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View Full Version : Elipical Idle and Strange Lighting Flicker???



John Valerioti
06-28-2004, 06:31 AM
Being a new SC owner.. And a prior Chevrolet man...I'm new to some "FORD" problems...I just bought my 90 SC a few months ago. I noticed that while listening to the engine idle, it has a definite elipical idle to it. Not only can you hear it, but you can see it if you watch the tach. The RPM will "bounce" from say 1000 rpm to 9000. Almost like you were "pulsing the gas pedal. If it is night time and you have the headlights on...the dash lights and headlights will pulse or flicker to the beat of the idle. I also noticed a strange wear patern on the belt tension pulley...it has a bumpie pateren that is consistant to the pulsing idle and light flickering..I'm not sure if the pulley pattern is causing the problem or what ...it is all so very strange to a chevy man...a SC new owner....I need advice....thanks,,,John

Mike8675309
06-28-2004, 07:56 AM
In general, the first thing to do is to pull the codes. If you're lucky, the computer (even if a check engine light hasn't come on) will have a stored code that will help track it down.

The SC motor uses a closed intake system. A Mass Airflow Sensor at the head end of the intake tract reports the amount of air entering the system. The air entering is controled by either the Throttle plates, or the Idle Air Control Valve.

If you have a vacuum leak or some leak after the MAF along the intake tract, that air is un-metered and the computer won't know about it. The computer will try to maintain the idle by adjusting the IAC, but if the leak is beyond the range of the IAC you will get a high idle. The back and forth you may be experiencing may be related to the EEC trying to take control of the idle by varrying the IAC orrifice.

Alternatively you could have an issue with the IAC valve itself not properly operating.

John Valerioti
06-28-2004, 07:32 PM
In general, the first thing to do is to pull the codes. If you're lucky, the computer (even if a check engine light hasn't come on) will have a stored code that will help track it down.

The SC motor uses a closed intake system. A Mass Airflow Sensor at the head end of the intake tract reports the amount of air entering the system. The air entering is controled by either the Throttle plates, or the Idle Air Control Valve.

If you have a vacuum leak or some leak after the MAF along the intake tract, that air is un-metered and the computer won't know about it. The computer will try to maintain the idle by adjusting the IAC, but if the leak is beyond the range of the IAC you will get a high idle. The back and forth you may be experiencing may be related to the EEC trying to take control of the idle by varrying the IAC orrifice.

Alternatively you could have an issue with the IAC valve itself not properly operating. Thanks Mike....I can understand the varing engine RPM being a possible vacuum leak..but what about the flickering dash lights and headlights...that has got to be electrical I would imagine....are you saying I have 2 seperate problems?,,,I was thinking the headlight switch might be the culprit....

TbirdSCFan
06-29-2004, 09:00 AM
As far as the electrical.. hook up a voltmeter to be sure you are getting a constant and solid charging voltage. Should be 14V at idle. Also, the meter should dip when the lights are flickering. From there, you start pulling fuses for all unessential circuits until you're left with just whats needed to run the car. Clean your battery posts and clamps with steel wool regardless of whether they look good or not; don't overtighten them. Check the connections at the Alternator and the starter relay. Lastly, take some jumper cables and ground the engine to the negative battery post using the black cable. (Yes, in fact electrons don't care about the color of the cable, its just really good safety practice to use the standard.) See if any of this helps. It could be the alternator is on the way out, or your 10" subs are pulling too much power ;) ;)

Mike8675309
06-29-2004, 09:48 AM
I would work them as two seperate problems. Electrical fluctuations shouldn't be causing the idle issue so start there. Work out the issues with idle. If the electrical issue then remains, work on that one.

You might need to use an analog meter to catch the fluctuations at the battery. Digital ones may not show the fluctuation in voltage if it's happening to suddenly.