My 1990 Gmc Vandura revs high in 4th and won't go in 5th and has loss of power.... BUT THERE'S A TWIST!!!
I drove the truck for about 4 months like this, drove slowly top speed was 90km/h, then one day it just kicked into 5th and drove perfect. Thought my problems were solved, until about 3 months into it, it went back to revving high in 4th and not going into 5th again. Any help would be appreciated!!
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@silkie ,Sean, I think your van probably has the TH700R4 transmission which has 4 gears, 4th being overdrive. As a first step check your transmission fluid level and check the color and smell. If fluid dirty and has a burnt smell, the transmission may have worn parts internally that need attention. Have the van checked for diagnostic/error codes as it is even possible for a plugged catalytic converter to cause shifting/power problems. Some of the auto parts suppliers offer free code reading like Pepboys, Nappa, Advanced auto parts,etc and some of the guys are very knowledgeable and may have seen the issue often and put you on the right track.
There are a lot of things that can cause up shifting problems low fluid,faulty govenor, TV cable faulty/out of adjustment(link below explains how to check), faulty/dirty valve body, burned 3-4 clutch plates, etc.,etc.. alot of which would take a trained eye to check. If not comfortable working on your vehicle take into a good transmission repair shop for diagnosis and estimate on repair.
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