97 Exp 4.6L Rough Idle

Hey Guys,

I have a 97 Expy 4.6L I just bought and was running fine when I bought it but a day later I had a coil pack go out which I replaced. It ran good after that then about a day later, it started idling rough when in drive - smooth when in neutral. When you normally accelerate from a stop, the engine is rough till about 2000 rpm, then it smooths out. it's not a bad rough like a cylinder not firing but it is noticibly "bumpy". If you floor it, it's fine - power seems good and engine runs smoothly. It only seems rough when idling in gear and running up to about 2000-2500 rpm. In neutral, if you gun it, the engine revs exhibit the same roughness as well. Cel is on. Had codes read. 1 code points to the usual gas tank pressure problem which I doubt is the problem. Cleaned MAF sensor - no improvement. Vac leak maybe?? Any ideas???

Have you changed the plugs and wires? A bad plug or wire could've burnt out both the old and new coil..

try looking for vacuum leak its free trust me, i just chased my tail for 2 weeks on same thing. about 3 ads down under your ad "misfire in cyl. #4" tells you how to check it the guys name is jhandwor he helped me understand alot!! this guy knows his ****! great site with someone like that helping others. hope this helps.........

Thanks, I just did that as when I was driving in the rain on the highway, the engine started running really rough and the cel started flashing. When I read the codes again, I got the infamous misfire on #4. Out of curiousity, I checked the #4 wire at the coil pack with an ohm meter and found about 20K ohms to ground which meant a short somewhere. I replaced plugs and wires. I noticed rust had migrated up the plug inside the wire boot on #4 which explained the 20K ohms. Water must be getting in and getting trapped in the plug well causing the rust. The engine runs great now except for the idle which now motor boats up and down and runs rough. Engine is OK when accelerating and cruising but once you come to a stop, it almost stalls out then recovers and then motor boats. I checked for the usual vac leaks and found none. Funny thing is when I pull the vac line off the EGR valve, it fixes the idle and the engine still runs perfect. I thought IAC valve was toast but when I disconnect the IAC connector, the engine dies either with or without the EGR vac line on which tells me the IAC valve is working. I am running with the EGR vac line disconnected but this obviously is not a proper fix. Anyone with any ideas??

hook a vac gauge to the line going to the EGR valve you should have no vac.