97 Expy 2x4, 5.4, towing package, rear load leveling, 450,000 miles
Well, this is a new one on me....while traveling at highway speeds around 70+ and the tach around 1900-2100, the tach will jump to 2200 and back to the previous rpm. The speed doesn't change, you can here the rev in the motor and it does it speradically.
At first I thought it may be the lockout on the tranny, but my tranny guy scanned it and found only an open relay on my fuel pump relay....I replaced it. I had recently changed my fuel filter and had unplugged it and the fuse. Then I thought the A/C may be locking up, so I shut it down, same results. I tried shutting down the O/D while driving and that kicked up 1000 rpm, so that eliminated that.
The odd thing is it only does it at 1900 up to 2200 rpm. Example, if I'm running 2000, it goes to 2200 and back down. At 1900 it jumps to 2200, ...at 2000 it may jump to 2200 or go to 2100 back down then back up to 2200 then back to 2000. If I'm running 2200 it doesn't do it, nor have I noticed it below 1900. After it returns to the previous rpm, it may do it almost immediately or maybe not until I get back on the highway again...extremely unpredictable. It will do this with or without the cruise set.
C'mon gang, I just rolled 450,000 and I'm trying to get her to a 1/2 mill.
Could the IAC have something to do with this?

 

I had a vehicle once that acted funny like that and it was the throttle position sensor. If you search for info on that it may fall under the emissions section of an autoparts website.

 

Well, a TPS isn't to bad on the wallet and if its not outputting the right voltage it would do that...hmmm...may have to meter it out to check for fluxuation. thanks aagitch.

 

update...it was the TPS aagitch, thanks... I've been out of town and finally got it replaced and tested.