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Sticky, sticky subject. Bottom line: while cruising around 40 - 45, if I step on the accelerator the car seems to lose power - falters. I have to step on the accelerator to shift down in order to gain power. And, I've noticed the tach occasionally may increase 500 rpms momentarily, and go back down while I'm cruising. ONLY occasionally, not all the time. Doesn't seem to happen when car is cold - only when warmed up. Typical climate - 40 to 75 degrees.

Longer story: I've replaced LOTS of stuff to try to run this down.

1999 Mercury Sable Wagon
3.0L 24 valve Duratec engine (232k total miles)
AX4N trans (rebuild 12k miles ago)
 

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What you are describing is characteristic of the transmission although the degree of issue may be exaggerated in yours given it's age / rebuilt status. Also note that at around 40mph, the throttle cable will resonate and you will feel a vibration in the pedal. There was a tsb to fix the issue.

I would consider doing either a full fluid exchange with licensed Mercon V or three or so drain and fills.

What parts have you thrown at the problem so far?
 

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What you are describing is characteristic of the transmission although the degree of issue may be exaggerated in yours given it's age / rebuilt status. Also note that at around 40mph, the throttle cable will resonate and you will feel a vibration in the pedal. There was a tsb to fix the issue.

I would consider doing either a full fluid exchange with licensed Mercon V or three or so drain and fills.

What parts have you thrown at the problem so far?
I exchange fluid with the jug shown in the pic. Takes about 20 minutes total suck out and refill. I remove the vent cap for this as it makes refill much easier. Vent cap needs cleaning and put back on carefully. Just on the first bulge, and wobbly loose. For vacuum, I remove the inlet filter from my small air compressor and use that for vacuum. I have sucked on it with my mouth and it can be done.
-chart-
 

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Discussion Starter #4 (Edited)
What you are describing is characteristic of the transmission although the degree of issue may be exaggerated in yours given it's age / rebuilt status. Also note that at around 40mph, the throttle cable will resonate and you will feel a vibration in the pedal. There was a tsb to fix the issue.

I would consider doing either a full fluid exchange with licensed Mercon V or three or so drain and fills.

What parts have you thrown at the problem so far?
RJacket - I'm sorry, I don't follow your comment about the vibrating cable at 40 mph.

Further detail : As I'm cruising, (doesn't happen while accelerating) at most speeds actually I. E., neighborhood speeds, 25, 30, 35, and even at 50, and 60 - so when I'm not accelerating up to speed, I encounter this faltering, like too much air in the fuel mix, a drop off in power, but no backfire or miss that I can tell. And, no hollow vacuum sounds, as it might make if it were truly starved of fuel. It's a little hard to explain.

So, the following is what I've done to determine if I'm dealing with a transmission issue, or something masquerading as a Trans problem :

Shop Fluid change with mercon V about 5k ago. Followed up with 3/4 of bottle Lucas transmission slip fix.
Changed out plug wires/plugs.
Changed coil pack (it has been missing upon startup and while cold). Still missing, but may be due to recent SES P0174/0171
Replaced VSS
Fuel filter
Air filter
Cleaned MAF with electronic contact cleaner and let air dry.

Fuel pressure is within spec.

Coil pack and VSS were suggestions given as fixes for missing /low power, as well as symptoms that mimic slipping Trans.
 

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Replaced VSS
I hate to be sound like one of those guys but when you replaced the vehicle speed sensor, did you do so with a Ford OEM sensor? I replaced one in mine, as the car was unsure of which gear to use, at parking lot speeds, with transmission hot. After replacement, it was the same thing with transmission cold. Swapped it out for another brand and it gave me the same issue. Swapped it out for an OEM, and haven’t had a problem since.

If it is happening at slower speeds, which you said it was at 25, 30, etc. bring it down to first gear. Is it happening where it’s not changing gears or is it only happening when the car needs to decide what gear to be in? That data should determine engine/fuel system versus transmission. If it doesn’t give you a problem at that point, double check the VSS. When I switched from the first replacement to the second replacement I also made sure to swap out the harness. The old one had seen it’s days.


Sent from some little thing that I hear voices from. Oh, and his name is Blüe Longroof
 

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How long have you owned the car and when did you notice this issue?

I would look at cleaning the throttle body and checking for slack in the throttle cable. Run a bottle of techron concentrate or gumout regane in the fuel to clean the fuel system and injectors. I would even go to Walmart and get the $20 fuel injector service which is more about cleaning up the combustion chamber. All this ensures that your fuel and air is operating properly. You could also give the mass air flow sensor a clean (with the correct spray).

The 500rpm rise you see when warm and not cold is the torque converter unlocking. The AX4N is programmed to lock up as soon as you're no longer accelerating for fuel economy reasons, even when you're going slow. The Duratec also has less torque below 2500rpm so once the torque converter locks, even at a speed of 60mph, it can feel unwilling to accelerate.

It's even worse at 25mph but the torque converter will unlock if it needs to and gears will shift down also if you press hard enough. Having slack in the throttle cable, a dirty throttle body, dirty injectors will mean you'll need to depress the throttle pedal even more when cruising to get a response.

One easy way you can get a more responsive experience, especially below 45mph, is to select the other "D" ie Drive instead of Overdrive.

The other thing is why is there a Lucas additive in a transmission that was rebuilt 12k miles ago? I would want to fully exchange the fluid so that it is virtually 100% new Mercon V. I would also want to (first) drop the transmission pan, inspect and clean it and change the filter. There could be all sorts of contaminants in the system as a result of the rebuild.

Lastly, disconnect the battery which will give the PCM a chance to relearn everything.
 

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Codes 171/174 = vacuum leak 99.8% of the time. Check LTFTs on both banks at hot idle and with engine under load with a good scan tool. High positive trims at idle that drop toward zero with engine under load = vacuum leak. Since you have both 171 and 174 codes, vac leak effects both banks. Look for split vac lines, split / rotted PCV hose, etc.
 
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