Automotive Forums

hello,
I recently bought a GMC savana G1500 Van 1996 with a 5l V8
It came to the Netherlands in 2000 so its an a car sold by bleecker USA,
now my problem !
its driving on LPG(natural gas ) too
If i.m driving it on the higway ,not accelerating for a wile ,at a constant speed ,it gives a rather hard bang in the driveline,now and than
Like it is trying to downshift in half a second,or ignition fals awaye completely for half a second
readings from OBD2 gives us only a drop in rpm,and a p0420
also the tranny is sometimes shifting hard in gears
it mostly happens on lpgi refresht the oil and filter already

the p0420 is from i think a missing catalyst
now my 2 guestions:

I wondered is the 4L60E shifting down on emmision because the previus owner told me the cat was missing,and therefore the computer is thinking that there is another load at these moments and is shifting the automatic transmission??

Or,is it posible that the crankhaftposition sensor is getting faulty??because there is nor fault stored in the computer


i disconected the lockup and the problem is gone

please help, we love the Van but ,this is no good for the drive line
regards willem
from the netherlands
see alsolease
http://www.chevytalk.org/threads/showflat.php?Cat=0&Number=1361822&page=0&vc=1&PHPSESSID=#Post1361822