Erratic RPM's when stepping on gas

I have a 1998 Chevy S-10 Blazer LS with 140,000 miles on it. About a week ago the electric fuel pump in the gas tank died on me. I had the vehicle towed on a flatbed to a mechanic I know and trust to replace the pump, as well as the fuel filter. When they started it up, the told me the transmission was slipping. When I start it in the morning, it drops into reverse and drive just fine. When I step on the gas in drive, the RPM's start jumping up and down between 1100 and about 2200. As it does this, it seems to be "bucking" a bit. Almost feels like Im running something over. As it starts to accelerate, the RPMs and bucking seem to settle down. The car did this a little bit last summer (not nearly as bad) and didnt do it all the time, and eventually it stopped until now. I have no engine lights on, or any other issues. What could be causing this? How do I fix it?

Even though you do not have a "Check Engine" light there may be codes in memory. Have the vehicle scanned for both engine and transmission codes.

Your mechanic should have a scanner capable of this.

Let us know.

Ok, so I finally got the truck in to get a diagnostic done. Just before I brought it in, the Service Engine Soon light came on. They said the only code in memory pertained to an Evap Module, or something like that. It was a relatively cheap part that they replaced, but it has had no effect on the initial problem. It continues to have the RPMs jump all around between 1100 and 1900 RPMs when initially accelerating and maintaining constant gas pedal pressure. It feels like it's lurching back and forth. Can't tell if its the engine, transmission, or maybe even the torque converter.

Have you checked fuel pressure and leakdown? Takes about 10 minutes and will tell you immediately if there are any problems. There are several parts in the fuel delivery system that can cause problems. Just because the pump is new, doesn't necessarily mean it's good. Fuel delivery problems will not turn on the SES light or set a DTC. The engine must be running correctly before any issues with the transmission are addressed.

Ok, I finally brought the truck to the dealer. They fixed an exhaust leak, diagnosed another code in the memory stating that a cylinder was misfiring. They said that at that point though, the cylinder was not misfiring, so they cleared the code and it hasn't come on since. They also checked the entire fuel system for leaks, and for correct pressure. They said the fuel system seemed fine and was at the correct pressure. This past weekend though, it didnt want to start. It cranked for a while, all the while it seemed to be jumping up and down a bit (Literally). It finally turned over and then was doing the erratic RPM's and lurching while driving again. This morning it wouldnt start at all, and I ended up having it towed to the nearest mechanic. I have to think it's a fuel system problem of some sort because everything started when the fuel pump died and had to be replaced. And of course, it has yet to duplicate any of these symptoms when a mechanic is looking it over.

Do you know what brand of fuel pump they used? Do you know what the fuel pressue was when they checked it?

I'd have to double check the receipt to find out the brand. I bought it at Autozone because the mechanic wanted 850 dollars for the pump. I got it at Autozone for 350. I think it might've been an Airtex Master, but like I said, I'd have to double check it. The warranty and receipt are currently in the vehicle, sitting in the mechanic's parking lot. The fuel pressure when running was 65 PSI, I think. I'm not sure if they did any of those other tests, they just told me the PSI, and said they couldnt find any leaks, or other problems with the fuel system.

Airtex is known to have "quality issues". Most of the time the check valve doesn't work properly. As a result they can't build pressure or leak down whatever pressure they do build. AC Delco or Delphi is the only way to go. Yup they're expensive, but it sure beats dropping the tank every other month. Wait and see what the shop says... it might be something totally different... cross your fingers.

Wow, wish I had known that before I bought it. Guess it's a learning experience. Well, if that's what the problem turns out to be, then I guess I'll buy the right brand this time. Although I suppose if that's what's causing it, it would explain why nobody has been able to figure it out, since it never screws up when someone is looking at it. Let's see what happens tomorrow....