When the engine warms up, the idle rises to 2100rpm.  It stays there until coming to a nearly complete stop(~3mph), where it suddenly drops to 900.

The car has no cruise or AC and has 62,000 miles.  We brought it to our general mechanic, and he can't make heads or tails of it.

Has anyone else had this problem before, or a good idea of what causes it?

Not sure about the 79, but maybe just the BCDD doing its thing. If it was suddenly cutting at 10mph anyway. Part of the emission control, consult factory manual for complete description and function.

Edited November 13, 20195 yr by Dave WM

1 hour ago, AutocraftCollision said:

When the engine warms up, the idle rises to 2100rpm.  It stays there until coming to a nearly complete stop(~3mph), where it suddenly drops to 900.

Is it a 5 speed or automatic?

Your description doesn't quite fit.  You're blending driving and idling together.  Why would you be "at idle" going over 3 mph?  Maybe add some details to what's happening?  Hanging idle speed is pretty common but has several possible causes.

Her's a service manual link. 

 

3 hours ago, Dave WM said:

Not sure about the 79, but maybe just the BCDD doing its thing. If it was suddenly cutting at 10mph anyway. Part of the emission control, consult factory manual for complete description and function.

I agree with Dave.

My bcdd kept the rpms hanging or high when i came to a stop. Adjusted it out, now works normal.

https://www.google.com/search?client=ms-android-americamovil-us-revc&source=android-home&source=hp&ei=Z6nMXfUegbaCB9CmstAH&q=bcdd+adjustment+classiczcars.com+&oq=bcdd+adjustment+classiczcars.com+&gs_l=mobile-gws-wiz-hp.12..33i299j33i160.5018.24393..25944...0.0..1.314.2789.1j19j0j1......0....1.......8..41j41i10j41i70i249j41i67j46i275j0j46j0i131j0i10j0i22i10i30j0i22i30.yUtRwNohHR

Edited November 14, 20195 yr by siteunseen

On 11/13/2019 at 3:14 PM, Zed Head said:

Is it a 5 speed or automatic?

Your description doesn't quite fit.  You're blending driving and idling together.  Why would you be "at idle" going over 3 mph?  Maybe add some details to what's happening?  Hanging idle speed is pretty common but has several possible causes.

Her's a service manual link. 

 

the car is a 5 speed. when I say at idle, I mean not accelarating. like when shifting gears or coasting to a stop sign

The guys were right, sounds like the BCDD.  But the symptoms are odd.  Usually they just stay at high idle, they don't correct on their own.  Seems like yours might just be gummed up.

It's the weird thing hanging under the throttle body.  Later ZX's had them on the manifold.

I tried to find that image Zed Head but couldn't, thank you.

When I lifted off the clutch pedal to coast up to a stop mine would stick around 1,500 to 2,000 rpm for about 15 seconds then drop down to normal.  I adjusted the BCDD in just a quick ride through my neighborhood.  When you get that locking clip thing off leave it at home and find some stop signs.  Leave the hood loose too.  Turn it one way and see if it gets better or worse.  It's really easy.

Mine also had a sticky gas pedal feel that would hang up or down.  I cleaned the throttle body bottom under the throttle blade with a toothbrush and lacquer thinner.