Airflite40

Helping out a friend with his 88 S4.

Symptoms:
-Hunting idle when warmed up and sometimes stalls
-When fully warm if you put the car in gear (rogerbox) and have your foot on the brake it wants to stall and die
-When driving if you brake hard it will stall

Done so far:
-Found minor leak from fuel pump crush washers, replaced washers and fuel filter. Problem persists.
-Found one injector wasn't seated all the way (minor vacuum leak), reseated injector and problem persists.
-Removed intake to inspect hoses for leaks and cracks. None found.
-Removed ISV and tested. With a 9v battery it wouldn't open all the way. With 12v source it opens all the way. In either case it appears that the ISV doesn't close all the way. There is a detent inside that the rotating cylinder stops against, and it leaves the cylinder partially open with maybe 2-3 mm gap. Tried to adjust the stop detent and it's frozen solid.

Next:
-Pour over wiring diagrams to trace paths for ISV and TPS to ensure no wiring issues.

Any input or help would be greatly appreciated!

Mrmerlin

is the ISV original?
spray a 20 second burst of WD 40 into the part shake vigorously then pour the liquid out into a cup inspect for orange brown deposits indicating corrosion.
Someone else here got a new ISV and found that it wasnt closing all the way ,
a new part cured the issue.

You can test the ISV on the 87 and newer cars with a pick what you want to see is if the vane moves freely in both directions, if these units get water in them then they will corrode

StratfordShark

This thread made me check I Lowe ISV I bought very recently from a German supplier.

I had tested it previously with 12V supply and it actuated fine. But the comments about it not closing fully made me look closer. Sure enough it is not fully closed with a gap of a few mm. There is an adjustment screw but the head is sealed by some kind of resin.

If I blow through one port air comes through!

Now I'm wondering if I should contact supplier. Can anyone please confirm whether the ISV is meant it be fully closed with no power applied?

Thanks

Airflite40

As far as I know the ISV is original.

We sprayed WD40 and carb cleaner into it, it moves freely in either direction. The wd40 coming out of it looked black and there was a bit of carbon buildup inside.

Thanks for posting the pic strat. That's exactly what this one is doing. I chipped the rosin away and there is a 2mm hex head underneath, and this one is frozen solid wasn't able to crack it loose before the hex stripped.

If I stick my finger inside I can move the cylinder to close it fully and I can feel where it stops on the detent leaving the 2mm gap.

Mongo

Definitely replace the ISV and reassemble. Do not forget to replace any other vacuum lines under the intake.

Perhaps this is a good time to paint it too, no?

Airflite40

Most of the hoses were replaced 2 years ago and intake was pc'd then too.

Mongo

They should be ok then the ISV, no.

PorKen

What's the condition of the O2 sensor? Bad or no adaptation will screw up the idle. Codes?

The S4 LH2.3 ICV armature is spring loaded. The LH just pulses it open, unlike the S3 LH2.2 which has to pulse its ICV open and pulse it closed.

In its resting position the S4-up ICV has a default opening like you see there. The S4 ICV rotates from the default position past fully closed to its fully open position. The default opening is a failsafe which the S3 type ICV does not have.

Airflite40

O2 sensor is unknown. How should I test it? I am pretty handy with a multimeter.

Also forgot to mention that we tried a new MAF, a different used MAF, no change.

How does adaptation play into this? Would the LH getting a poor reading from the O2 sensor cause the idle issue?

James Bailey

As the agent of doom and gloom.....you may want to check the crankshaft endplay after all it is a Rogerbox..

Mongo

Jim has a good point. Does the car crank slow when it is warm?

SeanR

Have not seen this addressed, is the TPS getting a good contact for idle?

Bertrand Daoust

A simple way to test the O2 sensor response:

- Disconnect the ground at the battery.
- Disconnect the O2 sensor.
- Reconnect the ground and start the car.

See post #20 here:
https://rennlist.com/forums/928-foru...ing-bad-2.html

It worked for me.

Airflite40

Crank end play was checked 2 years ago and also last Sept. Car cranks normally, actually quite quick compared to other 928's. Will recheck.

TPS was replaced 2 years ago, will check for adjustment and proper contact.

NC928S4

Temp II sensor check out ok? I had similar issue and new sensor fixed it.