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 Post subject: idle to working throttle transition
PostPosted: Fri Jan 03, 2020 12:11 am 
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Joined: Thu Jan 02, 2020 11:58 pm
Posts: 3
Model: 924s
Hi all, new to forum, my daughter and I have a 924s that we are steadily working through, biggest issue at the moment is the engine starts well and operates under load well but if the throttle is touched whilst the engine is idling then it will stuuter and die-makes low speed or in traffic operation very tricky!
Have checked the basics and they seem fine--leaking pipes/vacuum loss, ht leads, engine wiring etc, any thoughts or guidance would be very welcome! Thanks in advance


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 Post subject: Re: idle to working throttle transition
PostPosted: Fri Jan 03, 2020 10:21 am 
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Joined: Tue Dec 29, 2009 11:16 am
Posts: 4075
Location: Stoke On Trent
Model: Supercharged 924
Hi. Welcome to the forum! I would put my money on either a fuel pressure problem or ignition timing problem. I’m much more familiar with the 2L 924’s and they do this with either of those problems above and no reason why the S wouldn’t, accepting the control is more electronic than mechanical in the S.

Have you checked the ignition timing with a strobe light and the fuel pressure with a gauge? Measuring exhaust gasses might help but the ‘stall’ will register as lean due to unburned oxygen from the misfire so might not discriminate between a cause and effect lean mixture.

Someone with more ‘S’ knowledge will be along soon I sure to help with experience rather than just theory!!

Cheers,
John. :beer:

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924T engine with Eaton M90 Supercharger (DB7), 565cc injectors, coil pack ignition, Adaptronic ECU, Custom Air/water charge cooler, Wideband O2 + 4x EGT. 4pot Brembos, Spax adjustable dampers. 951 G'box with LSD & cooler, 944T style bumper. HID Lo-Pro headlights. 968 mirrors, handles + spoiler. Zender rear valance. Custom side skirts, Forged 17" wheels (8J front, 9J rear). Custom hubs (5x114.3 PCD), Twin circuit coolant re-route (2 pumps, 2 'stats, 2 radiators), 6 gauge centre console.


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 Post subject: Re: idle to working throttle transition
PostPosted: Fri Jan 03, 2020 12:34 pm 
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Joined: Mon Jan 04, 2010 11:24 pm
Posts: 17172
Location: Gloucester
Idle control on the S is by the idle control valve situated under the inlet manifold, problems with this can cause the symptoms you are getting.

Ignition timing is fixed, but managed by the DME.

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 Post subject: Re: idle to working throttle transition
PostPosted: Fri Jan 03, 2020 2:25 pm 
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Location: Birkenhead (UK)
Model: 924 na
Is it better or worse when the engine is cold or hot?

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 Post subject: Re: idle to working throttle transition
PostPosted: Sat Jan 04, 2020 12:09 am 
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Joined: Thu Jan 02, 2020 11:58 pm
Posts: 3
Model: 924s
Thanks all for the comments- I will check ignition timing dynamically although I think it is ok as engine runs and starts very well. The idle control valve works on the bench no probs but I should check the operation on the car. As long as the throttle isn't touched the car idles ok hot or cold. If anything the problem is worse when the engine is warm, I tried it tonight, leaving it to run until the cooling fan came on and then touching the throttle, engine dies everytime, the only way to make the transition is to either creep the throttle very carefully or keep your foot on the throttle and hold engine just off idle. It is idling around 1000/1050rpm according to tacho.

Exhaust at tailpipe is clean and clear although I thought I detected a whiff of petrol in it-- is engine fuelling not responding to increase in engine temp I wonder?
I have an old Sun 5 gas analyser kicking about so might resurrect that and see what it says.
There is a noise that sounds like an air leak but I assumed this is air going into the manifold via the idle control valve as I can't find any air leaks using either carb cleaner or a stethoscope.

Any more thoughts very welcome--

Ta!


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 Post subject: Re: idle to working throttle transition
PostPosted: Sat Jan 04, 2020 9:32 am 
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Joined: Thu Nov 30, 2017 6:21 pm
Posts: 3
Model: 924
Have you checked the throttle position sensor? Its not a true TPS, it only has an idle and a WOT position, but it needs to work.
1000 rpm idle seams a little high, are you sure the throttle closes enogh to engage the idle position.


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 Post subject: Re: idle to working throttle transition
PostPosted: Sat Jan 04, 2020 10:03 am 
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Joined: Tue Dec 29, 2009 11:16 am
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Location: Stoke On Trent
Model: Supercharged 924
safe wrote:
Have you checked the throttle position sensor? Its not a true TPS, it only has an idle and a WOT position, but it needs to work.
1000 rpm idle seams a little high, are you sure the throttle closes enogh to engage the idle position.


Very smart suggestion! Had this on my mother’s mx5 where it would do funny fuel cut things when throttle pressed as ecu thought still in idle mode but rpm too high.

Air leak (small) would also cause high idle and maybe lean tip in. The carb cleaner does a great job normally unless a vac line has come off in which case it isn’t always caught by the spray. I also like to put the compressor on one of the vac lines with the regulator down at around 15psi. With the air filter end of the induction pipe blocked it should hold pressure reasonably well and leaks often felt / heard.

:beer:

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924T engine with Eaton M90 Supercharger (DB7), 565cc injectors, coil pack ignition, Adaptronic ECU, Custom Air/water charge cooler, Wideband O2 + 4x EGT. 4pot Brembos, Spax adjustable dampers. 951 G'box with LSD & cooler, 944T style bumper. HID Lo-Pro headlights. 968 mirrors, handles + spoiler. Zender rear valance. Custom side skirts, Forged 17" wheels (8J front, 9J rear). Custom hubs (5x114.3 PCD), Twin circuit coolant re-route (2 pumps, 2 'stats, 2 radiators), 6 gauge centre console.


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 Post subject: Re: off idle stalling and brake parts
PostPosted: Mon Jan 06, 2020 12:11 am 
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Joined: Thu Jan 02, 2020 11:58 pm
Posts: 3
Model: 924s
Thanks for the thoughts there- I will check the tps -- I am hoping this problem has a simple fix as the car drives nicely on the road. I am able to take it for an MoT inspection next weekend so will be able to see the fuelling from the emissions, the tail pipe does look a little sooty but we have only been moving the car around starting and stopping it rather than giving it a good run.
On another note it looks like it could do with brake discs and pads front and rear--only looking for standard but decent quality stuff--any recommendations?

Ta!


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