Chevy_4_Life

Hello,
I'm new to the forums and I need help. I own a 1998 Chevy Silverado 1500 ext cab 4x4 with the 5.7 L vortec. It has 204,145 miles on it and has been taken really good care of. I bought the truck with 198,000 on it. Since then the only thing I've done to it is normal maintence, a new battery and dual 12-inch glass packs shortened under the bed with turn-down pipes. Just recently, I discovered a very strange RPM change while driving. Say if I'm driving at 55 mph, the rpms are around 2,000 give or take and then suddenly is drops to about 1,600 rpms then shoots back up to 2000 ( it does this multiple times in 3rd or 4th gear) and that's not me taking my foot off of the gas pedal. I first thought it was the transmission but then again it may be a sensor or something.

So when it drops to 1600 rpm, I don't really lose to much speed but then a second later it shoots back up to 2000 rpm (again that's not me doing it). I tried doing some research but couldn't find anything. If it needs to be taken into the dealer, then I will do so. The air filter really needs to be changed (It's an older K&N so I may change that this week). The spark plugs were changed at 186,000 so I don't think that has anything to do with it. So what do you guys think? Does the transmission need an oil change? Is a sensor faulty or something?
I really need some advice here, thanks in advance.

P.S.- I am not hard on my truck, I drive it like normal and don't "floor" it or anything. It's never been offroading or mudding. I take care of my truck and I will do anything in order to do so.

Allan In NE

Have you tried a new fuel filter? A plugged filter will drive the transmission crazy.

Allan

Chevy_4_Life

No but I did change the air filter last night and clean part of the air filter sensor (not the MAS). I also unplugged the sensor wires, clean the dust out of the connectors and plugged them back in. The K&N filter on it before was from 2002...and it had been cleaned multiple times. So I replaced it with a new K&N filter and doing that actually helped it a bit. The truck doesn't seem as sluggish as before, and it accelerates a bit better. I haven't notice the rpm problem since then, a little time will tell if I fixed it or not.

Thanks for the reminder though, I will change the fuel filter as soon as possible.

On second thought, I will probably do a transmission flush very soon and new oil in the rear end. Hopefully when I do a tranny flush, the transmission won't leak due to older seals. I plan on putting a programmer on this too.

by next year, I will do a complete engine rebuild. Once the engine is rebuilt, I can throw in a hotcam crankshaft to add some horses. I will need the $$$ to do that though.