97 Astro 2WD - rumbles on hard acceleration from a stop

I have no issues with the transmission of my 1997 Astro 2WD 4.3L other than after two hours of steady driving it shifts very roughly from first to second gear. The rough shift disappears instantly once I shift from DRIVE to PARK at a stopped position.

The van rumbles under hard acceleration from a standing start, no rumbling at all under normal acceleration, and no vibrations at all at any rolling speeds.

Is it reasonable to suspect a faulty U-joint ? (as I am hoping to avoid expensive tranny shop investigations)

More than likely your problem is not a u joint. Faulty and or worn universal joints will cause a driveline vibration that can be felt/heard throughout the vehicle and is most noticeable at approximately 35 MPH and then again at approximately 70 MPH. Usually not as noticeable in-between, above or below. Quite possibly a "clank" sound will be heard/felt when put into drive and shifting back and forth between drive and reverse while the engine is idling. Vibration becomes more severe as wear progresses, and may also be noticed at speeds BETWEEN 35 MPH and 70 MPH. It is rare to feel or hear the vibration below 35 MPH, unless the joint is near catastrophic failure. An early warning sign that a universal joint needs attention is a tell-tale "squeak", rolling under 5 MPH, that comes and goes with the revolution of the propeller shaft.

The rumble you are experiencing may be due to the exhaust system touching the body of the vehicle when engine torque raises the engine. Possible cause could be a worn engine support (motor mount) or the exhaust system needs adjustment.

As far as the harsh 1-2 shift, a transmission fluid and filter change may improve shifting. This should be done every 36,000 miles for best performance.

thanks for the help!
I checked under the van carefully and found some scrape damage on the exhaust pipe. I pryed the pipe carefully toward the scrape and have had no further rumblings.

I will change the transmission oil and filter at the next oil change. I suspect you are correct, since the rough shift acts only after steady use in Drive for at least one hour. As soon as I stop and switch to Park or Reverse, the hard shift disappears.

JFW

More than likely your problem is not a u joint. Faulty and or worn universal joints will cause a driveline vibration that can be felt/heard throughout the vehicle and is most noticeable at approximately 35 MPH and then again at approximately 70 MPH. Usually not as noticeable in-between, above or below. Quite possibly a "clank" sound will be heard/felt when put into drive and shifting back and forth between drive and reverse while the engine is idling. Vibration becomes more severe as wear progresses, and may also be noticed at speeds BETWEEN 35 MPH and 70 MPH. It is rare to feel or hear the vibration below 35 MPH, unless the joint is near catastrophic failure. An early warning sign that a universal joint needs attention is a tell-tale "squeak", rolling under 5 MPH, that comes and goes with the revolution of the propeller shaft.

The rumble you are experiencing may be due to the exhaust system touching the body of the vehicle when engine torque raises the engine. Possible cause could be a worn engine support (motor mount) or the exhaust system needs adjustment.

As far as the harsh 1-2 shift, a transmission fluid and filter change may improve shifting. This should be done every 36,000 miles for best performance.

I have a '98 2WD and had experienced the same HARD shifting from 1st to 2nd after extended running, mostly after extended highway use. This happened so infrequent that I kept saying I will have to get this checked out. After a year of this the transmition finally burned up and the rebuild cost us $1300. It sounds like it's not uncommon on these vans to have this issue. It's worth having a trustworthy transmition shop check this out. I was told in my case if caught early enough $300 would have avoided the rebuild. I hope this helps.

I checked into the transmission issue and you were correct. Apparently at about 100,000km there is enough normal wear in the transmission shift bushings that the computer thinks the transmission is failing, switches to code 1870 (or1780?) and cranks up the fluid pressure to operate in "get home" mode. Thus the hard shifting from 1st to 2nd gear. I can stop the van turn off the ignition for a few minutes and reset the computer when I notice hard shifting. However, running the transmission like this will eventually destroy it. I am installing an anti-code 1870 kit to stop this from happening.