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Revs surging cause on a 3.2?


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Hi Guys, perhaps tough to answer as my car is so far from standard (964 inlet, bored throttle, cams etc etc.) but is there a known culprit for making the idle on a 3.2 surge up/down? Mine revs up but frequently dies as the revs drop, wether hot or cold, making it very hard to drive. I have a Canems ECU and I have tried altering EVERY possible idle control parameter (and there are lots!), still does it, so is there an underlying reason that can cause this? I'm thinking too lean/rich? Air leak? I've cleaned out the idle valve a couple of times, so I'm convinced it's not that. 

Thanks, Chris.

 

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I had a problem of the revs holding and then climbing. Checked all the relevant sensors and cleaned the ICV numerous times. Found it was a blob of grease somebody previously thought was a good idea to “lube” the accelerator linkage rod with causing it to stick and hold the throttle open ever so slightly. The grease was on the linkage under the panel between the rear seats as it enters the routing tube forward. Cleaned the rod and then half a tin of WD to dissolve away / wash out any stuck up the tube and never had the problem again. 

Worth a check. 

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Hi Paul, no. But as I say the whole system is no longer controlled by the Bosch ECU but by an aftermarket Canems ECU which relies upon the ICV to control idle. But, I take your point and I'll do it again tomorrow and disable the closed loop idle control, set the bypass valve to try to stabilise the idle and re-enable the closed loop..... I suspect another rolling road session will be the answer in the long run sadly.

Chris.

 

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I’d agree with Paul, but difficult to assess on a non standard car.

Mixture or vacuum leak - a new oil cap helped some idle bounce in my previous 3.2 days.

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I recommend to get a Lambda/AFR gauge on the car and see what it's doing when it's surging. No need to go on a rolling road if the problem is only at idle - you'd be better off spending the money on an Innovate Lambda kit - you could then monitor your AFR whilst driving too. Have you got a Lambda boss in your exhaust ?

I found getting the idle right with the Canems ECU pretty tricky - adjusting the idle parameters alone was not enough. I had to tweak the ignition and base fueling map too - it took a few goes to get it right. I can look up my ignition settings tomorrow.

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Wingnonut, thanks for your thoughts, however with the laptop connected to the ECU I can see when the idle and WOT switches are activated. I know the throttle plate is set correctly, so when the idle switch is active I know the throttle has fully returned and the idle is under the ECU's control.

Hi Matt, thanks for your reply. I suggested a RR as it was leaking so much air it would continue to run while my hand was covering the air inlet to the AFM! Last RR session set the fuel/air ratio with this air leak, meaning it will now be too rich at least in the lower rev ranges, not just at idle, although just setting it at idle will at least make the car drivable. I do indeed have a lambda bung in my SSIs but I know it has 'welded' itself in and will take some removal! 

I'm glad I'm not the only person finding it tricky to set the idle with a Canems ECU. I'd love to see your settings for fuel and ignition, perhaps it could give me an idea how to 'tweak' mine a bit for a steady idle at least.

Chris.

 

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Remind me what cams you have?

You may find with cams and depending on the overlap in particular, that its very hard to get a stable idle full stop.  I had that on the 3.2 with Supercup cams and have it on the 930 (with, we think something like GT2 cams), the first with 993 type MAF + Motronic and the second on Motec.  On the 3.2, one partial fix was to simply raise the idle speed up to about 1000-1100rpm, but on warm up it would still do the same surge-die-catch routine, like some little demon was sitting there quietly revving the throttle.  It only stabilised at all when warm.  This is not unusual, I remember all the 964 Cup Cars imported to NZ back in the 90s were exactly the same going through this surging, unstable warm up routine.

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Hi Richard, thanks for your reply.

Pre modifications on a standard ECU It always did surge a bit during warm-up but never enough to stall, say 500-1200 revs. When warm it stabilised as you say. Now it surges and dies at traffic lights, roundabouts etc etc when fully warm, making it very tricky to drive! Indeed during warm-up it surges but rarely dies, it gets worse when hot.

 I have tried raising the 'target idle speed' in the closed loop idle ie. when warm from 850 to 900 or 1000 but with no apparent difference, it still surges between 1300 and stalling....

 Chris.

 

 

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If making changes to the idle speed settings in the software doesn't seem to be having much impact on the engine you need to check all that kit is working as it should. Have you watched the volts at the ICV while the engine is running? also do this while making adjustments and check you see changes to the readings.

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Thanks Jevvy, I haven't (yet) gone beyond checking there are volts at the ICV, feel it buzzing with ignition on and thoroughly cleaning it twice. Currently I can't leave the driver's seat without it stalling....!  

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15 minutes ago, Chris T said:

Thanks Jevvy, I haven't (yet) gone beyond checking there are volts at the ICV, feel it buzzing with ignition on and thoroughly cleaning it twice. Currently I can't leave the driver's seat without it stalling....!  

hook up a nice long + & - wire from the ICV to the drivers seat and connect to a multimeter so you can watch the voltage while you start it up. If the settings make zero difference I suspect you may have a wiring issue.

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