My '88 Grand Caravan did the same thing. After many years of never hitting wide-open-throttle, including many trips pulling an enclosed trailer that's bigger than the van, the problem started right after I DID hit wide-open-throttle. No downshift, (you don't have to put it in park, just put it down to first gear), delayed upshift, etc. Add to that, I only changed the fluid once in 213,000 miles.
Most likely there is a buildup of varnish in the valve bores. In my case, the problem gradually went away over a couple of months. Logic says it's the throttle valve that's sticking because that's the one affected by wide-open-throttle, but the governor valve is partly responsible too. If it sticks just a little, the transmission will think the car is still moving so it won't downshift.
If it doesn't improve in a week or two, try a cleaning additive. I've heard of Sea Foam, but never tried it. Just be sure it's something you can leave in. There are a lot of cleaners that will dissolve varnish but must then be drained out with the old fluid. You want a cleaner that won't affect the fluid's ability to lubricate bearings and seals.
Monday, September 7th, 2009 AT 3:35 AM