When I’m on the highway and accerate slowly it sounds and feels like the engine is missing or the transmission is slipping any time around 40 mph and above, most noteable between 60 and 70 mph. It’s especially noticeable when going up hill, even a small one. The tach does move at all when this happens If I hit the gas hard it doesn’t seem to happen and the tach goes up. Also one time the cruise control was on and when it was resuming the set speed, it didn’t seem to happen and the tach went up. Any ideas would be appeciated.

Correction to the orginal post. The tach DOESN’T move and stays where it is when the problem occurs

Model year?
Odometer mileage?

What can you tell us about the vehicle’s maintenance, particularly items like the fuel filter, spark plugs, plug wires, and air filter?

When was the transmission fluid last changed?

Please give us specifics, rather than something meaningless like, “it has been well-maintained”.

Also–Is the Check Engine Light illuminated?

Its 2003 windstar and the mileage is 69,000. Transmission fluid was changed at 60,000. The Check Engine Light is not on.

How about the other maintenance items that I mentioned?

If the tach doesn’t vary, I don’t think the transmission is the culprit. It’s not slipping. Why do people always seek the worst case scenario? Let someone who is knowledgable about cars take you for a drive, or take it to a reliable independent shop. DO NOT mention your (probably) erroneous suspicions. Let them draw their own conclusions.

The air and fuel filters were chnaged ever 15K and were last changed April, 2010. The spark plugs aren’t due for change until 100k and plug wires haven’ been changed.

"Why do people always seek the worst case scenario? "

You can thank the internet for some of this, I believe. People have a problem, search online, and hear horror stories, and many of them are actually very limited in scope. If you watch carefully, you’ll often see the same person posting the same problem in numerous places under the same or marginally different screennames. Then you get potentially hundreds or thousands of hits off the same complaint.

Then there’s the carryover effect - a vehicle used to have a certain problem, so therefore any symptom people believe might be a case of that problem occurring again.

Unfortunately, we then have unscrupulous mechanics who take advantage of these people. They see a Windstar come in with what the owner thinks is a transmission problem, they know they can tell them it needs a rebuild ($$$$ in the mechanic’s pocket) and the person will read online about other Windstar problems and think “yep, I had a defective transmission, too”… they’ll never be the wiser. And some of the chains are the WORST offenders here.

I’ve seen some stats, and they aren’t pretty… cars that, when serviced at dealers or independents, RARELY show up with transmission problems all of a sudden have very high rates of failure when you look at chains. Drive by a chain shop and see for yourself. I see early-mid 2000s model year Tauruses there all the time, but colleagues who work for Ford quality control have told me they see very low rates of failure overall for these transmissions… They did have a transmission range sensor problem briefly on some - but that’s a $200 problem that suddenly becomes $3000 at chain shops…

Someone should check out your fuel pressure - preferably under load - and at least have a look at the spark plugs & wires.

I would rule out a misfire problem before going on to any transmission issues. When you put a load on the motor (going up a hill) that is when a bad plug will misfire. A misfire would not show up on the tach.

If your motor has “coil on plugs”, or COP’s, a bad coil can cause a misfire also. The symptoms you describe are completely consistant with a bad coil or a bad spark plug(s).

The current plugs are 8 years old and certaily one could be going bad. With my Ford ('04 T’bird) I had a lot of misfiring before the check engine light finally comes on and it was bad COP’s.

I’d go for some new plugs first. If they aren’t hard to access, you can do this yourself. Install the exact brand and number plug put in at the factory. If the problem persists, then check into the COP’s. Ford uses COP’s on many of its motors and they often go bad.

Was the transmission fluid changed because of the problem or was it changed because it was due (a little past due IMO) but better than what most people do.

If your transmission was slipping, your tach would go up without the tranny downshifting. I also think you have a misfire under load and I would also start with the plugs.