Chasing my idle, part 2

[awolff280sl]

My linkage and timing are "perfect". The throttle dampener and throttle solenoid are set just right.
The situation is this: with moderate revs/moderate acceleration/moderate speed driving, the idle reliably comes to rest at 850. The throttle dampener eases the rpms down nicely.
However, with high revs/high speed or big acceleration, when the car comes out of gear the rpms often drop sharply (and without dampening) right down to about 400,  and that becomes the idle speed. But, if I then rev the engine about 500-1000rpms, the idle comes back up to 850 and stays there.
The FIP and throttle linkages are on their stops whether the car is idling at 850 or 400.
It feels like there is somehow a temporary low fuel situation that only happens when abruptly going from high fuel use back to idle. Temporary loss of some back pressure in the system?

[awolff280sl]

I have found the cause (thanks to Gernold) of this problem.
It is the tach or tach cable.
A lesson learned.

[awolff280sl]

After replacing my after-market tach cable with an MB cable ($209), I still had the same problem: an abrupt drop of rpms to below my normal idle when getting out of gear after high-speed driving or rapid acceleration.
For some reason, it felt to me like some sort of fuel problem. For whatever reason, I thought it might be a pressure problem occurring in the lines AFTER the pump.
I thought it might be a problem with the fuel outlet valve on the FIP or somehow with the fuel damper. I went for the fuel damper even though some of the info on the forum indicates that the fuel damper if basically for noise reduction. However, there was some info that its purpose is to smooth out rapid fuel pressure changes.
Anyway, this is what my damper diaphragm looked like, obviously not able to function properly with that fold in it.



I replaced the diaphragm using the method described on the forum to pre-tension the spring, and voila! The difference is dramatic, with the rpms settling down slowly to normal idle with the throttle damper no matter how the car is driven.
So, it appears that the fuel damper is needed to maintain a satisfactory "back" pressure (pressure AFTER the FIP) during times of high fuel use by the FIP.

[DaveB]

That's good troubleshooting, interesting about the required back pressure. And now you have a shiny new tach cable to admire!

[ja17]

Thanks

[ja17]

Thanks for the persistence Andy, we can all benefit from this information. This again illustrates how fluctuations in fuel pressure can effect the running of the engine.

[garymand]

Would not have guessed that.  Looks like the diaphram was originally installed way off center. 

[awolff280sl]

Something I am seeing now is that occasionally when going abruptly from high revs to idle is that the rpms will monetarily dip to 600, but then they come back up on their own to 850. This was not happening before changing out the diaphragm. Before, in order to ?"repressurize"? the system, I would have to tap the accelerator.