Idle is oscillating

[macipusy]

My idle has been getting worse over the last month. First it would sometimes oscillate, and now it does it all the time when the car warms up.

Before warm up it will be steady at 1500 RPMS...and after it reaches operating temp it will oscillate like a perfect sine wave from 950 to 250. THe duration from 250 up to 950 and back down is usually 2 seconds. The boost gauge is doing the opposite....at 250 RPMs it will float to about 5 Hg and at 950 it will be a 16Hg.

Most times if I just sit there it will get in such a groove that it will fall below 250 and die.


I took off the fat hose off the intake and the IAC and spreyed it with carb cleaner....no result at all

I checked for loose or disconnected vaccum lines on the drivers side of the car....nothing there

I will try to check for rippes or punctured vaccum lines and maybe take the IAC off and clean it.


What else should I check?

[Suigintou]

I had the exact same idle trouble with my Buick.

Check your voltage level at idle with a multimeter. If your alternator is only breaking even, the battery can get weak and things get a little crazy.

Also it wouldn't hurt to make sure all your vacuum lines are clamped on and not just pressed on.

See what putting on your brake does to the oscillations.

uhhh...thats all the other stuff I can think of. Replace the fuel filter?

[macipusy]

checked the voltage on the alternator....14.5 volts

My friend suggested pressurizing the manifold with a compressor and listening for leaks....I will try that

[macipusy]

I did the pressure test, by pressurizing the intake manifold with a compressor through the BPV hose, and I could hear all the air leaking out right away from under the drivers side fuel rail. I unbolted it and could see the problem right away.

The rear o-ring between the fuel rail and manifold was toast. It was missing a chunk .25" in length and half the thickness of the o-ring. Pretty much an area comparable to the ID of a small vaccum hose. All these orings were brand new 8 months ago, so I don't know how this happened.

I luckily have 4 spare various intakes laying around, so I just replaced the o-ring with a used one.

The idle was much better...still audibly oscillating but not visibally on the RPM gauge.

I just tuned that out by leaning it out slightly....was a little rich at idle.

[macipusy]

This are the pics of the oring between the fuel rail and intake manifold.

[wtdash]

I've had a return of an idle issue.

I'm going to try your pressure test. How much pressure did you use? I don't want to break anything!

[macipusy]

I just used the air attachment on my friends compressor...I think you should be safe with that sort of pressure, especially hand pressing the air hose.

I just hand pressed the attachment to the small hose going from the BPV to the intake maniflod and pressurized it for couple of seconds. You need to cap off the air intake at the turbo inlet, or else the pressure will escape through the air filter.

You will be able to hear all the leaks, and if you have really bad ones, the manifold will not hold pressure for more then a second or two.

[biggreen96]

Thats a good idea.

[yazmo]

hello i have the same problem as describ here
whiile i push the brake pedal do affect the idling

i chqange the alternator and starter one year ago
but nothice tha the wire from the alternator to the battery have few wire broke ill get you pic of it
someone told me that should not be a problem

[Danny-G]

i have a similar problem but the idle doesnt low enough to kill the car and sometimes its just a really low idle which makes the car shudder at times