I have a 1990 F350 with the 460, and it's been giving me some trouble lately. For the past couple weeks, when it first starts on a cold morning, it will occasionally idle rough and run rich, almost like there's a misfire, but then after about a minute, it smooths out and runs fine. This week, it really started acting up, and I don't exactly know what the problem is. The initial symptoms led me to believe that it was something in the ignition system, but I also have reason to believe that there is an issue with the air intake systems as well. For the past couple months, the truck has been throwing a code for a bad IAC valve, causing it to suddenly drop RPM at idle when it's warmed up. They might be two completely separate issues, but I just need some pointers so I know where to start.


The issue really became bad on Tuesday. It started up and ran rough, and didn't smooth out. I shut the engine off, let it sit for a bit, and then I went to start it again, only to have it crank and sputter at me. When I finally got it started (done so by flooring it), it was not any happier. So I drove a different vehicle that day. I left it plugged in overnight and the next morning, it started fine, no problem. Ran like a top. And then the morning after that, no go again. Ran rough, idled poorly, almost stalled a couple times. The thing that was strange is that the engine did smooth out after a little, except for at idle it was still running horribly. When I got home that day, I checked the computer again for codes, and now it was yelling at me about an IAC fault, a MAP sensor fault, and an EGR fault. After I had the computer perform a self-test, it ran fine again.

Basically, this intermittent fault is really confusing me. One morning it runs fine, the next it will barely idle or will run roughly. I really don't know where to start, because I don't want to drop $140 on a new MAP sensor and IAC valve, but at the same time, trying to pull the plugs out to inspect them in 20 degree weather doesn't sound like a party either. Forgive my *******ery and laziness, I'm just looking for info before I begin buying parts and scraping knuckles.

 

I would not worry a ton about the egr valve. but try cleaning the connection on the iac and also unplug it and see how it runs with it disconnected. As far as the map sensor goes, I am not totally sure how to help you with that but you may as well clean that connection as well. If cleaning the connections does not help I would start by replacing or cleaning the iac and checking if that makes a difference. In addition, check for vacuum leaks. Also you may as well pull out a plug and take a look at it. It can be done in a really short time on the 460. Sorry I can't help more!

 

Try a blocking plate to eliminate your potentially 27 year old EGR Valve from leaking exhaust into the intake intermittently at idle.

Many other things to try.

What are your Code #s ?

 

Turns out it was none of the above. Pulled the distributor cap off, and all of the terminals were corroded. New cap and rotor later, and boom! runs fine. Although I should still probably replace the IAC at some point.

 

Just proves my motto;

"Do All Scheduled Maintenance Prior to Troubleshooting"