"Lumpy" or "erratic"??? - GT-R Register - Official Nissan Skyline and GTR Owners Club forum

Jim27 · 1st November 2005, 05:40 PM

If you say you've got an erratic idle, everyone sucks through their teeth and says "Ooooh - must have an air leak somewhere on the intake side."

But if you say you have a lumpy idle, everyone says "Yeah, must be the cams."

'scuse me for being thick, but can someone please explain to me the difference between a lumpy idle and an erratic idle? Even better - link to some audio or video clips where you can actually hear the difference - that'd be great!

Still trying to decide whether my idle is lumpy or erratic!

Cheers,

Jim

R32 Combat · 1st November 2005, 06:20 PM

I'd say an air leak would result in a higher idle and running a tad leaner than usual. The idle would be smooth but higher.

An erratic idle would be one that goes up and down, a little or a lot. Say between 900 and 1100 or 800 and 1600. The fluctuations would be regular. Like your doing it on purpose. Why, I don't know, but it could be a faulty wiring or maybe an incorrectly set up ECU.

As for lumpy idle, I've never heard a lumpy 6 cylinder. If it is, then turning the idle speed up sorts it out. A description of what it sounds like would be like it keeps trying to stall, the revs drop some, it sounds rough, it picks up again but still sounds rough. These RPM fluctuations would be irregular because the flow across the top of the cylinders is too slow and it hardly runs at this speed.

I don't know if you've ever heard a top fuel car/bike, but they suffer from cronic lumpy idle or cammy idle and 2-3000 rpm.

Eagle Eyes · 1st November 2005, 10:02 PM

It might have something to do with what they call too much valve overlap.
Just like my engine had. It sounded very strained and not smooth at all when
idling. I advanced the exhaust cam by about a tooth and re-adjusted the timing (set to 15 deg) and it was a lot better.

skyline501 · 2nd November 2005, 05:59 AM

I would describe mine as 'lumpy'. Although it idles at a rock steady 1100 rpm, the whole car seems to pulse or rock, much like an American V8. I assumed it was the asymetric cams (HKS 264 /272) as it only appeared after the stage 2 upgrade (550bhp). The noise (and I mean noise) from outside the car 'pulses' too. Sounds marvellous.
I'd be interested if anyone has an expaination.

Vincenzo

Jim27 · 2nd November 2005, 08:30 AM

Any sound/video recordings would be great

mattb · 2nd November 2005, 09:35 AM

In my experience a lumpy idle, on my car, used to make the revs bounce from between 600 to 1100 and stalls coming up to roundabouts in hot weather. I tried all sorts to remedy this, cleaning AFMs, changing plugs, checking pipework, carb cleanering the inlet manifold. None of it helped. Eventually I had to give in and admit that it was a leaky exhaust manifold, which is a common problem on a GTST. Got that fixed and everything was absolutely perfect. Unfortunately the sucky teeth noise comes from the £900 bill I got for the change of gasket, drilling out of studs, new high tensile studs and the eventual head gasket change that occured when I had to take the head off to get at one of the snapped studs

skyline501 · 5th November 2005, 01:23 PM

Best I could do Jim.

http://media.putfile.com/Lumpy-Cams

http://media.putfile.com/Steady-Idle--1100-rpm

Vincenzo

Jim27 · 5th November 2005, 03:44 PM

Perfect - mine is doing the same idle fluctuation as the first media file there - the "lumpy cams".

Cheers Vincenzo!

1st November 2005, 05:40 PM	#1 (permalink)
Jim27 GTR Register Member Join Date: Jul 2004 Location: Bolton Posts: 4,478	"Lumpy" or "erratic"??? If you say you've got an erratic idle, everyone sucks through their teeth and says "Ooooh - must have an air leak somewhere on the intake side." But if you say you have a lumpy idle, everyone says "Yeah, must be the cams." 'scuse me for being thick, but can someone please explain to me the difference between a lumpy idle and an erratic idle? Even better - link to some audio or video clips where you can actually hear the difference - that'd be great! Still trying to decide whether my idle is lumpy or erratic! Cheers, Jim

5th November 2005, 01:23 PM	#7 (permalink)
skyline501 GTR Register Member Join Date: Jun 2001 Location: Rugby Posts: 760	Lumpy Sound? Best I could do Jim. http://media.putfile.com/Lumpy-Cams http://media.putfile.com/Steady-Idle--1100-rpm Vincenzo

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1st November 2005, 06:20 PM	#2 (permalink)
R32 Combat New Users Join Date: Jan 2004 Location: Berkshire Posts: 4,357	I'd say an air leak would result in a higher idle and running a tad leaner than usual. The idle would be smooth but higher. An erratic idle would be one that goes up and down, a little or a lot. Say between 900 and 1100 or 800 and 1600. The fluctuations would be regular. Like your doing it on purpose. Why, I don't know, but it could be a faulty wiring or maybe an incorrectly set up ECU. As for lumpy idle, I've never heard a lumpy 6 cylinder. If it is, then turning the idle speed up sorts it out. A description of what it sounds like would be like it keeps trying to stall, the revs drop some, it sounds rough, it picks up again but still sounds rough. These RPM fluctuations would be irregular because the flow across the top of the cylinders is too slow and it hardly runs at this speed. I don't know if you've ever heard a top fuel car/bike, but they suffer from cronic lumpy idle or cammy idle and 2-3000 rpm.

1st November 2005, 10:02 PM	#3 (permalink)
Eagle Eyes GTR Register Member Join Date: Aug 2002 Location: London Posts: 725	It might have something to do with what they call too much valve overlap. Just like my engine had. It sounded very strained and not smooth at all when idling. I advanced the exhaust cam by about a tooth and re-adjusted the timing (set to 15 deg) and it was a lot better.

2nd November 2005, 05:59 AM	#4 (permalink)
skyline501 GTR Register Member Join Date: Jun 2001 Location: Rugby Posts: 760	I would describe mine as 'lumpy'. Although it idles at a rock steady 1100 rpm, the whole car seems to pulse or rock, much like an American V8. I assumed it was the asymetric cams (HKS 264 /272) as it only appeared after the stage 2 upgrade (550bhp). The noise (and I mean noise) from outside the car 'pulses' too. Sounds marvellous. I'd be interested if anyone has an expaination. Vincenzo

2nd November 2005, 08:30 AM	#5 (permalink)
Jim27 GTR Register Member Join Date: Jul 2004 Location: Bolton Posts: 4,478	Any sound/video recordings would be great

2nd November 2005, 09:35 AM	#6 (permalink)
mattb GTR Register Member Join Date: Feb 2002 Location: Milton Keynes Posts: 1,416	In my experience a lumpy idle, on my car, used to make the revs bounce from between 600 to 1100 and stalls coming up to roundabouts in hot weather. I tried all sorts to remedy this, cleaning AFMs, changing plugs, checking pipework, carb cleanering the inlet manifold. None of it helped. Eventually I had to give in and admit that it was a leaky exhaust manifold, which is a common problem on a GTST. Got that fixed and everything was absolutely perfect. Unfortunately the sucky teeth noise comes from the £900 bill I got for the change of gasket, drilling out of studs, new high tensile studs and the eventual head gasket change that occured when I had to take the head off to get at one of the snapped studs

5th November 2005, 03:44 PM	#8 (permalink)
Jim27 GTR Register Member Join Date: Jul 2004 Location: Bolton Posts: 4,478	Perfect - mine is doing the same idle fluctuation as the first media file there - the "lumpy cams". Cheers Vincenzo!