I have a 1.8 petrol C-max that has developed an unstable idle problem when the car is moving.
When I?m slowing to a stop the rev's will bounce from around 500 to 1000 rpm until the car comes to a complete stop then the engine will return to normal idle revs.
The problem has gotten worse over time and now the car is nearly stalling with the battery light coming on. I have scanned the ECU for but there are none registered.
The car is also often a pain to start normally when been standing overnight the car will start then the rev's just drop off to 0 causing a stall.
I was wondering if anyone on here has had this problem before I start spending £££'s on parts that do not need changing.
Was thinking of changing the Intake Manifold Runner Control valve first
Any help or advice would be much appreciated
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To my (untrained!) eyes/ears sounds like a sensor of some sort.
Here's my understanding of how it hangs together, but someone will chip in to clear up any confusion.
When you're driving along there's a closed feedback loop -- the MAF sensor measures the amount of, and temperature of the air coming in. These tend to fail slowly and take forever to actually die and register a fault code.
They can cause unstable revs by incorrectly under-reading the amount of air that came in. This would give similar behaviour to a vacuum leak (bouncing revs). However, when in closed feedback loop mode, the lambda sensor can detect lean exhaust (too much air) and instruct the injectors to fire in more fuel. Provided the lambda sensor continues to work, this underperforming MAF could be largely disguised from you, with no codes logged either!
Then there's the combustion bit (injection of fuel, spark etc.). The coolant temp sender could be telling the ECU the engine is colder than it is, causing the ecu to request more fuel. This would give juddering on the overrun too.
Then theres the lambda sensor, when you come off the throttle i think a lot (most / all?) ECUs go "open loop" for a while and disregard the lambda sensor. That would suggest to me your lambda sensor is working ok and telling your ECU how to run the injectors when in closed loop. When it switches to open loop, whatever's wrong upstream is too much for the standard ECU map to disguise.
I'm guessing MAF or Coolant temp sender, but would appreciate someone more knowledgable chipping in with guidance on my logic here!
To test MAF, disconnect it and the car should run the same way (might be fine, might be a bag of nails) in all situations with no change during the overrun as you experience now.
A dying CTS typically shows a lazy temperature needle on the dashboard -- car takes a while to get up to temp etc. not sure about unplugging this to test it -- i know you can do it, but i'd be more inclined to go for live diagnostic readout from a laptop and glance at the CTS readout when the car is behaving like this.
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Thanks for the info,
I have looked at the live diagnostics with my code reader and the coolant temperature sensor looks ok.
The MAF sensor being at fault would fit in with a couple of other symptoms, when the is car warm and idling the idle speed is around 700rpm if you try and raise the rpm to say 1200 you tap the accelerator and the rpm rises to around 1600 and will sometimes keep climbing. Also the Long term fuel trim is often above +10% indicating unmetered air is entering the engine.
Going to get the air box out at the weekend and have a look at the MAF sensor.
I was also wondering of the MAP sensor could cause the same issues? I know it is integrated with the intake temperature sensor.
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