Diesel engines don't rev as high as gas engines and electronic controls don't allow over running the engine in neutral
SOURCE: rev counter
The idle speed controller is having trouble regulating the idle RPM to the speed the computer is asking. Go to the parts store and get some Seafoam (about $8/can). Pop a vacuum line off the intake manifold, while the engine is running, and feel for suction. When you find it (it should be sucking air into the manifold), put a tube into the can of Seafoam, and allow the engine to **** up about half the can. You may need someone in the car to hold the RPM around 2000 (the engine will bog and may want to stall out while it's taking in the Seafoam). When half the can has been pulled in, pull out the tube, wait for the engine to stabilize a bit, and put the vacuum line back on. Let the car idle for about 15 minutes so it can burn away the carbon deposits that the Seafoam is freeing up. Rev it up to about 4000 RPM every 10 seconds or so for a minute, and then periodically while the car is idling. You won't believe the smoke that comes out the back of the car, but that's what is supposed to happen - that's the Seafoam burning away the carbon that has gotten all over the inside of the engine, and is likely clogging up vacuum lines, the idle controller, EGR system, etc. It's great stuff.
Save the other half of the can to treat the car again in a week or so, or dump it into the oil, drive 30 miles, and change the oil and filter (it can break down oil sludge and deposits as well). Don't drive more than 30 or so miles though because the oil will be thined out and too much more mileage than that may compromise the oil's ability to properly lube and protect the engine.
You can get a second can of the Seafoam and pour it directly into the gas tank as well, and it'll de-resin the tank and lines, clean the injectors, and clean out the fuel pressure regulator as well. The stuff is as close to magic as you're gonna get and it works great. I've done about 20 cars with it now, and not one person has called back saying that they're car isn't much more peppy, efficient, and smooth.
SOURCE: vw golf mk4 2.0
If the check engine light is on look at the faults. It could be a defective cam sensor, the timing belt might not be in the place, or a defective mass air flow sensor.
SOURCE: Rev counter jumps whilst idling and then engine cuts out
Your problem sounds as if the thermo sensor....which controls the air
fuel ratio when engine is cold....is faulty. It is a common fault on
this model.
This is not an expensive part, and takes only a short time to fit.
Good luck
John
SOURCE: high rev, rough idle in golf
If your car is fuel injection,try changing the cold start temp sender.
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