RPM surge at exactly 19mph, 2000 SE

I've had this issue for several years now and thought for a while it was just a Ford "feature" but don't see its benefit. The symptom is 100% repeatable under every condition imaginable - hot/cold engine - in gear or coasting.

The easiest way to see this and describe it is as follows. We live on a hill with a stop sign at the bottom about 1.5 blocks from our driveway. I pull out into the road and usually just select neutral after backing out of the driveway and start to coast down the hill. RPM's are steady and read anywhere from 700 to 1000 at this point depending on cold or hot engine. As I watch the speedometer, at a point just shy of 20mph (I guess 19 mph) the rpm's will quickly surge to 2000 rpm and then fall back down to the normal.

What's most annoying about this (and potentially dangerous) is when I am starting out in traffic while in Drive. If I am on a flat to slightly downhill surface and just get her moving - I can let off the gas and the auto tranny will maintain speed to slightly increase speed. Again just as in neutral, at 19 mph I can see and feel the rpm surge and get a quick little acceleration burst. I often have to tap the brake to stay off the car in front of me's bumper.

Anyone know what would be causing this? It has to be getting a signal from the PCM to bump the rpm's but for what reason?

A little background on the van. I am the original owner, 2000 model SE 3.8L with 50K on her. Recent Windstar hobby repairs include: isolator bolt mod last fall to stop the lean bank vacuum leak P0171/0174, recently put in a new battery and alternator try and eliminate a random dead / weak battery condition, put in new fuel filter, new DPFE sensor, engine coolant temp sensor, blend door actuator motor, flushed and filled the pwr steering fluid, replaced a slightly leaky brake pressure safety switch, I use top tier regular gasoline and run fuel cleaner in every third tank or so.

Hmmmm... not a single reply after 5 days. That tells me:

1) I've done a poor job explaining the issue.
2) I've stumped the panel with this one.
or
3) It's not a significant issue that warrants a reply.

Questions are welcomed, please - if I've left out anything just ask. I usually check this site daily so I'm not one to post and then go away for good. Thanks for any help, comments, or questions...

have you considered testing the TPS (Trottle Position Sensor) to see if it is giving out false readings at a certain position? that is the only thing I can think of. These have been known to have dead spots i.e. the rpms drop to idle at certain speeds. I don't see why it would not have the opposite problem of giving out higher voltage at a certain spot.

One test you can do is to disconnect it (it will trigger the CEL and will disable TractionControl) for a test run and go up past the 20mph mark and see if it results in the same surge. if it does, then we know it is not the TPS that is the problem.

Basically as you turn the TPS, it should be gradually increasing in voltage.

Post back and let us know how this experiment went.

Thanks Mundy - I'll try disconnecting the TPS and see what that does and report back. My wife is out with the van all day today so will post tomorrow.

Okay - I removed the wire connector going to the TPS and started the engine. The engine was fully warm and was idling around 700 rpm. I backed out of the driveway and onto the road. Slipped selector into N to start the coast down the hill. Without ever touching the accelerator pedal, at 19 mph the rpm's surged to 1600 and fell back to 700. Symptom is still present.

I drove the van for 2 miles like this and the CEL never came on. I did notice the tranny shift much harder on both upshifts and downshifts. In fact I pulled into a parking lot and shut off the engine to reconnect the TPS for the drive back home because of the hard shift.

according to the experiment that eliminates the TP sensor as the culprit. maybe others here have something to add. i need to think about this a bit.

I found 3 things that could cause this on other vehicles.. Idle air control valve,Mass sensor and O2 sensors.. I looked for some same type issues and the closes were those.. May just want to clean your IAC and Mass sensor first.. Be careful when cleaning the Mass sensor.. Some use electrical cleaner.. I have used the Mass cleaner from gunk or CRC. I think who makes it, with no issues.. I think I recall some having some issues but if you let it dry I can't see there would be a problem.. But each one of these issues did not produce a CEL on various cars. One of them was a Ford but not a Windstar.. Others ended up being a fuel pump problem but was different in some ways as of not running well and surging and stalling.. It did not fit your problem.. But the 3 I mentioned above did. Other issue referred back to a bad PCM.. But lets not think that..

Thanks for the replies... Last fall when I did the TSB for the isolator bolt kit, I bought some CRC MAF sensor cleaner and thoroughly sprayed the MAF and AIC while I had all those parts in easy access. I observed no difference in this speed / rpm surge problem after those were cleaned. Cleaning the IAC did make a difference as the idle speed would never drop below ~950 rpm even when engine was warm. After cleaning, rpm's drop to around 700 rpm.

The van otherwise runs well, starts fine and idles / accelerates very smoothly. The only complaint would be gas mileage as it has slowly deteriorated over the last 4 years but I attribute that mainly to my wifes driving habits and her mostly shorter trips. I keep every fill up logged at an online website to monitor cost / mpg's for your cars. Below is the last 4 yrs:

2005 17.78mpg
2006 16.72mpg
2007 14.41mpg
2008 13.58mpg
2009 14.29mpg (ytd)