tombtaylor

I just finished replacing sensors & hoses on my 87 928 & it now surges at idle...
could an incorrect hook up on a vacuum hose be causing it?
Thanks,
Tom Taylor

NickT

I would certainly double check all of your vacuum lines and sensors. "False air" is a huge issue when it comes to a smooth idle. Could also be a sticky ISV, there is a trick with WD40 that can help that issue. Do a search for it and you'll find it pretty quick.

A bad LH computer can also do this (ask me how I know )

Did it have a smooth idle before you starting replacing things?

no doubt

I had a huge idle surge then lag (repeat ad nausium) issue earlier this week on my 88 S4. Disconnecting the battery for a minute, then reconnecting it seems to have reset my LH and EZK computers and cured that problem.

Easy.

Free and quick test, too!

Mike Frye

Tom,

I'm not an S4 guy, but you said 'sensors and hoses'. You should probably be more specific in listing exactly what you replaced.

There are lots of interconnected components that can cause weird (and often similar) issues. Idle issues are the most common problem reported on here IMO and MY, engine type, and type of recent maintenance will give people a lot more to go on.

I think you're on the right track, leaking vacuum hoses are a major cause of idle issues. Bad/loose grounds can also cause unpredictable results. Loose connections on anything you replaced will also affect things. More info will help the gurus...

Good luck.

TheoJ

you'd be surprised how unstable the idle is when the car is overfuelled (when hot obviously). This is often not considered. There is always talk about false air. The Lambda sensor (O2) may be lazy causing the ecu to respond way too late, and cause slow moving corrections which you will result in erratic idle behavior. (i.e. fuel too rich or too lean)

False air often also shows a high idle rpm. Resetting the ecu by disconnecting the battery seems to help for just a minute or two. Then the idle climbs up and fluctuates. If that happens, look for false air.

Last but not least, a faulty LH ecu can make the idle oscillate up and down from 1400rpm to almost zero.

regards

tombtaylor

This all started from my needing to replace the spark plug wires....I had a rotted breather hose from the top of the passenger side valve cover to the bottom of the oil fill neck...I removed the intake & discovered the knock sensor covers were disintergated & the plug for the sensor which is obove the flywheel the plug had fallen apart...these I replaced & put it all back together...but I'm not positive about the placement of the vacuum lines...there is one which rums from the rear of the intake to the front of the engine& another which comes out the front of the intake...these I'm not sure about...the rest I think are corrrect.
Tom

tombtaylor

What is the ISV,& where is it located . What is the "trick" with WD 40....also where is the Temp ll sensor located..
Thanks,
Tom Taylor 87s4

VehiGAZ

When you removed the intake, the ISV (idle stabilization valve) was right there under it all. Mine was just replaced in a full-blown intake refresh - what a wonderful improvement! Anyway, it's stabilizes the idle, as the name implies.

It could be part of your problem, as could any air leak, particularly at the intake gasket. Did you replace it when you removed your intake? Actually, even if you did, if there is a gap, the unmetered air would lead to a bouncy idle.

My advice to you would be to do that intake refresh again, replacing the ISV, throttle position switch (WYAIT), the intake gaskets, and all imperfect intake hoses. Make sure you get it sealed up again perfectly, too.

Also, I still suggest to swap out the MAF, as discussed in the other thread.

Good luck!

no doubt

WAIT!

Before going to major effort and expense to replace parts like the ISV that are buried under the intake...do the tests first. Test to see if that is your problem, first.

Test.


Test.

Test...rather than replace, replace, replace.


One test for the ISV (there are surely tons of which I'm unaware) is to barely raise the idle speed with your foot on the accelerator pedal while parked to see if the surging stops.

After all, a vacuum leak or an air intake leak will still be there whether your foot is on the throttle or not, so raising the idle speed with the gas pedal pressed a little bit won't change the surging if the problem is with vacuum or air intake leaks...but it *will* change the surge behavior if the ISV is clogged.


Always, always, always suggest tests first before suggesting replacing components! Testing makes you smarter...whereas replacing/guessing just makes us lucky (or not)!

VehiGAZ

Yes, yes - test first. THEN do an intake R&R...

tombtaylor

That may well be the problem, as I can hold the rpms at aboout 1100 & it runs smooth....does the isv have a vacuum line attached which comes out at the front of the intake?..what about squirting some wd 40 through the vacuum line ????

Mrmerlin

you might want to remove the air cleaner and the MAF and look very carefully at the plastic junction that plugs into the boot that the MAF plugs into. The one that seems to fracture is the plastic junction on the right (passenger side)
I found a similar problem with a surging hi idle and was going to remove the MAF boot when I moved the plastic junction I found it cracked and it also had a hole in it letting in un metered air.
This is the part I ordered 928 110 690 01 for the right side and the one that seems to not break is 928 110 689 01 (driver side)
This is pertaining to S4 cars and later