I have a stock 89 F150 with the 302.The truck only has 115k miles but has issues at startup. I drove the truck around town quite a bit this evening making numerous stops. Each time I would start it up there was a pretty bad surging idle for 5-10 seconds and it cut off a couple times. The longer it sat between stops the worse the startup was. The IAC is brand new. After it runs for 10-15 seconds and the idle clears up the truck runs and idles perfect. The problem is only at startup.I recently had the exhaust done and cut the CAT off. Would that missing 02 sensor be the issue? Any help would be appreciated

S

 

The information from the O2 sensor is not used for the first several minutes of running after a cold start. When you replaced the AIC, did you also clean the bore in the throttle body of accumulated carbon which the AIC resides? A missing O2 sensor will definately affect how well your engine runs warmed up.Other things to look at are vacuum leaks, sticking throttle body, cable or someone messed with the idle hard stop screw, which should never be touched. It is not there to determine the idle speed.Idle issues will occur is that screw is contacting the throttle lever and holding it open.

 

I did not replace the IAC, I had it done while the truck was at the shop for other issues. I will check the bore though. Does the 02 sensor that was deleted when I cut the CAT off have to be plugged in the CAT or can it plug in the exhaust anywhere? As far as the hard stop screw, I have no idea what that is or where to find it so I know I haven’t touched it. I am curious where to find it though.

 

The truck runs perfect once it gets done surging a few times. If it was the 02 wouldn’t it run like crap all the time?

 

I realize now the hard stop screw is the setscrew in the butterfly

 

I think I found my vacuum leak, I cleaned the intake through the throttlebody last week and unknowingly the upper hose was bent in when I reinstalled it.

 

Nope not the problem, still surging not on a cold start but after the truck has warmed up then shut off and restarted.

 

street my 4.9 does that. i know its running lean. check pcv? its your cold start enrichment and immediate fuel trim correction. the fuel trim SHOULD start to work fine, but when it attempts to use purely data from sensors it notices it doesnt run right, so it has to richen. i noticed that tune up helped mine be less severe, and im gonna fix the leak here soon.

 

That screw should not be holding open the throttle. Just check to make sure. Pull off the throttle cable and make sure the throttle body is not binding and returning all the way to it resting point.
As far as the O2 goes, your engine will never run efficiently with out a working sensor in its original place.

 

The closer you keep the truck to stock, the better it will run. Don't attempt to re-engineer it until you're a qualified engineer.

With the engine idling, pull the Red vacuum line off the FPR and check for the presence or odor of gasoline.

(phone app link)