1998 Ford F-150 Rough Idle

SEBASKI

MEMBER
1998 FORD F-150
V8
2WD
AUTOMATIC
14,800 MILES

After a fill up on a trip this summer my F150 (5.4) suddenly began to intermittenly idle rough and the check engine light came on. The idle is rough like a miss, the RPM's drop (500) then suddenly will come back up, sometimes the idle is fast (11-1200). Under load the engine runs fine, with the MPG unaffected. The codes indicated a MAF sensor or O2, my mechanic checked both O2's and they're OK; the MAF looked marginal was replaced with no effect. He is stymied, says he thinks its an intermittent vacuum leak but can't locate one; also the computer doesn't register a misfire when it runs rough. I've had 3 coils replaced in the past, prior to this, losing a coil made it run rough constantly.

1 Reply

JAMES W.

MECHANIC
2,395 POSTS

Sounds like you have covered most of the bases. Usually a spark problem will show up more under load and you have pretty much checked all that. The only thing that comes to mind is an injector with a weak coil that is hanging up during the short pulse of idle. The problem is, if I'm right, is which one and how do you isolate it. You could try, when the missing condition exists, pulling injector connectors one at a time to see if you can narrow it down. Keep me posted.