pmb692

Hello all, I have a 1989 S-4 with a 5 speed and I have an idle issue that I can't solve. When I start the car everything runs and it idles smoothly, but once it gets warm and the fans turn on the idle really fluctuates. It goes back and forth from around 600 rpm up to about 1100 and back down. It does this for as long as the fans are on, sometimes even dropping low enough that it feels like its going to sputter out and die. It shows over 12 volts constantly, and as I mentioned runs very smooth as long as it's cold. Any suggestions?

Speedtoys

Check the temp2 sensor specs.

Mongo

Erratic idle when warm also points out to a failing idle control valve too.

Good luck to you.

Speedtoys

Yup..fergot that one.

Mongo

Found that out after replacing everything BUT the ISV and doing the intake service.

No wonder my car runs like it has 10,000 miles on it.

worf928

Steady cold idle that fluctuates when warm points to a problem with the idle control loop. These are the primary input sources to the control loop:
- idle switch
- temp-II sensor
- o2 sensor
- mass air sensor
- engine speed sensor
The single actuator is:
- the idle stabilizer valve

The ISV rarely fails. Sometimes they are sticky. O2 sensor and Mass-Air sensor are 'wear items' and need to be periodically replaced or rebuilt, respectively.

If you can use a multi-meter and can fashion some test leads my inspection guide, linked in my signature block, has directions for testing the idle switch, ISV, and temp-II sensor.

pmb692

Thank you guys for the info. MAF was replaced with a rebuilt one from Roger less than 100 miles ago, so I don;t think that's the issue. O2 sensor is also pretty new. I have a new TPS, and just ordered a new ISV. If I have to take the intake off, I want to have them on hand.

Speedtoys

If you take the intake off, plan to replace _everything_, it wont run well piece-mealed afterwards.

pmb692

Did an intake refresh last summer, maybe 200 miles ago. Have new gaskets just in case as well.

worf928

Just because your idle/WOT (aka TPS) switch was replaced doesn't mean that it was set-up correctly and is working.

Do the tests before spending money and time on parts.

Ricardo Vega

That's the same issue I'm having on my 87, I replaced MAF and now crank position sensor and its gotten worse. I can't it. I do have a bad idle valve which I had to disconnect tube leading under he intake.

Ricardo Vega

Speedtoys

Everything you get within 8" of, you're breaking without knowing it.

Do the intake refresh properly.

Rodrv6

I went through a similar problem on my 88 S4. After replacing:
mass airflow sensor
idle stabilizer
throttle position sensor and checked adjustment
crank sensor
O2 sensor
temp II sensor
had both engine computers replaced with refurbished units
full intake refresh with hoses and vacuum lines
verified the integrity of all vacuum components
pin by pin check of every wire in the fuel injection harness

I still had a problem with the warm idle being unstable and occasionally causing the engine to stall. I did notice that the problem became better or worse depending on the electrical load. Turning on the headlights and air conditioning seemed to make the problem worse most of the time. I had previously cleaned the grounds and had treated the central electric panel with Deoxit. What finally fixed the problem was replacing the battery positive cable! The old one checked OK with an ohm meter, but installing the new one completely cured the idle issue! Now it runs like a new car!!

SwayBar

Wow, what a discovery that was...!

Shark_gts

Yes it's actually easy to look at the basics first:

Which is test for voltage swings the buss that feeds the the LH computer and engine circuits ( i suppose)

And (although not related to this thread) Don't forget about fuel pressure.

distortions in Fp will also cause stability issues.