bgrabner

Hi all - I'll be the first to admit I am no expert on these cars, but this has been an ongoing issue for my 87 S4 Auto that I really want to resolve. I've had a rough idle on the car pretty much since I bought it, and have done a lot of work on it and nothing has really improved it. I don't drive it often, and since it has an electrical drain I usually disconnect the ground when I am not driving it, which I'm sure doesn't help the situation.

Basically, the car starts up and idles kind of high while cold - around 1100 RPM. When the car gets warmed up after a few minutes, the idle starts to oscillate between 300 and 1200 RPM. It occurs constantly when in Park or Neutral - when in a drive gear at a full stop it tends to stay steady at around 800 RPM unless the car is really hot, which is not uncommon during a Sacramento summer.

What has been done - full intake refresh, w/replacement of all hoses and vaccuum tubes, so I believe I have most of the vaccum leaks resolved. I replaced the ISV and Temp II sensosrs while doing the refresh, so I think i have eliminated all of those items as potential issues.

What I have done today that stumps me. I disconnected the O2 sensor a few weeks ago, and the idle issue seemed to mostly go away, although when the car is really warm it comes back. So, i ordered a new O2 sensor, hooked it up today, and the issue is the same - oscillating idle, but reduced/goes away when the O2 sensor is disconnected.

So, I did some searching on RL, and came across Dwaynes idle and WOT test in this thread:

https://rennlist.com/forums/928-foru...re-w-pics.html

So i disconnected the battery ground and pulled the EZK plug and did the test. What I got was not what I expected, and I don't know if this is the cause of my problems or if I am not doing it correctly.

I first did the Idle switch test. According to the manuals and Dwayne's excellent writeup, with the Ohm meter in the #8 (throttle valve switch idle speed) and #18 (Ground), i should get less than 10 Ohm's with no load on the accelerator, and when i press on the accelerator the meter should go to infinite Ohms. Instead, I get the following Ohm readout both with no load on the accelerator pedal and WITH the accelerator fully depressed - there is no change:



That didn't seem right, so I also did the test for the WOT, which according to the manual should read infinite Ohms with no load on the accelerator and go to less than 10 Ohms with the accelerator depressed. This is the readout I got that also did not change regardless of whether I depressed the accelerator pedal:



Am I doing something wrong or is this symptomatic of my idle issue? Is there something that is causing the computer to not recognize when there is no load on the accelerator and hence use the idle mapping? I'm not sure where to go here - I've replace almost every component that has caused issues for other people on the list, but that hasn't helped. I could either get the MAF refreshed or try a new EZK, but I'm not sure if that is the right direction to go.

Sorry for the long writeup - I really want to get this issue resolve - this idle makes driving the car not very fun. Thanks in advance for any help on this.

Bill

Mrmerlin

first we need more info..
Did you replace the knock sensors ,and the hall sensor, and the throttle position sensor, and the crank position sensor and the flappy pod when you did the intake refresh??

bgrabner

Thanks for the reply Merlin - i did replace all of those with the exception of the hall sensor - I'm not familiar with that - would that be a normal part of an intake refresh?

86'928S MeteorGrey

Without more information...

1. Clean all grounds on car.
2. Test vacuum system with gauge.
3. Double check ISV and it's connections including wiring.

Good Luck!

Mrmerlin

Hall sensor goes on the back side of the passenger side cam housing , it will take out 6 deg of timing if its bad, usually replacement is done because the connector has crumbled..
With more info I would suggest that you first make sure the TPS is adjusted properly and the throttle cable is adjusted so the TPS is actually closing, from your post the throttle cable may not be loose enough.
The throttle cable i am referring to is the one thats connected to the quadrant and then goes around the wheel

worf928

Do the test on the LH ecu also. Pin 17 is ground. 2 is idle. 3 is WOT.

Just to make sure.

But, if you replaced the Idle/WOT switch during your intake refresh, then....

the switch was not adjusted corrected AND you have some cable issues preventing the WOT switch from closing.

OR you've got some harness issues between the Idle/WOT switch and the ECU plugs.

When you do your plug tests make sure you are using the right pins. Here are the diagrams:

John Speake

Double check you have the correct pins for the test, as Dave suggests.

The fact that the idle speed osciallates up and down implies that the idle loop is being activated by the closed throttle switch OK, but the loop is not able to get control. As the ISV loop nad O2 adaptation are linked in the LH software, this may be the reason that things are improved when you disconnect the O2 sensor.

Try to beg borrow or steal a diagnostic test, and run the idle adaptation process. Some cars just seem to need that. See if that process with run and complete OK, and if it fixes the problem. The bad news is that adaption paramters will be lost when you disconenct the battery when you store the car.

bgrabner

Thanks for your suggestions everyone - I will work on those this week and report back what I am able to find.

John - by diagnostic test, do you mean the spanner tool? Will that work on an 87 S4? I don't have a problem purchasing one - I also have an 88 so it would be useful for sure.

Bill

John Speake

Hi Bill
For an 87 you need to fit MY 88 EPROMS in LH and EZK. I am sure someone can make you a set, or I can send you some.

The '87 car is wired for diagnostics, the same as your '88. So just the newer EPROMs are required.

I do have a few Spanners left. Other tools will also do the job.

worf928

With the EZK/LH system, a failed always-open idle switch will result in a hunting idle once the engine is fully warm and the 02 loop is engaged. (Assuming nothing else is wrong with the engine management system.)

The hunting is due to the LH oscillating the fuel mixture around the 14.7:1 middle of the narrow band sensor window with no idle control loop functioning. I would surmise that once you disconnect the 02 sensor the hunting stops because the LH quits oscillating the fuel.

In any case, always-open doesn't appear to be your problem if the test method is correct.

Make sure you are doing EZK tests on the EZK plug and LH tests on the LH plug. And that the orientation is correct. Use the absent pins as markers for checking.

John Speake

You're saying that an O2 loop in control is swinging the mixture so far as to affect idle speed ? And it's only the idle loop that prevents this ? I don't think so...

Brett Matthews

One thing I do not see mentioned, is fuel pressure testing. I suggest this, as I had similar, but far worse idle speed fluctuations.
Turned out to be my fuel pressure regulator sticking open, and then closed, back and forth. Do have your pressure checked, if not already done.

I also notice your point about electrical discharge, when left sitting... I also have this problem.
One item I have finally noticed, is that when the alarm system is working, i.e. when I lock a door, and the other door also locks, my battery keeps it's charge for a long while longer. When I lock one door, and have to walk around and lock the other, the battery's charge can go away in as little as two days.

I have determined I will need to replace the two door plunger switches. I've known for years, that the driver's side one has failed entirely. How hard will this job be, anyone?

For all those years, to open the hatch electrically, which I had to do, because the hatch latch key mechanism was worn too much to work, I had to open the passenger's side door, and "pop" the plunger switch.
Brett
Also with an '87 auto S4

worf928

I willing to debate the cause but not the effect or cure.

I've seen this specific effect twice with S4/GTs with absolutely no intake leaks and all other components functioning. In one case - my own car - I didn't replace the idle/WOT switch (the only thing I didn't replace) when I did the intake and the idle switch failed shortly thereafter (doh!). In another case a new idle/WOT switch I installed as part of an intake R&R failed intermittently to close.

In both cases, at idle, the cars would very noticeably hunt. In the second case, since the failure was intermittent, blipping the throttle would usually allow the idle switch to close at which point the idle went rock steady.

In both cases, my ARM-1 showed the cycling of the fuel mixture at "idle" and in both cases the frequency of the hunting was identical to the frequency of mixture oscillation shown on the ARM-1.

In both cases, a functioning idle switch completely cured the condition.

Lizard928

Your readouts for the pins are strange, I would confirm the pins for sure. If that doesnt solve it then I would unplug the ISV and see what readings you get. If you get no change then IMO you will have frayed and partially touching wires just before the connector.

You need to get the ohm meter to get the proper readings with WOT too before the functionality of the TPS is working correctly.

An in op TPS is unlikely to give these readings. But as Worf has noted a failed TPS will cause an idle hunt.

If you are seriously wanting a spanner/hammer then I know someone who might have one for sale. PM me and I will put you in touch with him.