Idle Issue

BELONGA68

MEMBER
1985 PLYMOUTH HORIZON
4 CYL
2WD
AUTOMATIC
80,000 MILES

Anyone know of any common issues with lopey idle on this vehicle. Been doing it for years with many individuals and shops trying to smooth out the idle and stop it from dying at stop lights. When cold it dies at idle no matter what's been done. When heated above normal operating temperature the problem almost disappears but comes back with intermittent lopey idling. Almost to the point of dying and just barely "hangin on." Any ideas on what to check first? Fuel pump/pressure is fine, new gas, new plugs, new wires, new distributor cap/rotor. Idle is controlled via computer so it's not adjustable to my knowledge. Could it be a bad O2 sensor or maybe a leaking vacuum line? Any good/simple ways to find vacuum leaks?

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

I think you mentioned your issue in one of the things you mentioned which means we need to test them all.

Here are a number of guides that will take us through this. First is to check for a vacuum leak.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Also, here is a guide that will help with idle issues:

https://www.2carpros.com/articles/engine-idles-too-high

Lastly, we need to check the fuel pressure.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

I assume you will find the issue with one of these guides but if not, we should check the basics starting with compression.

https://www.2carpros.com/articles/how-to-test-engine-compression

The only thing I will caution this with is that this appears to be the year they moved over from carb to throttle body so if you have a carb then we need to not only check for a vacuum leak but use starter fluid in the carb to see if it smooths out.

If it does when you add fuel, then we need to rebuild the carb.

Please let us know what you find with this, and we can go from there. Thanks