89f1504wd

Just got a beater truck for the winter (89 f150 4wd, 5.0) and I've been getting her road ready, already fixed a few things and now I've noticed this idling problem. Right now I'm not driving it daily, so today I decided to go put some gas in. I noticed when I got to the station and I hopped out the truck had a surging idle, up down up down, seemed like it was going to die but it didn't. So I pressed on the gas a little and it went away and idled just fine for 5 minutes till I turned it off. Filled up and started it (no surging idle) but when I got home it did it again it was surging up and down, When I was driving and I was off the gas slowing down and braking, it was also idling funny felt like the truck was going to kill but it didn't. Then I would press the gas and it would idle fine. Or if I turned it off and restarted it, it would idle fine, Any ideas? Check engine light isn't on, and I'm not very mechanically inclined. All help is much appreciated. Happy holidays

1990 5.0

Sounds like a possible throttle position sensor or an idle air control valve.

fltdriver

You can pull the IAC valve off and clean it with throttle body cleaner, wouldn't hurt to pull the air hoses off of the TB and get the valves, use a toothbrush and a rag, Get the backs of the valves and be extra vigilant on the edges as they are the most important part. You can use a pipe cleaner to help with the small parts on the IAC. If you have a multimeter you can test the voltage to the TPS, it should read .9-.97 with throttle closed.

My money would be on the IAC, but it could also be a fuel delivery related problem, clean the IAC and TB first, if this doesn't take care of the issue then check TPS and fuel pressure, if you get the loaner fuel pressure tester from oreillys it comes with a multimeter (if you don't have access to one). If pressure is a little low a new fuel filter would be the next step. After that the only thing would be the pump or fuel pressure regulator. It may be caused by a vacuum leak but those would usually cause continuous problems.

89f1504wd

Cleaned the iac and added a bottle of seafoam and now it idles good, I think the problem is fixed. Thanks guys

fltdriver

Cleaning the IAC will get you by but usually once it needs cleaned it will keep needing to be cleaned, don't know why, so if you don't mind doing it you will be good for a quite a while but it will eventually need replaced (could be a year or two down the road).