Speedtoys

I had in my hand a brand new "not OE" idle valve for my daughters 85, that passed so much air I could snorkel with it.

Michael Benno

I tested three ISV's using a variable power supply, they are all pretty close in the way they operated and the voltages required at the various stages. No smoking gun. They all went fully closed at about 2.5v and were all halfway open at 6.5v, and fully open at about 8.5v. They all had about 0.5v of variance if depending on if it was rotating clockwise vs counterclockwise to get a given test point. Not the results I was expecting. I cleaned the used ones thoroughly with PB blaster and that didn't have any impact. they all seem to move very smoothly and respond similarly to voltage changes.

My next move will be to re-assemble the intake using a different ISV than before despite the lack of significant differences.

I followed Stan's suggestion and put some DC111 in the seals on the intake side covers. I rechecked all the hoses and clamps.
I rechecked the throttle plate for gaps with a feeler gauge and was not able to get the smallest gauge to fit anywhere (0.006mm) so that seems extremely tight.
I'll do another pressure check once everything is assembled.
I'll check the PN on the dampers and regulator to make sure they are correct.

If you all have additional suggestions to go over as I re-assemble the car, let me know.

Darklands

Is the power supply of the ISV good? Corrosion or so?

Michael Benno

I think I have a tentative victory. I am able to get the idle down below 700rpm and all other signals look good! Here are some plots from the Sharktuner, these were taken after the car was warm. O2 adaptation was disabled.


RPM is in the black line and you can see it's mostly below 700rpm

The O2 signal looks totally normal. The ISV % value is averaging about 37% which is a little lower than I would want. Jim said a target of 45% is what I want.

I'm pretty excited about these results. Thinking about what I changed to "fix" the high idle issue. I did two things. I replaced the original used Bosch ISV with another used Bosch ISV from a local 928 owner. I decided against the Löwe ISVs because the diameter of the openings was not the same and the preset gaps were not the same and I was being superstitious. The other main thing I changed was I sealed the intake side covers with DC111 per the recommendation from @Mrmerlin .

The evidence points to the ISV because when I unplugged the old one the idle dropped significantly to 600ish. Meaning there was not extra air being sucked in. And, the intake passed numerous pressure tests. However the bench testing of the ISVs did not show a significant difference in performance for any of them. Maybe the bench test was not an effective simulation of the Pulse Width Modulation that the LH does.

However, there is a distinct possibility that there was a vacuum-only leak from the intake side covers that did not show up in pressure testing. I had new seals in there and the insides of the covers were also blasted.

In any event, the idle seems much better and all the systems are reading totally normal. So thank you all for this very helpful and supportive troubleshooting. I am very appreciative for your patience.

Next up, I'll be replacing the old OEM injectors for some modern 30lb injectors and some Sharktuning. But that will be another thread.

Mrmerlin

FWIW you might have found a plenum leak with the no flame torch method,as suggested by PorKen
by running the torch head around the periphery of the plenum.while the engine was running.
I am glad your running .
I would suggest to not mess with new injectors till you have put 1000 miles on the car.
Right now you dont know what your systems are doing,
or how they are performing and thus no real need to make more changes.
IOW you need to establish some base lines.

Michael Benno

i did follow this suggestion and it did not show and indication if leaks.

GregBBRD

More the "norm" and not the exception, these days...

Michael Benno

@GregBBRD thank you for the expert guidance on adjusting the throttle plate!

android606

Congratulations! That was quite the fox hunt!

FredR

Michael,

A good result for sure and hopefully you already appreciate the insights that ST2 gives the owner as to what is going on in the motor and had you had access to it earlier it would have saved you a lot of trouble.

Disconnecting the ISV is a simple way or telling you whether the ISV is working [or not as the case may be] and the acid test is what happens when you turn on the ac once the motor is warmed up. Because the ac places a load demand on the engine the rpm's will drop and/or quite possibly stall the motor because there is no incremental fuel/air. In your case you had incremental true air and fuel so it confuses matters.

Trying to diagnose problems with two or more concurrent issues makes diagnosis exponentially more difficult albeit I am not sure what the exponentiating factor is- if two it would be 4 times more difficult, if three it would be 9 times more difficult etc.

Excellent thread- represents what this forum is theoretically all about- mutual cooperation and support to enhance knowledge and learning for all.