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Hunting for the Idle

So my little beast was in the shop for 4 months messing with the fuel injection system. Most of that time was spent waiting for the fuel distributer to get rebuilt......twice. I found a used one on ebay and that seemed to fix it. In the mean time, my mechanic went through almost every component in the system with new or used parts.

It runs really well now except it is way rich in my opinion and sometimes it will not idle. It will hunt from around 300-900 RPM and sometimes just die after running around town and stopping at a light.

On a cold start, it runs really rough for about a minute and then it smooths out correctly. It blows white smoke and then once it smooths out, all is good.

Any ideas on what needs attention? I am pretty sure that I need to visit the shop and get the AFR's adjusted. I am blowing a fair amount of carbon on the garage floor which I do not recall doing with my last 930 several years ago. I am just wondering what to expect on the next 930 Doctor's visit.

I need her to run as good as she looks!


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1986 930 Ultra-Smith RUF Slant Nose conversion, RUF VB, K-27 HF-S, B&B Headers and Exhaust, RUF Short Shifter, RUF Interior
Old 12-16-2011, 02:11 PM
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Don't know if this will help, but mine had a hunting idle as well, but only fluctuated about 1-2 hundred RPM from a set 900rpm idle.

Mine turned out to be a leaking deceleration valve, I took it off completely, a capped off the hoses as needed. Idle has been rock steady ever since.

Your stumbling & dying sound more like a vacuum leak???

Mark
Old 12-16-2011, 02:28 PM
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ask for helios help, he is the man when it comes to idle and vacuum issues and yes they seem to be related!
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Old 12-16-2011, 05:39 PM
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Thanks for the replies.

I thought that it might be vacuum related, but it seems to manifest itself after the engine is warm now. I run it for about 20 min and go inside the store and then after running about 10 min, it does the hunt at a stop light. I give it a rev and it mellows out where it should around 900 rpm. Very strange indeed.
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1986 930 Ultra-Smith RUF Slant Nose conversion, RUF VB, K-27 HF-S, B&B Headers and Exhaust, RUF Short Shifter, RUF Interior
Old 12-18-2011, 07:30 AM
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In my experience, an overly rich condition at idle can cause "hunting" as the lamda system is trying to correct it - or the lamda system is not reading correctly due to problem such as a bad oxygen sensor . . .

Last edited by Ronnie's.930; 12-18-2011 at 08:18 AM..
Old 12-18-2011, 08:01 AM
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Mine started doing a similar thing, only it idles around 2000rpm. I assume it's AFR related as the outside temp was in the 30s, no apparent vacuum leaks and gets full boost.
Any ideas?
sorry for the hijack
Tia
c

Last edited by Tt surgeon; 12-18-2011 at 08:41 AM..
Old 12-18-2011, 08:38 AM
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Quote:
Originally Posted by Tt surgeon View Post
Mine started doing a similar thing, only it idles around 2000rpm. I assume it's AFR related as the outside temp was in the 30s, no apparent vacuum leaks and gets full boost.
Any ideas?
sorry for the hijack
Tia
c
Do you still have the deceleration valve (also know as the vacuum limiter valve) in place? If so, the diaphram in it likely has a hole or tear which lets unmetered air pass through it, thus increasing idle speed. Mine did as you describe and when I took the valve off, discovered it was full of water . . .

I removed it - I like the "normal car" throttle responce now, but closed throttle popping is very pronounced (could be considered anoying by some).

There are many good threads on this (the decel valve and its functions).
Old 12-18-2011, 09:07 AM
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Hunting is usually idle mixture related...usually. If you have other stuff wrong like vacuum leaks, etc. then that will show as a continuous poor running condition vs. "hunting" through the rpm's.
If you think you're running way too rich, then dial it down just a tad. Get out your allen wrench (damn, my memory is gone...I think it's a 4mm) and turn your mixture screw just a scant degree or two counter-clockwise to lean her out. Listen to your ear, let it tell you what the engine is doing. Don't be afraid to do this small adjustment, it won't mess with your overall mixture much beyond idle and may just cure your hunt.
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Old 12-18-2011, 09:19 AM
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Little update
took it out again, idled at 900 initially. Got it warmed up, started stumbling like crazy at wot, idles at 2000 again.
no idea about the decel valve. I have an Andial IC , have had no issues until now. Gotta be vacuum.
Old 12-18-2011, 09:45 AM
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As you replaced/rebuilt the fuel head I would say it is mixture related. The system pressure may have changed.

Do you have an adjustable WUR? Use the mixture screw to lean it out and get it idling smooth at operating temp. I would bet it is running way rich.
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Old 12-18-2011, 02:09 PM
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Adjusting themixture??? Sounds easy and makes sense. I thought that mine had a stick that is adjusted vice a hex nut. I will take a look.

Thanks guys!!!!
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Old 12-18-2011, 03:43 PM
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Dang Mark... are you drunk??? Everyone knows it is a 3mm wrench.
I'm not sure I would just go changing the mixture willy nilly though. But then again, what's the worst that could happen???


Quote:
Originally Posted by mark houghton View Post
Hunting is usually idle mixture related...usually. If you have other stuff wrong like vacuum leaks, etc. then that will show as a continuous poor running condition vs. "hunting" through the rpm's.
If you think you're running way too rich, then dial it down just a tad. Get out your allen wrench (damn, my memory is gone...I think it's a 4mm) and turn your mixture screw just a scant degree or two counter-clockwise to lean her out. Listen to your ear, let it tell you what the engine is doing. Don't be afraid to do this small adjustment, it won't mess with your overall mixture much beyond idle and may just cure your hunt.
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Old 12-18-2011, 06:49 PM
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Looks like my IC o ring at the turbo side is loose.
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Old 12-18-2011, 07:04 PM
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That would could cause the stumbling at WOT that you described but not the high idle as it is before the throttle plate.
Old 12-18-2011, 07:13 PM
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Quote:
Originally Posted by DSPTurtle View Post
Dang Mark... are you drunk??? Everyone knows it is a 3mm wrench.
I'm not sure I would just go changing the mixture willy nilly though. But then again, what's the worst that could happen???
Ah crap, a little tweek on the mixture never hurt anything. Just don't go too far and remember where you started.

Yep, I'm still mentally challlenged after celebrating my B-day yesterday. Heck, I left my allen wrench permanently installed at the mixture screw and can't remember for the life of me what the size was (is). 3mm sounds good to me.

This particular diagnostic thread is confusing, since we're attempting to answer two separate issues for the original poster (1986 930) and TT Suregeon's issues. But then, being another year older now, confusion is my second name.

As The Stones once sang "when you're old, when you're old....nobody will know, that you were a beauty, a sweet sweet beauty....a sweet sweet beauty but stone stone cold" - which has absolutely nothing to do with anything
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Old 12-18-2011, 07:37 PM
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I will have what Mark is drinking!!


I am going to swing by my shop and see if he will give it tweek of adjust.
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Old 12-19-2011, 11:22 AM
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Quote:
Originally Posted by mark houghton View Post
Ah crap, a little tweek on the mixture never hurt anything. Just don't go too far and remember where you started.

Yep, I'm still mentally challlenged after celebrating my B-day yesterday. Heck, I left my allen wrench permanently installed at the mixture screw and can't remember for the life of me what the size was (is). 3mm sounds good to me.

This particular diagnostic thread is confusing, since we're attempting to answer two separate issues for the original poster (1986 930) and TT Suregeon's issues. But then, being another year older now, confusion is my second name.

As The Stones once sang "when you're old, when you're old....nobody will know, that you were a beauty, a sweet sweet beauty....a sweet sweet beauty but stone stone cold" - which has absolutely nothing to do with anything
Happy B-Day!!
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Old 12-19-2011, 11:23 AM
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Thank you John, are guy using any of the smog stuff, if not , can you post some pic's of your engine bay too see what we are working with, especially how the vacuum going to and from the distributor and the throttle body and so on,
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Old 12-19-2011, 06:26 PM
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I dropped by the shop yesterday and the adjustment to the mixture seems to have done the trick! She is running fantastic.

Thanks for the heads up. It is nice to complete a fix and not spend any money for a change!!

Thanks again!!
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Old 12-21-2011, 03:56 PM
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9 times out of 10, it's simple stuff. Glad it did the trick for you!

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Old 12-21-2011, 06:21 PM
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