trouble.  Feel with fingertips on the underside as that often seems to be where they crack worst. Post a photo or two from above/below ideally.  

 

This is a particularly bad one I found on a similar engine:

 

Hi Pete, 

Thanks for the speedy response. I had a look and a feel all the way along this hose yesterday afternoon, but couldn't find any cracks - pics below. 

 

I'm going to attempt to take the throttle body out today to give it a thorough clean in case there is some gunk I couldn't get to from the top. 

Relating to this - when I push down on the valve to open it up to clean, there is some squeaking as the valve rotates. Is this normal, or could this be part of the issue? I might be barking up the wrong tree here but thinking that as the engine warms up and the parts expand, something that is squeaking/having a hard time moving when cold might get stuck.

 

TIA

I'm not sure about the throttle body, I wouldn't think moving it by hand is a very good idea, the gears are all plastic as far as I recall, and could get damaged.

 

The other place to look for an air leak is on the output hose coming from the PCV valve. Here are a couple of photos from above and below showing where I mean.  The soft rubber coupling pieces don't appear to be sufficiently oil resistant to last:

 

You may find that moving the hard plastic pipe around while the engine is running affects the idle by opening/closing such an area of degradation.

OK I've left the throttle body alone. Had a look at the PCV valve and associated hose - it's different to the one pictured above. The tubing is hard, ribbed black plastic, and I don't have the soft rubber couplings. I checked all along the hose, including the connections at both ends, and can't find any splits. 

 

I'll be honest, this is new territory for me - out of curiosity I disconnected the PCV valve to have a look at what state it's in. From what I could see of the inside without taking the cap off, it looks like there's a fair amount of oil built up in there:

 

I've had the car ~6 months and only have a limited service history, so I have no idea when or even if this has ever been changed. Time for a new one?

 

Ah, sorry, I made a 50/50 guess on which variant of 1.2 engine you had and guessed wrong.  

Normal for it to get oily.

With it back in place, engine idling, try blocking that little hole just above the main entry, nicely visible in your last photo.

Doing so shouldn't make any difference to how the engine runs, but if it does, it probably indicates that the rubber diaphragm inside there is bust or leaking.

 

No worries! I probably should have specified at the start it's engine code BME. 

Once I'd put it back, I blocked the little hole both when the engine was cold and when it had warmed up, but there was no change to the idle either time.

 

I didn't have any issues over the last week or so with the unseasonably cold weather (the temperature outside was consistently below 5°C, at least while I was driving). I thought maybe I'd unknowingly fiddled with whatever was causing the issue, but today it's 15°C and sunny, and I did notice the revs hanging again a couple of times. I have no idea if that's a coincidence or genuinely related, but thought it was worth mentioning :') 

Does the temperature gauge rise up to an indicated 90° fairly quickly from a cold start and stay there? Say within 3 miles of driving.

I hadn't ever really timed the temperature rise as such (just knew that it got up to 90° in the first 5-10mins), but made sure to pay extra attention to it today. After 1 mile it was at 60°, 2 miles was 85°, and 3 miles was 90°. Stayed at a constant 90° after that (another ~35 miles).

 

Probably a stupid question, but may I ask what this is to diagnose/rule out? 
Thanks! 

That all sounds fine.  

The question was asked to try to see if the engine coolant temperature sensor might be reading wrongly. This sensor reading has an impact on the fuelling that the engine ECU demands.

This was a thought generated by your observation that the misbehaviour presence/absence varied with outside temperature.

 

If it is reading wrongly, I would say from your observations that it can't be very far out from reality, so is probably not causing the problems. 

 

Can probably safely rule out that idea.

I have the same issue. I replaced the throttle body but it's still doing the same thing. If you've managed to fix it. I'd love to know what you did. Thanks