Dodge caravan 98 suddenly running slow on freeway ...

Could be a fuel pump which is beginning to go. Thus the fuel is not regulated properly and affecting the usual acceleration. Lack of maintenance such as spark plugs, plugged filters, carbon build up..

This could be a result of the fuel pump as the answer before me suggested but it could also be a more serious problem. You should have your van hooked up to a computer to get the codes. This could be a problem with the compression, a blown head gasket, transmission problems. There are many reasons as to why your vehicle is unable to go above 40mph and it may be a very serious issue. I would recommend not driving the vehicle until you can have it properly checked out. Most auto parts stores have a computer that they can freely hook up to your vehicle to find out what is wrong with it. It is more than likely the compression from the engine.

forestry trucks

Check your fuel filter, catalytic converter to start with. There's a multitude of things it could be. Need more info.

If you are still checking this link, first check your catalytic converter. W/the check engine light on and the symptoms you describe, this is your best bet. A local repair shop should pull your codes for free, and if it is a catalyst code (usually P0420/P0430), or an O2 sensor code, have them do a backpressure test on the converter. Only if it passes this test should you then look other places for the cause.

Remember, though, if the catalytic converter turns out to be the problem, that the converter did not self destruct. Something in the way the engine was running killed the converter--bad plug, shorted plug wire, timing off, stuck fuel injector, bad oxygen sensor, etc. Have a diagnostic performed, or the situation will re-occur.