Idleing

[Sorny]

I have a 89 Bronco with the 5.0L, and in the mornings when i start it, it will run fine but then after a min it will idle down to 600rpm and it will jump from 500rpm to 700rpm. It will do this for about a min or so then it will quiet and idle fine. So does anyone know whats up with this?

 

[Txquadhunter]

first try a tune-up if you haven't in awhile. fresh plugs, wires, cap and rotor fixes lots of these little problems.

 

[Shadow_D]

Don't forget the forgotten tune-up part, the O2 sensor. Also an oil change is always a good thing. Now the oil change may not solve the problem but it will most likely stop it for a few miles.

If the tune-up seems to slow it down but it comes back in a few hundred miles then your next step should be to check / replace the EGR valve and other sensors related to that.
This was a problem with my truck and I had the dealer fix it when they replaced all of my drive-line seals.
Hope this helps.

 

[Justshootme84]

Try to check the Throttle Position Sensor (TPS) for proper voltage.

 

[VegasBronco]

I've also got an '89 with a 5.0 and mine does kinda the same thing. It starts up at about 1250 for maybe 20-30 sec then to down to 1000 for 20 then to 900, then it drops to normal idle. I just think of it as warming up.