Author Topic: Wobbly turbo gauge - ng900, 140k  (Read 797 times)

jimmym105

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Wobbly turbo gauge - ng900, 140k
« on: 11 July 2019, 03:37:20 PM »
Chaps - any pointers of where this probloem might lie gratefully received. The turbo gauge wobbles when idling. Rev counter static, but can hear the tiniest change in revs. Any thought? Thanks in advance

carrera

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #1 on: 11 July 2019, 03:43:01 PM »
I'm going to guess that the turbo gauge is old school pressure tube operated and it has a small hole in it which can open / close allowing the fluttering and also a small variiation of airflow into the engine.

I know virtually nothing about NG900, but just an idea until someone who knows more comes along
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sgould

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #2 on: 11 July 2019, 05:18:26 PM »
Yes, the early cars have a direct tube from the gauge to the engine.  There should be a plug in the tube with a small hole to stop it all fluttering too much. A cheap damper.  Has anyone replaced the tubing and just left it full bore all the way?  At the age of your car, it's likely that quite a bit of the rubber tubing has been changed, or is in need of changing.
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jimmym105

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #3 on: 11 July 2019, 05:36:56 PM »
You guys are brilliant, thanks for quick responses. I'll see what I can find

carrera

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #4 on: 11 July 2019, 06:07:24 PM »
You could also nip off the pipe near the offtake point as a test. If the tickover becomes smooth then it's definitely a hole in the tube that is causing the idle variation
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jimmym105

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #5 on: 20 July 2019, 05:43:21 PM »
Chaps, by way of an update, and again, thanks. Have done 400 successful miles in the old beast since I last posted and although I cannot spend too much time looking at the wobble for obvious reasons (!), it seems to dissapear when car is hot, and has been for a while. I've now taken a look at all the hoses.

The one I assume goes directly to the gauge - as it dissapears into the bulkhead on the drivers side - is original, judging by the amount of grime on its small white sticker and the zip tie looks ancient. It's in good condition. What I did find was that the clear hose that comes off the breather nipple is not attached to anything. So it looks like it should go between the two large pipes beneath the turbo (attachment) so I have attached it and fired it up and after some initial wobble when idling it calmed down and was still (or almost still, the kind of wobble one would never spot unless looking).

I am a true novice on cars, does this sound right?!

Just spotted Carrera's post - will give that a go ASAP.

carrera

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #6 on: 20 July 2019, 06:06:19 PM »
Did I mention I don't know much about ng900's

Well anyway, that clear pipe is connected to the crankcase breather, and the other end of that pipe should I believe be connected onto the throttle body, so that excess crank pressure gets fed back into the engine. Look for a metal spigot with nothing connected

........... or is that someone's bodge with a screw and piece of rubber on the left hand side of the inlet pipe, blocking off the air leak because the pipe was not connected at some time ??
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jimmym105

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #7 on: 20 July 2019, 06:34:10 PM »
You'll know more than me! Thanks, we might be alright then. That screw with rubber, well, it's got a round head and looks like you couldn't turn it...

carrera

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #8 on: 20 July 2019, 08:22:22 PM »
I have been reading around. That rubber bush / metal bit is standard.

This page should be useful for you

http://www.saabwiki.info/index.php?title=NG_900/9-3_Throttle_Body

I suspect that the clear pipe should go to 5 in diagram 7.

Note particularly figure 3. There is an idle control circuit, so you need to make sure that those hoses are in good condition with no leaks.
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jimmym105

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #9 on: 21 July 2019, 08:53:28 PM »
Many thanks for this, will take a good look around asap

jimmym105

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #10 on: 12 August 2019, 06:11:43 PM »
Hi everyone, the wobble continued but I've managed to take another look and recheck the hoses. During a much more thorough search in better light, I have found a small slash in the hose running from the turbo bypass valve. It was in one part pretty perished.

I've managed to replace it with some spare hose and the car started very well and idled fine. A miniscule wobble on the
the turbo gauge remains, but much, much better and I think the primary issue resolved.  The spare hose was silicone and smaller so it was hard to get on at the throttle body end - worth replacing with some bigger?

The hoses carrera refers to in figure 3 are fine. Might check the ICV and see if TB needs a clean when I have time.

Thanks again for the help, you bunch of Saab legends.

sgould

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #11 on: 12 August 2019, 06:54:56 PM »
If the hose is tight on there, it should be OK.  If you bigger you will probably need to clamp it to stop it blowing off at high boost.

The risk is "easy on, easy off"!  You don't need a bigger pipe.  There's no flow of air, just transmission of pressure

You may find the wobble is still due the lack of a restrictor in the pipe.  1 mm is usually enough.  But only as a small plug, not the whole pipe length.   As I said earlier, most gauges come with a restrictor that pushes into the end of the tube at the gauge before you put the hose on.  It's there to stop the flutter.  People think that removing the restrictor will give a quicker response at the gauge. That isn't so. You will be getting the turbo pulses which cause the flutter and actually remove the accuracy.
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phoenix

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #12 on: 12 August 2019, 07:18:30 PM »
IIRC the restrictor is on the input to the gauge on the dash binnacle- or at least it is on a C900 and a 9000 so I would expect it to be the same.

sgould

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #13 on: 12 August 2019, 07:25:45 PM »
Indeed, but I've seen a couple where the owner has drilled the restrictor out, thinking that it was a moulding fault in the casting!! ::)
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jimmym105

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Re: Wobbly turbo gauge - ng900, 140k
« Reply #14 on: 13 August 2019, 08:23:23 PM »
I'm obvs not looking hard (well) enough, but the only things going into the bulkhead towards the driver's side are cabling - hard etc. Will have anpother look

Anyway, I've had another close inspection and I think the short hose from the air pressure sensor to the throttle body if not leaking now will be soon - it's way too fiddly for me to change so might need some help on that one but I'll do that next. The short hose from the fuel pressure regulator looks pretty good and I assume has been changed proactively at service time or something.

Thanks again!