Any GM techs here? Engine surging!

[Charger4404spd]

 I am having a problem with my 90 Silverado. The problem is the rpm is erratic at a certain accelerator position. Driving down the road at say 50 mph, the gas pedal is approx. a 1/4 way down. The engines rpm's goes up just a little and goes back down. It is a continuous up and down. Just little surges. It quits surging if I let up a little on the gas pedal or if I give it more gas. I first suspected the TPS switch. After replacing, problem is still there.

I then thought the tv cable was the problem, so I adjusted it per my manual. After this, the trans shifts a little harder and shifts into overdrive at approx. 48 mph instead of 42 mph as it has always done. I adjusted the cable again and had the same results.

One other thing, not sure if this matters or not, but on occasions my speedo will quit. Simple fix is to slap the top of the dash and it starts working again. I also checked for fault codes and retrieved the code for the Speed Sensor. I am assuming it set this code because of the speedo quits.

I dont care much about the speedo quiting, but the rpm surging is driving me crazy. Any thoughts?

90 Silverado 4x4, 350, 700r4

[rt green]

could be speed senser or coverter lock up switch. just a guess. i havent worked in a gm dealership since '95.    good luck. if i think of something else, i'll let you know.   bruce 

[Charger4404spd]

convertor lockup switch?

[BigBlackDodge]

Speed sensor went on our intrepid a few years ago, speedo needle wouldn't move and it would not let it shift into 3rd gear or higher.  Sensor replaced, problems solved.

BBD

[The Mad Scientist]

not sure if it's got the same emmissions systems as my friends astro van, but his 95 4.3L had very similar symptoms and it turned out to be a sticky EGR valve. 

[Charger4404spd]

Sticky EGR? Now that I havent replaced. I will check it out. Thanks y'all!

[andy74]

i would have to agree, the egr is probably the culprit-also, try unplugging the convertor wire,that will remove the electronic lock up function,and although youll get worse fuel economy,youll know if thats the problem-the speedo on that truck should still have a mechanical cable,they changed around then,ill check my books when i get home,if you need me to-Andy

[Charger4404spd]

Thanks Andy, no there isnt a cable, it has the crappy circuit board dash.