Can't Lower Curb Idle - Ford Mustang Forum

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post #1 of 3 (permalink) Old 08-22-2013 Thread Starter
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Can't Lower Curb Idle

I have a 1985 Mustang LX 5.0 with 4bbl, 5 speed, 108kmi, and only drive it in the summer. Can't lower the curb idle w/o adjusting the timing. Can't get curb idle below 900RPM, easily. Any thoughts?

Background info, sorry in advance for writing a novel, but ....
About 6 months ago my car wouldn't start and figured out it is the pick-up coil in the distributor. Since replacing the coil required removing the distributor, I bought a rebuilt distributor exactly the same as mine. I put in the new distributor and "eye-balled" the timing by setting the distributor about half between rough idle and pinging, all was fine. Now, I knew the car needed to be inspected so I checked the timing and noticed my initial setting was 8deg BTDC and the spec is 10 BTDC. I incr the timging to 10 BTDC and noticed that the idle increased a little, I figured that was a good thing. The car is running great, I got 20MPG the last two tank fulls, not bad for a 30 year old car. The idle was about 1000 RPM, I knew that was a little high but took it to the DMV and it failed for high HC's at curb idle, 700 vs 200ppm. However, since fast idle emissions are fine, I think it is just related to the idle setting.

Now, I backed off the idle screw (drivers side) and can't lower the idle, I can only increase the idle. I remembered that the idle increased with timing incr from 8 to 10 deg.. So, I lowered the idle to 6 deg BTDC and now I have more control over the curb idle. I was able to set the idle below 750RPM. but ...... I notice when the car is sitting alongside the curb for a long time the idle drops to my setting of < 750RPM. However, when I drive it and stop at a stop-light or stop sign, the idle drops < 750RPM, but immediately climbs to >750RPM. Not quite as high as a 1000RPM, but it definitely doesn't remain <750RPM. I don't recall this happening in the past and I am concerned I will fail curb idle emissions again.

Any thoughts?


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post #2 of 3 (permalink) Old 08-23-2013
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You car uses timing to help adjust the idle. You have to make sure it is installed correctly with #1 cylinder at compression TDC.

2014 V6 Deep Impact Blue Mustang, Borla Type S, 2.73 (Uughh need 3.73)
1989 Sonic Blue Mustang LX Coupe, 349 ci,
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post #3 of 3 (permalink) Old 08-23-2013 Thread Starter
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yes it is, otherwise I would expect the car wouldn't run right at all. I re-installed the distributor with rotor pointing exactly where it was previously. In addition, I was able to set engine to 0 deg timing mark and the rotor was pointing at #1 plug.

Any other thoughts?
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