worf928

Again, any constant and small amount of false air, by-and-of-itself without any additional failure mode manifestation, that effects the entire engine will not cause a surging/hunting idle. The spindle bearings are such a source: very small, constant, effects the entire engine and don't otherwise directly effect fuel, spark, or timing.

This small constant leak will not cause a hunting/surging/unsteady idle. It may result in a slight (~10 rpm) higher idle. But, something like that is going to be difficult to measure unless you do before and after measurements and change nothing else except the bearings.

The GT hunting idle problem I diagnosed was a fairly-new perfect-looking CPS that was providing a poor signal to the EZK. This poor signal caused the EZK to 'believe' that the engine speed was hunting through the idle timing map. The EZK was thus, changing spark advance based upon incorrect speed values, thus *causing* the idle to hunt between 600 and 900 rpms.

Isolating this fault took a Hammer, messing with idle timing with the SharkTuner on a second set of ECUs, and a timing light with an RPM display.

After I installed a new CPS the hunting disappeared and the idle was consistently 775 +/- ~12 rpm.

Interestingly enough, when I reconnected the tank vent system to the air guide, the idle rose to 800 +/-. So, there is something not quite right with the tank venting system on this GT. (It was disconnected and plugged because a faulty tank vent system can cause a fluctuating idle.)

But, the idle is still rock-steady (see below) even though it is a bit higher than I would like.

This is the second time in 20 years I've encountered this CPS failure mode.

I define a rock-steady idle as an idle that, with your human senses, you cannot perceive a fluctuation under steady-state warmed-up conditions. I'd go so far as to say that a fluctuation that is at the edge of perception - as in if you are paying attention you don't sense it, but when you get distracted you think you sense it, maybe it's there, maybe it's not - is ok too. You will sense an idle fluctuation when the fans kick or or off, or when you engage/disengage the A/C compressor; basically any change in load will cause a transient idle fluctuation and possibly a slight change in the steady idle.

In the case of this GT my measurement devices show slight fluctuation (+/- ~<20 rpm) but my ear has a hard time convincing myself that I hear the fluctuation.

Remember that the spec is 775 +/- 25 rpm.

No. A perfect idle doesn't exist. Even on new Porsches. All of our newer Porsches exhibit idle fluctuation that are at the edge of perception.

My perception is not as good as what's displayed on my Hammer, SharkTuner, Timing Light, and Durametric.

My perception seems to be able to consistently detect rpm fluctuation in excess of 30-ish rpm only.

GregBBRD

My Bosch supplier now has a replacement idle stabilizer for the 928 application, so you don't have to use the LOWE unit.

worf928

Tellin' the wrong guy

The Bosch part has always been available. For the last several years it's been ~$300. Is your supplier's replacement less than $300? It this 'new' part different from the original?

FredR

This issue has me intrigued in a number of ways. I have been chasing a similar concern for time to no avail. I have a slight stumble in the idle. A few months ago I decided to change out my knock sensors that were still working but had connectors that were dropping to pieces so it was an opportune time to tr the Lowe ISV. I found this unit to be visually excellent and when installed it performed exactly like the stock item have used in my motor for the last 19 years of ownership. Thus nothing much lost $$$'s wise and something gained knowledge wise. I consider the used ISV a "working spare" in case the Lowe unit packs up prematurely. This left me wondering if the Lowe unit is made in the same factory as the Bosch unit- just a different name on the box it comes in [and a different price].

I consider my idle not bad but somewhat less than perfect. The slightly uneven idle I detect could be a slight misfire for whatever reason and I intend to do further investigation shortly. In the days when we had a number of 928's over here various owners commented how the idle was a bit on the erratic side- in particular a friend of mine who had a GTS from new. He complained about the idle as part of the initial warranty and the local agents took a number of steps to resolve his concerns but never managed to change his perception.

We know that the GT idles notably higher than the S4/GTS and one wonders why this is needed. Needless to say the GT cam is going to be a bit lumpier because of the higher lift but it is nothing outrageous lift wise. Whilst using ST2 I noted that the idle on my S4 motor was jumping around a bit cell wise- whether this was the system reacting to the slight rpm change or whether it is something going on with the MAF I am not sure but my suspicion is that there may be some impact from reversion pulses given the load values were bobbing around a bit as was the rpm value and the cursor was crossing between two adjacent cells.

I suspect this issue is a limitation of the system and the sensitivity of the owner is the main variable- I tend to be very sensitive to minor fluctuations.

merchauser

sounds like a real PITA.

will check that out once I get everything back together.

merchauser

I have this disease too...lol