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1999 Taurus DOHC with 246,000 miles, only 185,000 miles on transmission. The car is well maintained. When A/C failed to engage one warm day last spring I parked the car and drove another for several months, only periodically driving it every few months. I began to notice rough idle. Been through that before. Vacuum lines. Time to perform some in-depth maintenance again.

Last July I personally replaced coil pack, wires, plugs (again), air filter, fuel filter, cleaned MAF sensor, checked EGR for proper function, cleaned EGR ports in throttle body, cleaned throttle body, replaced IAC with OEM unit, and replaced every vacuum hose easily reachable from the top of the engine. Sprayed carb cleaner around intake plenum gaskets, but no vacuum leak detected. Still the engine loads up against the brakes while in gear at full stop, and begins bucking and surging.

Engine runs fine once the car is rolling. You could say it pulls like a raging bull, but it was running good before all the work at a few hundred RPM above idle. Engine idles perfectly, out of gear. Correct RPM, smooth and steady. Drop it in gear, and that is when the trouble manifests. Some times I can release the brakes and the engine will begin gradually accelerating the car until I'm steadily cruising down a neighborhood street with my foot off the accelerator, engine at 1200-1300 RPM at just under 30 MPH, but not always. Inconsistency doesn't help.

When engine and transmission are cold, aka recently started, drop trans into reverse and much of the time it may back out smoothly, no engine RPM changes, controlled, but not every time. Within a couple minutes of driving I am forced to slip the trans into neutral at every stop sign and stop light. Drop trans into 'Drive' when time to go and it feels like I'm driving an automatically shifting manual transmission, though minus the clutch, if you get my meaning. Engine RPM climbs as the engine builds torque against the brakes, then begins to buck.

Once the car is rolling/accelerating above 1200 RPM or so, it's zoom zoom, smooth and strong acceleration. Approaching a stop light or stop sign with traffic, I can hear and feel the trans downshifting, although a bit later than I would think it ought to given the engine loads up against the brakes, fighting the brakes as it were. Engine tends to buck and surge against the brakes at a complete stop, either immediately, or within a couple seconds. Lots of surging will eventually trip an unrelated SES light. Don't recall the codes I've had read 3 separate times, but the codes are symptomatic of something else, not the problem itself.

I've gotten very good at slipping the trans into neutral as I roll to a stop light. It's necessary to put it in neutral, or you may kill the engine, or get pushed into the car in front of you if you're not mashing the brakes pretty hard. Slip into neutral, and the engine always recovers its composure and idles smooth as silk.

Can't help but wonder if it isn't related to the car sitting up for an extended time. A former Ford trained technician who started his own shop suggested a trans/torque converter rebuild. Said it was common. I can't find a word online on the subject. Must be searching with the wrong words/terms.

Ideas folks? I've been driving it for several hundred miles in the last 3 weeks, and very little improvement. The only real improvement came from following a suggestion the service tech made, which was to 'try' some Lucas trans fix. When I got the product in the trans, there was a subtle improvement, aka the car would backup in a civilized fashion on a cold engine for a change.

So again, I hope these fine forum members have some ideas.

Thanks in advance.
 

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I'd normally suspect the IAC valve or the torque converter. Being that the car idles fine out of gear, it isn't likely the IAC. Does it raise the RPM on a cold start and settle down to normal as it heats up? If so, the IAC is functioning properly. This really sounds like a torque converter issue. The fact that the Lucas Trans Fix had an effect also points to this being torque converter and/or tranny related.

I'd be curious to know what those codes were though.
 

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I'd normally suspect the IAC valve or the torque converter. Being that the car idles fine out of gear, it isn't likely the IAC. Does it raise the RPM on a cold start and settle down to normal as it heats up? If so, the IAC is functioning properly. This really sounds like a torque converter issue. The fact that the Lucas Trans Fix had an effect also points to this being torque converter and/or tranny related.

I'd be curious to know what those codes were though.
Thanks for the reply madscientist!

When I start the car it idles high for a few minutes, gradually coming down automatically. Usually I don't have the time to wait, unless I'm scaping snow or frost recently, so I rev the engine a bit over the established high idle to 'let the engine know I'm here' and it will idle down quite a bit more quickly. Don't really want to drop it into gear while idled up, so I have to be patient. As the engine warms the RPM will eventually drop to around 800 RPM.

I will make a point to have someone read the current code, perhaps Wednesday, and post it for feedback. Killed the engine the other day by waiting too many seconds to pull it out of gear at a stop sign, and it finally popped a code as a result. Usually I can avoid the SES coming on if I get it out of gear quick enough.

Yes, the fact that the trans fix did make a subtle change is concerning. I still hope for a reasonable correction to the problem. I've hoped that driving it again would result in it 'loosening back up.'
 
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