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My (new to me) 89 SSE is a splendid car and drives very nicely, but . . . . .

When I bought the car and drove it to a nearby town for a set of tires and a little local driving, it ran perfectly for about 40 or 50 miles. Then I filled the tank and within 2 miles it started making a squalling sound under load and ran really flat like the spark was pulled way back until it surged ahead with additional throttle and the squalling would start. The squalling is a constant pitch, but gets louder as you crowd it. It seems to be coming from the base of the EGR valve assy, so I have resealed those joints as well as I could - no change.

Disconnecting the EGR valve (digital type) returns the pulling power to normal, or close to it. It still squalls. It will maintain cruising speed on the highway without squalling, but accelerating starts the noise. With the EGR valve active, it won't maintain cruising speed without considerable throttle pressure and some squalling.

During my inspecting for exhaust leakage, I noticed that the wire bundle to the EGR valve had been misrouted by previous work and the 4 wires and the corrugated sleeve had melted together at the base of the EGR valve assy. I don't know if the wires shorted among themselves or to ground. They were really melted together. I fixed the wires and it made no difference. Could it have geeked the ECM if one or more of those wires shorted to ground?

And to add to the mystery, now and then it gives a serious chug or hesitation like a momentary fuel cut. It's harder and a little longer than an ignition misfire. What on earth is that about?

Using a stethoscope, I can hear exhaust leakage at the joint in the EGR valve where the plate that holds the solenoid pack attaches to the base plate (not a servicable part evidently. I can't find that gasket on line anywhere.) The gasket material there is a high temp type like you'd find between an exhaust manifold and the head. I reasoned that an exhaust leak there might cause the perforated metal layer to vibrate like a clarinet reed, causing the squalling sound. So I tried to reseal that joint with some high temp putty (2400 degree stuff) with the now useless gasket removed. It's a little bit better, but not much. So much for clarinet reeds.

And now, at full throttle, I can hear a substantial whooshing sound from under the hood. Anyone have any clues as to what is going on with all this?

you can test the egr - http://easyautodiagnostics.com/gm/4.3L- ... ve-tests-1

Thanks for the reply, but my EGR valve is the digital type, not covered by the link suggested.

The answer to the squalling is - blocked cat converter.

The ceramic core in the cat was badly broken up and many pieces turned sideways to the flow, effectively blocking the flow at maybe 80 to 90%. Ran OK at low speeds, but as the back pressure built up with application of throttle, the exhaust available to the EGR valves was at such a pressure that it overwhelmed the mixture in the intake side and it ran really flat. At yet greater throttle, the pressure started pushing the spring loaded EGR solenoid stem seals off their seats, chattering and causing the squalling. At even more throttle, the seals were pushed fully off their seats and the whooshing sound resulted. Trouble codes don't reveal such matters.

The exhaust tone now has a pleasant growl to it which seemed to have been masked by the cat restriction.

As an aside, the OBD reader ID'd a faulty cam position sensor which was replaced. That assures the timed injection coincides with the intake valve sequence and it runs smoother.

The EGR valve now seems to work effectively so the melted wire bundle did not damage the ECM in any way apparently.

Now, with the converter issue resolved, it runs strong with no more strange noises.

Thanks for posting your findings.
Glad you fixed it.