Carrera 3.2 idle fluctuation

[Zef]

I need some imputs down here...The babe is a 1984 Carrera 3.2 stock (SW chipped)...Since two weeks, the engine idle fluctuate a lot on starting...it goes from 250 rpm to 2000 rpm...only when it's cold. Before that...the idle was stable at 850 rpm... When I go on the road for warming...it tend to stop...and when the engine is warm...all is ok...Any clue...??? thanks...

[DonMo]

There is a ton of info on this, do a search. To help you on the short term. My guess is you either have a bad O2 sensor or a air leak that has developed changing your air to fuel mix. Our year is notorious for leaks on the intake gaskets. Get some fuel injector spray cleaner and while parked and at idle, start spraying around intake base, rubber boot and vaccum hoses looking for air in leakage. To see what it sounds like when you find the right spot, spray some in the air inlet to your air cleaner and hear the increase in rpm. If you haven't changed your O2 sensor in a long time, it is probably the culprit.

[Zef]

Thanks DonMo...I will try this...my O2 sensor is two month old...I will get a fuel injector spray cleaner can...and will keep informed on the results...

[DonMo]

The other think is to remove and clean the idle control valve. You can use the fuel injector spray to clean it and I spray some wd40 or silicon based spray in their to lub it afterward. Shake it and if you can hear the ball move it should be fine.

[dentist90]

Syl,
I had a similar problem a couple of years ago. Started well, idled about 800rpm but if you touched the accelerator it would lope up and down from near stall to 1200 rpm for several seconds until it stabilized again. Once the car was warm it was gone. Could be a vacuum leak, but I took off the idle control valve 1st and gave it a good cleaning. Problem solved, could drive cold now without it stalling! The rotary valve inside the ICV was stuck from years of oil vapour being drawn past it. Give that a try if you haven't already.

Bon Chance, Syl!

[Zef]

Cooool...I'll go to local auto part store this PM to get the fuel injector spray...I'll keep you informed...Thanks a lot...

[dentist90]

Plain old carb cleaner will do it.

[Wil Ferch]

Check with Steve Wong.... his chip for 84's will *likely* by-pass the closed loop functionality of the O2 sensor...so a bad O2 sensor will likely then not be cause for the idle instability. You're likely running open-loop and the O2 sensor is not a player anymore. There is a closed loop chip option that he provides if you have the later 4k or 8k DME PROM....the 84 and 85 cars have the 2K prom DME and that is why I think you're open loop. Better check this first.

[Zef]

When Steve sent me my chip...he gave me info on how to do the mod for the DME from the original 2K to a 4K setting...That's what I've done...but the idle fluctuation problem came in after a complete year following the chip installation...so I don't think the SW chip is related to the idle fluctuation I got now...

[450knotOffice]

My guess is that you have an air leak somewhere in the intake system or your Idle Control Valve is going bad.

[dentist90]

Q: if an aftermarket chip bypasses O2 sensor, is removal of the cat converter recommended to avoid any issues with clogging it up by running rich? It wouldn't pass visual on smog test then.

[jbrinkley]

man, I would love to have an old eagle talon to mess with, i thought those were pretty cool when I was 20.
my money's with scott and an air leak/idle mix

Do they do a visual in BC?

[Steve W]

Hi Syl,

For the most part, Don and Scott are probably right. Something changed in the last two weeks that caused your idle mixture to change - usually attributed to some type of intake air leak. When a car is cold, the O2 sensor is cold, and if the mixture is off, the car is dumb to the mixture and the car will want to idle at a speed outside the adjustment window of the idle stabilization valve. This will cause the idle control valve to oscillate frantically, often out of sync with where the rpms are at the moment, causing the idle to hunt all over the place, until the O2 sensor warms up and tells the DME unit to correct the mixture, bringing stability back to the idle speed. Important is the DME unit is not adaptive in any way, so the mixture has to be correct from when the car is cold in order to have a stable idle, weather there is a connected O2 sensor or not. So if you still have a cold start hunting problem with the O2 sensor disconnected, it is not the O2 sensor.

A sudden mixture change can be attributed to either a intake system air leak, or a component failure such as the head temp sensor. To test for intake system air leaks, I prefer to use engine starting fluid (ether) as it is volatile and the slightest leak will cause a sudden rise in engine speed. To test, disconnect the O2 sensor and ISV, and at idle, spray around all suspected areas of the intake system, including intake manifold gaskets, fuel injectors, manifold joints, vacuum lines, and around the fuel pressure regulator and fuel pressure damper. Any leak will cause a sudden rise in the rpms. Also, test the impedance of the head temp sensor which if I recall should be around 2500-3000 ohms cold, and 100-300 ohms hot.

When the culprit is fixed and traced, the idle mixture needs to be adjusted to the correct ratio, so at start up, it will naturally idle nicely at it's expected speed, and need little to no correction assistance from the idle control valve and O2 sensor. Idle CO measured before the cat should be 0.6-0.8%, or use a digital volt meter and measure the voltage off the O2 sensor at idle and normal running temperature disconnected from the car. The idle mixture properly adjusted should swing evenly between 0.2 and 0.7 volts. Mixture adjustment is performed with an air bypass screw under the air flow meter with a 3 mm allen wrench. After setting the correct mixture, the base idle speed needs to be adjusted to the correct operating window of about 880 rpm. This is done at normal operating temperature on the throttle body. Jumper the b-c terminals on the test socket on the left side of the engine compartment to lock the idle stabilization valve at 50% duty cycle so you can adjust the base idle speed to the middle of the ISV's operating window.

Other things you can do to help, although not the cause of your problem, is to remove the ISV and clean the gunk out of the insides out with brake cleaner and lube with a light spray oil like TriFlow, and check that your idle microswitch is properly and consistently activating when the throttle is returned to rest position. Good luck and let me know if you have any other questions.

Steve

[Steve W]

Wil:

The only chips now that bypass the O2 sensor at the ROW/Euro Carreras that do not come with an O2 sensor, or specially requested ones for race cars or cars where guys want to run without one (ie. backdated cars with headers). All the standard ones for the U.S. spec cars for the past few years recognize a sensor if connected.

Originally Posted by Wil Ferch

Check with Steve Wong.... his chip for 84's will *likely* by-pass the closed loop functionality of the O2 sensor...so a bad O2 sensor will likely then not be cause for the idle instability. You're likely running open-loop and the O2 sensor is not a player anymore. There is a closed loop chip option that he provides if you have the later 4k or 8k DME PROM....the 84 and 85 cars have the 2K prom DME and that is why I think you're open loop. Better check this first.

[Zef]

Wow...thanks for this troubleshooting task Steve...I will go step by step as described...I will let you know my findings...Again...Thanks a lot...Syl

[Lorenfb]

"A sudden mixture change can be attributed to either a intake system air leak, or a component failure such as the head temp sensor."

Not really:

1. An air leak around the idle valve will not cause a mixture change
and if around the AFM (e.g. manifold leak-gaskets) will cause a lean
condition (low idle/miss).
2. A temp sensor failure will cause a continous mixture problem
in most cases, which is NOT the case here.

To not waste time and to avoid the hyperbole of the long post:

1. Eliminate the idle valve as a problem by simply disconnecting it when the
engine is initially started. The idle should be very low and NOT hunt if it's
the problem. Then squeeze the idle hose, if the idle doesn't drop then there's
an air leak around the idle valve AND throttle butterfly but after the AFM.
If the idle drops, then check that the idle switch is initially closed at startup
and that the idle valve is not sticking (vane moves freely).
2. To check for a bad temp sensor (most likely not the case here) just disconnect
the temp sensor and replace it with a paperclip (keep one in glovebox).
3. An intake air leak (that which bypasses the AFM) will cause a lean mixture
(unlikely here) and will not cause hunting but an idle-miss.
4. Make sure that the AFM is not sticking, i.e. the "flapper" is moving freely when cold,
as a sticking AFM can cause a rich mixture resulting in hunting.
5. Make sure that the fuel pressure is 35-38 psi at cold startup.

Other possibilities, but VERY rare:
1. Overcharging alternator at startup (cold)
2. Intermittent DME ECM
3. Intermittent idle valve

[Wil Ferch]

Steve:
I will send you PM on the question of whether or not a 2k prom car runs open loop...as in "my" chip that I have from you.

[DonMo]

Excellent post Loren.

[Zef]

Thanks for the light up Loren...the more we know...the best we are...Syl.

[stlrj]

As long as you have carb cleaner, you might as well spray the base of your intake manifolds at idle to see if, indeed, you have vacuum leaks there from a sucked gasket.

Cheers,

Joe