94 Mercury Grand Marquis transmission slips, engine flares

My wife's grandparents gave us their 94 Grand Marquis as a gift but the transmission slips and the engine flares. There doesn't seem to be any rhyme or reason to when it happens, it just does. We have
taken it to 3 mechanics who have run various diagnostics tests. The first one found that the fuel trim was running at 11 so he replaced the fuel pump. The second and third mechanics both agreed that a new transmission was needed, so we had a rebuilt transmission and torque converter installed.
The problem is still there. Basically what happens is that we'll be driving and feel the car slip into neutral for 5-10 seconds before re-engaging in a gear. The car also sounds exactly like you would if you revved the engine while in neutral during this time. Most of the time we stop pushing the accelerator. Sometimes we experience this when it seems to be changing gears, but many times it will happen when we are simply in cruise control, or maintaining a speed. Replacing the fuel pump and transmission has made HUGE differences in improving the problem, it is nowhere near as violent or scary now. However, it is still slipping regardless and we either want to fix it or sell the car while it is at maximum value.
Apparently her grandparents have had this problem ever since they bought the vehicle in 1998. They also had the transmission rebuilt in 2000, we just found out, and nobody they took the car to in the last 6 years has ever solved the problem. Does anybody have any idea what the problem might be? Any idea what to check? I don't know much about cars but am willing to learn. I also own a Hayne's book, so I could theoretically look for some of this stuff on my own if I knew what direction to go in. Thanks in advance!
- sterling
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

...snipped... Just a quick basic question - have you taken it to a Ford / Mercury dealer ???? I'm serious in asking because that's the first place I'd start with. Linda
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
we have not taken it to a dealership ourselves, but I've been told that her grandparents have taken it there many times before...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
If you ever find out what's causing this problem let me know. I lived with this problem for years on a 93 Grand Marquis. I took a stab at trying to find out what caused it monitoring various inputs such as solinoid signals and Vss etc. Never did get to the bottom of it. Best of luck.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 17 Aug 2004 11:52:49 -0700, snipped-for-privacy@gmail.com (sterlingsmith) wrote:

The nuetral safety switch or MLPS (manual lever position sensor) or whatever it was called that year. I had the problem with my '94 CV and a couple of others. It will not usually give a code until it is about completely failed. It is mounted on the left side of the trans at the selector shaft or lever. It tells the computer what "gear" position you have selected. The original models were prone to getting moisture in around the wire harness connector causing corrosion and an inconsistant signal to the ECM. They are readily available at most parts outlets and include an updated harness connector to be installed which will prevent moisture ingression. The part is not difficult to install if you have an 8mm wrench and a couple of other basic tools along with a way to safely lift the vehicle to work under it. The switch must be timed or adjusted properly which is very easy with the instructions included with the switch. It should cost you $25-30US - more at a dealer.
If the trans has not been serviced in the last 30K miles or so, it should be completely serviced including a new filter and draining the torque converter which is easily done through a plug in the converter accessible through the rubber plug in the bottom of the flywheel housing. If the converter is not drained, more than half of the old fluid will remain in the trans. When you get the trans serviced, add a product like "Shudderfix" or Lubegard red as recommended. Do not be tempted to use Mercon V or other synthetic fluid in your '94 trans - a good quality namebrand Dexron III which meets the Mercon spec is the best choice. It should take about 13 qts. of fluid with a torque converter drain. The additive will help prevent torque converter shudder which is aggravating as hell. These services/repairs are probably best done by someone with good to advanced DIY ability or left to a professional. The switch should correct your immediate problem.
Good luck
Lugnut
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The transmission range sensor is most likely bad or out of adjustment. It will not cause a code. The fuel pump replacement did nothing for a transmission problem, it was nothing more than a placebo.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.