OT: 1989 GMC 350 Emissions

I have a backup vehicle, a 1989 3/4 ton Suburban with the 350/auto. It's emissions exempt in GA because of the high GVW. It no longer has a cat converter because the previous owner installed a nice dual exhaust. The 350 still has the EGR system in place. I had heard that if I were to disable it, it might improve my MPGs. Is there any truth to this? Just trying to boost gas mileage any way I can without having to part with it.

As counterintuitive as it sounds, EGR actually has a cooling effect on the intake charge:

"EGR has been successful in reducing NOx in gasoline engines. When an engine uses EGR, a percentage of the exhaust gases are drawn or forced back into the intake, at the direction of the engine's electronic control module (ECM), and mixed with the fresh air and fuel the engine is ingesting.

"The purpose of using EGR is to lower the peak flame temperatures inside the combustion chamber. Forcing some exhaust back into the cylinder slightly reduces the concentration of oxygen and slows the burning a bit, making things a little cooler. EGR also absorbs heat, further cooling the process."

So eliminating your EGR will probably increase combustion temperatures within the cylinders. But it may come at a price, as your engine may actually run hotter...and any tiny MPG increases MAY be offset by evaporative losses. Or they may not.

The two largest factors on MPG are vehicle weight--which you may not be able to affect significantly--and final gear ratio. If you already have a 700R4 overdrive automatic [0.8:1 overdrive ratio] and a differential gearset lower than 3.30:1, you may have already seen all the MPG you're gonna get from a vehicle that weighs what a Suburban does.

For what it's worth, my '89 Chev heavy half, 4X4, Xcab, long box started idling rough and got horrible gas mileage.

Turned out the diaphragm had ruptured in the EGR valve, giving the same effect as pulling the vacuum hose off the valve and not plugging it. Plugging the vacuum hose made it run only marginally better. Wasn't until I replaced the EGR valve and got the system working properly that the pickup ran properly again.

I'd say leave it alone.

The thumnail sketch on an EGR valve on a gas engine is it reduces part throttle detonation (ping) allowing for a little more aggressive timing curve. Disable the EGR and you will likely see a reduction in mileage.

Probably not. With the EGR plugged the car will probably run lean and ping a lot at part- throttle operation which is most driving. Could do it with older less integrated emissions systems such as with a carburetor which can rejetted, but fuel injection almost always requires reprograming the ECM.

Boost gas mileage and Suburban don't really go together.
My '89 Suburban has had the converters and mufflers and smog pump removed. I never paid any attention to the EGR valve. It gets 16 MPGs pretty steady and pulls like a locomotive. 5.7 - 700R4 - 3.42 ratio - 1/2 ton 2 wheel.

Buzzman72 got it nailed and i spec its already pluged its self up if the intake has never been off and the egr ports cleaned...