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  #11  
Old 16-08-2016, 03:52 PM
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Checked rubber bung which is fine also oiled throttle linkage and springs, checked throttle cable and cruise cable all look ok. Engine starts at around 1200rpm on fire up and then dropped down to 600rpm when warm.
A/C on or off and steering turning makes no difference to revs. Will keep an eye on it and see if it happens again.




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  #12  
Old 17-08-2016, 07:12 AM
JuhaV JuhaV is offline
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Hello Dave,

That is good news.

I think there are many possible reasons to have an slightly elevated, higher than normal idle in some situations without having any real engine issues. For example, after longer period of continuous hard driving it seems that my car takes a little longer (20 sec or so) to settle down into normal 600-700 rpm idle. Again, if the outside temperature is somewhere between + 5-10 C it seems that finding the correct idle when warming up the engine takes somewhat longer than in lower or higher outside temperatures. Might be related to some short term adaptive values in the DME/ECU ?

Regarding the Vanos issue that I described earlier in this thread, my theory is the following below. I have not yet checked this out by disassembling the old unit that is still on my workdesk at the garage. Intend to do so at some point.

I believe that when the internal Vanos piston seals deteriorate to certain level, they will start leaking internally which will slow down the response time of Vanos when changing the camshaft positions. This will happen especially near idle where the output of the Vanos oil pump will be lowest. The feedback from the camshaft position sensors helps the system to compensate for these leaks and the system will try to drive the camshafts towards the target positions but the delays will eventually start to grow and at some point the target positions cannot any more fully reached near idle with low pump outputs. This makes the idle to start to wander around and at certain point will start to cause rough running.

This could be tested/found out by making the Vanos test where test computer shifts the Vanos and camshaft timings around and measures both the response times and difference between the actual and target camshaft positions. This will reveal also the internal leaks in the Vanos pistons or if the Vanos oil pump cannot produce enough pressure at idle. I did not have this capability so I have no data to show about this.

My current Vanos is an original factory (Rolls-Royce ?) serviced unit and it worked beatifully the first 20 000 km but already shows some slight signs of fatigue. All other seals are OEM except the solenoid O-rings which I changed into the M5 version at about first 10000 km when the original type seals started to generate a slight external leak. The next Vanos unit will be most likely prepared from my original unit and it will rather likely not use original seals in order to improve the long term reliability of these thingys.

That having been said, I have been lately driving some first generation water-cooled 911's equipped with the 3.4 liter engine. Thanks to Vanos, our cars feels much more responsive in the low rpm range than those Porsches :) The Vanos really helps and therefore so some challenges to live with it are easier to tolerate.

br, Juha
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Last edited by JuhaV; 17-08-2016 at 07:17 AM. Reason: some typos and clarifications
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  #13  
Old 17-08-2016, 02:06 PM
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Quote:
Originally Posted by JuhaV View Post
Hello Dave,

That is good news.

I think there are many possible reasons to have an slightly elevated, higher than normal idle in some situations without having any real engine issues. For example, after longer period of continuous hard driving it seems that my car takes a little longer (20 sec or so) to settle down into normal 600-700 rpm idle. Again, if the outside temperature is somewhere between + 5-10 C it seems that finding the correct idle when warming up the engine takes somewhat longer than in lower or higher outside temperatures. Might be related to some short term adaptive values in the DME/ECU ?

Regarding the Vanos issue that I described earlier in this thread, my theory is the following below. I have not yet checked this out by disassembling the old unit that is still on my workdesk at the garage. Intend to do so at some point.

I believe that when the internal Vanos piston seals deteriorate to certain level, they will start leaking internally which will slow down the response time of Vanos when changing the camshaft positions. This will happen especially near idle where the output of the Vanos oil pump will be lowest. The feedback from the camshaft position sensors helps the system to compensate for these leaks and the system will try to drive the camshafts towards the target positions but the delays will eventually start to grow and at some point the target positions cannot any more fully reached near idle with low pump outputs. This makes the idle to start to wander around and at certain point will start to cause rough running.

This could be tested/found out by making the Vanos test where test computer shifts the Vanos and camshaft timings around and measures both the response times and difference between the actual and target camshaft positions. This will reveal also the internal leaks in the Vanos pistons or if the Vanos oil pump cannot produce enough pressure at idle. I did not have this capability so I have no data to show about this.

My current Vanos is an original factory (Rolls-Royce ?) serviced unit and it worked beatifully the first 20 000 km but already shows some slight signs of fatigue. All other seals are OEM except the solenoid O-rings which I changed into the M5 version at about first 10000 km when the original type seals started to generate a slight external leak. The next Vanos unit will be most likely prepared from my original unit and it will rather likely not use original seals in order to improve the long term reliability of these thingys.

That having been said, I have been lately driving some first generation water-cooled 911's equipped with the 3.4 liter engine. Thanks to Vanos, our cars feels much more responsive in the low rpm range than those Porsches :) The Vanos really helps and therefore so some challenges to live with it are easier to tolerate.

br, Juha

I did feel at least a couple of times over the weekend the car felt not 100% and the power delivery on occasion was stepped and not smooth so may be signs of a vanos issue.
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  #14  
Old 17-08-2016, 08:50 PM
Aceman Aceman is offline
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I had a similar issue Dave but would also present itself with intermittent high idle hot or cold. Eventually located to the throttle bodies having developed a gum line around the butterfly valves causing them not to close properly. I can only presume this was a result of little use but after stripping them down and a good clean all was back to normal.
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  #15  
Old 18-08-2016, 12:33 PM
Rags Rags is offline
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Is the idle fluctuating and stuttering? Any signs of an air leak in the intake rubber?

Last edited by Rags; 18-08-2016 at 12:34 PM. Reason: Typo
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  #16  
Old 18-08-2016, 02:26 PM
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Quote:
Originally Posted by Rags View Post
Is the idle fluctuating and stuttering? Any signs of an air leak in the intake rubber?
Idle was rock steady at 800rpm , most hoses are new including intake rubber
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