As much as I hate to start another thread on this subject, I am stumped and I just can't afford to pay any more mechanics to guess (wrongly!) about what is causing this issue.
The problem first manifested a year ago. While high-speed freeway driving I would occasionally feel a momentary misfire. On secondary roads (40-60 mph) the car would randomly surge and lose power, then pickup and run smoothly for a while. Over the last year the problem has got much worse--to the point where it will suddenly start surging and stumbling in in-town traffic and will sometimes just quit at red lights. My no. 1 mechanic (30-yr relationship) replaced coil pack, plugs and wires, IAC
, had the intakes off and checked gaskets, did a smoke test and over time did a bunch of other stuff that made no difference. The car runs perfectly for him. It stumbled, surged and quit on the way home. I let my no. 2 mechanic take it home several times (he has a 45 minute mixed city/highway commute). He experienced no problems. Lately the problem can even appear when starting the car in the morning--it will 1) start and run perfectly, 2) start and rev up to about 1500 rpm
and immediately quit and not fire again until I wait a few minutes. 3) Start and run on what sounds like 3 or 4 cylinders and limp along for a minute until it finally quits. With option 3 there is black smoke out the tailpipe and a strong smell of gas so I don't think fuel delivery is the issue. I broke down and paid my Ford dealer for a mechanic to ride with me for 2 hours with his laptop all plugged in. There was ONE stumble that registered on his computer with no indication of why. It almost died on me on the way home. I have changed the computer (used), the throttle body (used), the cam position sensor (the one in the end of the head, not the old-style distributor shaft kind) fuel pump, coil pack (again!). Probably some other stuff too but I can't remember right now.
Now, when the car decides to run it will run perfectly and smoothly and it goes like hell. It will sometimes run for days without any issue. I noticed that any time I disconnect the battery and discharge the computer it seems to run well for about a week before it starts acting up again. (Just long enough to make you think you fixed it!) Then the problem manifests slowly and, over the course of a few days builds up to undriveability again. Like the computer is taking a week to learn bad behavior again. That's the only way I can describe it.
Note: During this time there has NEVER been a check engine light or any codes displayed. NEVER!
I'm stumped. And broke. Any advice would be welcome, short of being told to go out on the freeway and stop in front of a Peterbilt. (which I have been tempted to do. LOL)