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Hi all, new user here looking for info for my partner's car. She has a 2014 Astra J Limited Edition 1.4T (A14 NET, 140ps) with approximately 60k miles on the clock and FSH.

A few months ago, the car developed a rough idle which seems low to me (600-650rpm). The idle vibrates through the car. When driving, the car seems reasonably happy when your are a constant throttle or light acceleration, but if you accelerate hard or labour the engine in too low a gear, it produces quite a pronounced coarse growl from the engine. The car hasn't done as many miles as normal as she is still working at home mainly, but the problem hasn't got significantly worse either. The worst thing about this issue is that the car has not thrown a CEL or any DTC codes.

After checking some of the basics myself, she booked it in with a Vauxhall dealer (who I won't name), who suggested the gearbox had a worn differential (and suggested she should consider trading the car in rather than pay the quoted £2,500 - they didn't really have an answer when she asked why that would affect the idling). I really don't think this is a gearbox issue.

From what I can remember, I've checked (from general experience and reading countless Vauxhall and Chevrolet Cruze forums/YouTube videos, in no particular order);
  • Spark plugs - good condition, gapped correctly, all coloured same (leaner side of what I'd expect, but good).
  • Air filter - remarkably clean.
  • Throttle body - removed from car, small amount of oil present, cleaned and refitted.
  • PCV system - 3 valves checked and confirmed working correctly as per instructional forum posts/video.
  • Fuel injectors - removed connectors one at a time, all made a noticeable difference to how the car ran, I couldn't isolate a single one as making more/less difference. Measured resistance of all injectors at 14.5 ohms and resistance back to ECU as 2 Mohms. Using the stethoscope method, all sounds ok, but being tightly coupled on a common fuel rail means I'm probably hearing them all go simultaneously.
  • Brake vacuum line to intake manifold - removed and plugged hole in intake manifold, no difference.
  • Evap purge valve - noisy when operating, draws a very mild vacuum when connector removed and it should be shut, but plugging the hole for it in the intake manifold afterwards made no difference to engine idling, so I doubt it to be this.
  • MAF sensor - checked condition and cleaned with some carb cleaner spray. Looks ok and gives stable reading on my little OBD2 code reader device.
  • Turbo pipe-work - inspected this as I previously had this cause an unmetered boost leak on a Saab 9-3 TTiD I owned, nothing obvious.
One thing I haven't checked is the coil pack, but being a single moulded lump, I'm not sure the best way to test this (and would've thought it would've registered as a misfire DTC if it was open-circuit or HV breakdown.

I'm reluctant to take it to a garage without at least narrowing the issue down, as it's one of those problems that I think a mechanic (with the best intentions) could sink a lot of time and money into. I'm no mechanic, but I am an engineer and tinker lots with motorbikes and this has me completely stumped after countless hours of reading/videos and time under the bonnet of the car.

Sorry for the massive post, but any suggestions, diagnostic tips or experiences on similar issues would be much appreciated.

Cheers,
Joe
 
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