99 Blazer RPM jump

1999 Chevy Blazer LS, Auto 4x4, ~78,000 miles

Engine starts without any issues and idles at about 500rpm. RPMs jump from ~1000-1500 back down to about 500, sometimes lower when slowly accellerating with light pressure on gas pedal (think driving through a residential area/parking lot) accompanied by a moderately loud clunking from under the center of the truck. This occurs 3-4 times or until I'm able to turn into traffic and use more pressure on the pedal, then problem occurs up to about 40mph (engine rpm dropping from 2000 to 500 or below, usually along with a drop in oil pressure). This lurching/rpm jumping is beginning to occur more frequently now, like if in drive and the breaks released rps will jump from 500 to about 900 and cause the truck to lurch. Also, if I put the gas pedal to the floor engine rpms only go to about 3500, no higher and give me the power that I'm used to when flooring the gas pedal. No SES light.

I haven't a clue if this is relevant, but I also have difficulty shifting from 4hi to 2hi. If I engage 4x4, I can drive 50 feet before I get a service 4x4 light and cannot shift back to 2hi w/o shutting the engine off and restarting. I have a hunch that these problems may be related and any input would be greatly appreciated.

Note: You can probably tell that I'm not a car guy, so forgive me if I ask an inordinate ammount of questions

-v

Without illuminating the SES light, the problem could be secondary ignition or fuel pressure related. OBDII does not directly monitor either one. I would start with fuel pressure.

Fuel pump operation:
The fuel pump circuits are designed to energize the fuel pump continuously any time the key is in the START position. When the key is in the RUN position the fuel pump will operate provided the engine is running. If the engine stalls, or is not running for any reason, the PCM will shut off the fuel pump after approximately 2 to 3 seconds.

Fuel pressure test:
Install the fuel pressure gauge to the test port near the distributor. Turn the key to the RUN position but do not start the engine. While the fuel pump is running, fuel pressure must be 60-66psi. After approximately 2 to 3 seconds, the fuel pump will shut off. Fuel pressure must remain between 55-60psi for several minutes. A drop in pressure below 55psi indicates a leak. The most likely place is the fuel pressure regulator, or the lines under the upper intake manifold.

The two problems are more than likely not related. Hope this helps. Post your results.

If fuel isn't it a check cap/rotor would be my first step - get a Delco or I've had good luck with standard motor products parts also - stay away from the cheapo ones they fail! Also look at the vac lines 5/32" on the drivers side for rot and replace as necessary - they all do it and check to the reservior in the fender - you can reach it take your time.