Hi guys, I'm hoping that you can give me some advice prior to taking my car to the mechanic. I have a 2008 Roomster 1.9tdi with 62,000 on the clock, I have had a problem with an erratic idle for around the last 12 months / 12,000miles, I've been ignoring it, but it could be getting worse.

If when you've read this you think it's in the wrong section of the forum, hopefully it can be moved ?

I've owned the car 'almost' from new, it was a dealer demonstrator. It's had few replacement engine parts yet, no injectors, glow plugs etc, the one part that was changed was the manifold flap motor/controller which was causing a 'bump' at shutdown and turned out to be covered in black carbon / coke crap, but even this had been operated for some time whilst that issue persisted (otherwise the car is serviced on the button)

The idle can randomly decide to jump from 950rpm to maybe 1300rpm, this can be at any time including initial startup or as you're driving in traffic, it's often noticed when you're rolling up to lights, it doesn't go to rest where it should, it can go down to 950rpm after a few seconds by itself or sometimes a blip of the throttle can set it back to 950, other times four or five blips make no odds, it's like the choke on old cars. Rattling the pedal makes no difference, nor does trying to lift the pedal up with your foot behind it. (Note, there are no other abnormalities in the operation of the pedal) This can happen with the engine cold as well as hot, and after any condition of driving.

The car was remapped to 153bhp by Awesome performance in Manchester, but this was more than 4 years ago, and both Awesome and my mechanic feel that this problem is unrelated to the remap. My driving is generally sedate town work with mainly short journeys but also about 10% motorways (the same routine that caused no issues with my previous Astra 1.7td over 140,000miles) I do generally use the cheaper supermarket fuels, and don't use a fuel system cleaner (I'm not putting suggestions for the problem in your head, just being honest) there are never any issues with emissions tests on MOT.

This 'changing of idle speed' has been known to effect the car when in gear, it did this last night when slowing for a roundabout in second gear, there was a bump as the car effectively reacted to the accelerator being pressed when it hadn't been, it's possible that this intermittent problem could be dangerous ! (No, it's not me accidentally pressing it, or bad heel and toe

Now, something further that must be connected to this problem. When you drive, say in 4th gear at 30mph, and you have about 5% or even, lets say 3% gas, you're following a car and coasting, maintaining speed only, the car starts to judder. (Please don't tell me I'm driving too slow in gear, I've been driving for 20 years, I'm stupid, but not that stupid !) the judder can be felt by the passenger and gets worse to a point, but disappears if you let up on the gas or press it even a couple of percent more. I sound 'picky' with this comment, but it's real, and something that anyone would notice given a little time with the car. These two problems must somehow be connected.

I could 'try' to get a video of this, but I might be limited to taking it on an iPhone, for now, any suggestions would be great.

Please, please post your ideas or if you have ever experienced anything like this, it's getting really annoying now that it's revving on 'high choke' in traffic queue's :(

Many thanks

Paul

One thing that springs to mind immeadiatly is the Accelerator pedal, sticking internally so it raises the revs then after a few pedal presses it returns to normal. Does it sometimes stick/raise when you start the car up? next time it happens raise the pedal with your foot to see if it returns to normal, if so then you have your answer - replace the pedal.

 

Seen similiar on Fabias and Ibizas.

 

the other thing that can cause idle issues is the temp sensor or if the DPF is mid way through a regen. Yours i dare say does not have a DPF.

I would spend £15 and ten minutes of your time to change the fuel filter. I had an 'erratic idle' on mine and a missfire that then developed, having replaced injectors, wiring loom etc. the part that made the most impact was the £15 filter.

 

Re-reading your post about the juddering, very similar symptoms to mine had.

The filter wouldnt cause the rise in idle speed though.

Its a simple case of a failing/blocked EGR valve.

I'd be checking MVB for pedal position, start and stop the engine a few times with the pedal disconnected to see if that makes a difference and try running the car with the egr disabled also.

Failing Coolant Temperature Sensor caused erratic idle problems on my old Octavia.  Is it recording any faults at all?

Thanks guys so much for your replies ! There are no warning lights, must there be a warning light for some fault codes to show up ?

The opposite mate, there can be fault codes stored but non OBD faults and 2nd priority codes can take a long time before they show a light.

Also it's worth doing a full scan as faults effecting one system might be stored in another control module, for instance a Temp sensor fault can show in address 17 - dash pod rather than in 01 - engine electronics.

Hopefully your mechanic has VCDS or equivalent (not just a generic OBDII code reader) and can hopefully pin down the source.

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Thanks guys, I've got a note of all of your suggestions, I'll be speaking to the mechanic in the next few days. I'll come back and post when it's sorted, let you know what it was

Cheers

Paul

Coolant temperature sender failure does give erratic idle but its always accompanied with a genuine fault code.

EGR does not give a fault code unless it's completed stuck shut.

It it's juddering open and closed it will really mess up the throttle.

For the £20-30 its worth having your mechanic cleaning it out.

I found a failing CTS threw a fault only periodically. As long as it gives a reading within an acceptable range the ECU will accept it. Only if it comes up with a daft reading does a fault get logged.

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