My rpms are going way up at highway speeds and check engine light comes on. I’ve replaced the muffler, cat, had the fuel injection flushed and replaced fuel filter in the past two weeks but still having the issue. Today I unhooked the battery for 30min or so, and still no change

 

probably either the OD solenoid on/in the trans or the back half of the trans needs freshened up....if I had to guess.....
How many times do you hear/feel it shift?

 

And, of course, what codes are produced by the Check Engine Light?

(Use the key dance - take a deep breath, then turn it on/off/on/off/on within something like 5 seconds, and then count the flashes of the CEL. You'll have at a minimum 5 blinks, a short pause, 5 blinks, and no more; you may have a blink, short pause, two blinks, longer pause, then the 5/short/5 pattern - 12 is "Lost Battery Power Within Last 50 Drive Cycles" and 55 is "No More Codes".)

RwP

 

 

 

OK.

What does that have to do with what I typed?

This is a 1st Gen; the error codes are shown by the CEL (Check Engine Light) blinking after the key dance.

The one that you say comes on in the original post?

Do the key dance, and it'll start blinking at you. Note the pattern of blinks; I'll bet you'll get a 12 and 55 at least, and probably others too.

RwP

 

I'm having same issue it's more then likely your governor solenoid in the trans pan I'm dealing with same issue and thats ehat the code pulled is bad tcc sensor aka governor

 

on this series of trucks, most problems I see, don't produce codes...… not like the newer vehicles do. I get more "false codes" like the 37 comes to mind, I think that one is the part throttle unlock for the lockup converter.... another one is something for the AC.

 

91 doesn't have a governor solenoid.

 

nope. that didn't start til 96.. still a solenoid in there to control the lockup converter
on the speedo not working they had an old school speedometer cable til 93.