RPM Surging ???

My ’98 Jimmy with a 4.3L and an automatic will show a change in cruse RPM of 200 to 300 rpm when below 50 MPH. The tach shows an increase of 200 rpm for about 1 second and then returns to its original reading. You can feel the change but only slightly. This happens about every 10 to 15 seconds until you get about 30 to 50 miles down the road. I’m assuming that everything has warmed up by then so it stops. The engine starts and idles fine and there are no problems when cursing above 50 MPH.
There are no shown or stored trouble codes in the computer. Any ideas?
Thanks, Steve.

Welcome to AF.
Can you feel engine or transmission do anything?

You need to get it on a engine/transmission capable scanner and check TCC up converter for kicking in and out.

Nope, I don’t believe it’s the transmission. I don’t feel the TCC unlocking and most of the RPM changes are too small. I only feel a very slight change in engine RPM and it shows on the Tachometer.
It will do this “surging” at just about any speed between 35 MPH and 50 MPH and only when everything is warming up.
And, NO, the coolant temperature comes up to normal within a few minutes. I removed the stepper motor (idle control valve) and cleaned its face and the cavity thinking that maybe carbon was causing this. No change.
My next thought was the TPS but why would it work better after 30 to 50 miles down the road?
What about the EGR? Could it be causing this?
My scan tool can give me a real time reading of the O2 sensors but I don’t know what I should be looking for there.
And thank you for the Welcome to the Board.
Steve.

Nope, I don’t believe it’s the transmission. I don’t feel the TCC unlocking and most of the RPM changes are too small. I only feel a very slight change in engine RPM and it shows on the Tachometer.
It will do this “surging” at just about any speed between 35 MPH and 50 MPH and only when everything is warming up.
And, NO, the coolant temperature comes up to normal within a few minutes. I removed the stepper motor (idle control valve) and cleaned its face and the cavity thinking that maybe carbon was causing this. No change.
My next thought was the TPS but why would it work better after 30 to 50 miles down the road?
What about the EGR? Could it be causing this?
My scan tool can give me a real time reading of the O2 sensors but I don’t know what I should be looking for there.
And thank you for the Welcome to the Board.
Steve.

I would watch the TCC solenoid on the scanner on the road.
200-300 is about the rpm change of lockup going in and out.
Also the rpm changes in scanner.
And you can watch the egr valve desired and actual position when rpm changes.

Any codes or check engine lights.

OK, I’ll see what I can do. My Scanner is an old “EZ Link” by Auto Xray and I don’t think it will display anything from the transmission. However, it does show TPS, Mass Air Flow, % Power and Timing Changes.
I just serviced the transmission by changing the filter. But, the problem has been there for a long time.
If the Torque Converter is unlocking would it be caused by something in the transmission or on the engine? Why would it be an “In the Morning” thing? Not enough coffee? :eek7:
Thanks. Steve.

OK, I’ll see what I can do. My Scanner is an old “EZ Link” by Auto Xray and I don’t think it will display anything from the transmission. However, it does show TPS, Mass Air Flow, % Power and Timing Changes.
I just serviced the transmission by changing the filter. But, the problem has been there for a long time.
If the Torque Converter is unlocking would it be caused by something in the transmission or on the engine? Why would it be an “In the Morning” thing? Not enough coffee? :eek7:
Thanks. Steve.

Hard to say.
You need to pinpoint the problem before you can fix it.
Also check for engine missfires to.
Good Luck