There is a TSB for poppets sticking open due to dirt or low-quality fuel. Try the fuel injector cleaner...

The TSB says there's a procedure for cleaning the poppets under pressure that involves unhooking the fuel lines at the filter and return behind the the plenum, cutting the tank out of the circuit. Then blasting GMs top engine cleaner through. Then adding GMs port fuel injector cleaner to the tank and 'advise the customer to buy higher-quality fuel'.

So yes, the poppets could be the problem. But you'd think they would cause a problem both cold and hot.

Once it warms up is when the EGR valve opens. And the O2 sensor gets into the act. Are you getting ANY codes? Like O2 sensor detecting a rich condition? Or the EGR code? The light won't come on until the truck's warmed up; for me, it came on as soon as it started running rough, about the time it hit 210 on the temp gauge.

If you bend a paperclip into a U, push one end into the upper right-hand pin of the diagnostic port under the steering column and the other end into the next pin to the left. Turn the key to run, but do not start. The SES light will start flashing and will flash each code 3x.

First code should be 12, or flash-off-flash-flash, repeated 2x. Then it'll do any others. When it returns to 12, that's the end of the list.

EGR is code 32, O2 sensor detects rich exhaust is 45.

 

Here is the link to the paper clip trick
http://www.troublecodes.net/GM/
I will check and see if I can pull some codes.

I found that the fuel pump changes speed in the tank.
I can hear the pump slow down and speed up. The pump does not sound good .
I would expect the pump to be a constant sound to maintain a constant pressure.
Does that sound normal ?

It would not be a bad idea to change the fuel since this is a 93.

 

Disconnected the EGR .

Now there is no more surging or loping .
There is a slight Black fire from the exhaust when the throttle is released.

Now What ?

A new thread cause this thread was about fuel pumps.

 

Its your thread, you can take it where ever you would like. Really should have chosen a better title which covered your problem instead of a rather simple question. Just a suggestion for the future.

 

Now it sounds like your EGR valve is malfunctioning.

I *think* AutoZone and like places will test the EGR valve for you if you bring it in.