When I go out in the morning to warm it up the engine will fast idle for a minute or two then decelerate to idle, then the revving begins. Almost to a stall, then revvs back up. Irritating. But it runs great(slight spark knock from cheap gas) and gets normal(lame) 5.0 gas mileage. After you drive a bit the revving is gone entirely and there is a smooth, even 900rpm or so humming along. I can drive 1 mile and it's gone and will idle normally for hours(literally). 60mph at 1500rpms.
I think its called 'IAC hunting' or 'idle hunting'. You usually see it on 5.0's with lumpy cams that are still running the stock MAF and ECM. Maybe a vacuum leak or IAC motor issue?
Guy across the street has a mustang with FI that does that. Drives me nuts. he thinks it is ok. I want it to hold a steady idle from start to finish.
clean the throttle body and iac dissconnect battery and do a relearn on pcm also check pcv valve for sticking poss iceing up from cold
usually the pcv valve has come out of the intake manifold (still efi i assume). thats what i have found more times than not.
Yep same thing mine used to do and this was the fix, do not go with an auto zone or checkers replacement for this part go directly to ford or it will not fit well. Mine was just so dried out it was such a loose fit causing my problems. After I replaced mine with a factory piece ALL surging gone, esp the cold start surging.
My 88 use to do it. They called it "Erratic Idle" since I worked at a Ford dealer, they would always fix it. They changed the throttle position sensor, and clean the throttle body.
It runs great otherwise. I have taken the amazingly poorly designed airbox and hoses off to access the radiator hose once and again to adjust timing and repair a serious short.
If your sure there is no vacume leaks then i would check or replace the Idle air control (IAC).Very common Ford EFI problem.My 5.0 F150 use to surge and stall,replaced the IAC runs perfect. Also pull the air silencer out from behind the air box its in the wheelwell good for a few extra HP. IAC..
Vacuum leak is most common but are you still using EGR? A piece of carbon or trash in the valve can cause it as well as the IAC, or an air leak between TB and MAF. TP sensor is easy enough to check, it should have about .95-1.00 volts on the green wire. The PCV grommet mentioned above is often overlooked but don't forget about the screen underneath. If it clogs with carbon or sludge, oil will pool there on engine shut down and get sucked back up upon startup which can also cause an erratic idle. btw: are you running a MAF conversion or is it still MAP sensor? MAP hose could be loose or deteriorated or possibly a bad sensor. IF it eventually goes away, I doubt it's the sensor, but possible.
