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Engine idle ticking goes away when ac is turn on


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Hi All, new member here and most happy to join the community! Ive been able to find much information and the DIY's are spot on, so thank you all. Can use some help, first the particulars, I'm from Northern California and I have had my 1999 911 996 C2 Cab for ten years, 6 speed manual, 50,000 miles. I was told the IMS bearing had been done.  Recently,  I have changed the oil and filter (mobil 1 0w-40), filter clean. Air and cabin filters, fuel filter, plugs (not coils) water pump, low temp thermostat, upper idler pulley and serpentine belt.  Now the problem, the engine idle ticking, after 2-5 mins after start, (I believe its just at idle) has been going on for a few thousand miles or more, all my searches point me to piston slap, bore scoring, sticky lifters....etc. However, while trying to listen for the source, I read somewhere to turn on the AC and bingo the noise went away, or it was drowned out by Ac compressor, but I don't think so, turn it off, noise back comes back. I was going try to run it without the belt to try and rule out some of the accessory items, but the noise doesn't start for a couple of minutes and that's to long wait  before I cause major problemos.  What should be my next trouble shooting steps, or am I'm just grasping at straws in hoping for a simpler fix other then bore scoping and the like.

 

Thank you Paul

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A mechanics stethoscope comes in handy when checking for noises that only occur while the engine is running.  The AC compressor could very well be the source of ticking.  When you turn on the AC, the compressor electromagnetic clutch engages and stops free wheeling.  So the tick could be from the clutch when it is not engaged.  You can also check the other pulleys, power steering pump and alternator for noises as well.  You want it to be something like this and not something internal to the engine.

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Yes, started to do that, via the old fashion way with a long screw driver, since ordered up a mechanics stethoscope, should be here by the weekend.  Incidentally, I can get better access if I remove the air box, but in doing that, the AC will not  function, I'm assuming that has to do with the MAF being disconnected? Is there anything else I should be looking at? Many questions, but I'll hold off for now until i have more data to report.

 

Thanks Paul

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Is it possible to upload a video or sound clip? My guess is the elevated rev (cold start or a/c ON) makes the noise go away. Based on your description, it's more likely scored cylinder(s). You may want to use an endoscope to check for scoring.

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I've seen the clip before and yes that's sound like the ticking I'm getting, no need to upload lol. A endoscope is in my future. incidentally I was able to use a "mechanics ear" and listen to all the belt driven items, unfortunately/fortunately they all check out fine with regards to any funky noises.  The slightly higher revs, like you surmised must be drowning out the ticking. I would of thought the opposite, that the ticking would get louder as rpm's increased?  Once scored, do they continue to get worse or can it be managed? I do not have any abnormal smoke or soot inside either tailpipe, no oil level fluctuations and all looks clean so far,  although its was changed just 200 miles ago.  Looks like I'm in a heighten monitor mode. Other than a bore scope, or there any other items I should be checking or monitoring?

 

Thank you both for helping out, it is much appreciated.

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