I have had a 89 and 93 both with the 4.3. Both saw a considerable difference with new (high flow) muffler. I think older mufflers often become restricted over the years.
If it has a catalytic converter, theres a good chance that is getting about time to change that as well.
Higher octane in the older 4.3's never seemed to do much for me - even 110 octane merely seemed to "feel" a bit spunkier at take-off. Of coarse, you may have better results.
I *have* had good luck changing out the upstream o2 sensors (just because they are not throwing a CEL at 150K+ does not mean they are not reading the correct voltages).
Depending on your emissions testing in your county, you could consider removing the catalytic converter and moving the downstream O2 sensor to a housing the sits just out of the exhaust stream (has worked great for me every time). If you don't do any welding, a shop is not too high for this. Older trucks (pre-94??) do not have a downstream sensor - so simply removing the catalytic converter does the trick.
I have also had excellent results in removing the EGR valves. Elimination is fairly simple on the older trucks.
One last and fairly inexpensive thing you can do is add a cold air intake. This definately seems to help out a bit. If you don't wanna spend the money on a good kit, go to Lowes and buy some flexible alumunum dryer duct and route the air filter inlet to the front of your grill (theres usually a hole the exact size for this already there). To keep water from getting in, I have used the dryer flap housing (that goes outside your house) as the breather just behind the grill. Requires a bit of dremel work, but works nonetheless. I can send some pics of my own mod if you want.
All of the above has typically "felt" like it gave me an extra 20HP give or take on both 4.3 chevys I have owned.
BTW, removing the catalytic converter is... illegal. In fact, defeating any emissions equiptment is afaik. I only recommend doing this for ehh.. "testing purposes".
If it has a catalytic converter, theres a good chance that is getting about time to change that as well.
Higher octane in the older 4.3's never seemed to do much for me - even 110 octane merely seemed to "feel" a bit spunkier at take-off. Of coarse, you may have better results.
I *have* had good luck changing out the upstream o2 sensors (just because they are not throwing a CEL at 150K+ does not mean they are not reading the correct voltages).
Depending on your emissions testing in your county, you could consider removing the catalytic converter and moving the downstream O2 sensor to a housing the sits just out of the exhaust stream (has worked great for me every time). If you don't do any welding, a shop is not too high for this. Older trucks (pre-94??) do not have a downstream sensor - so simply removing the catalytic converter does the trick.
I have also had excellent results in removing the EGR valves. Elimination is fairly simple on the older trucks.
One last and fairly inexpensive thing you can do is add a cold air intake. This definately seems to help out a bit. If you don't wanna spend the money on a good kit, go to Lowes and buy some flexible alumunum dryer duct and route the air filter inlet to the front of your grill (theres usually a hole the exact size for this already there). To keep water from getting in, I have used the dryer flap housing (that goes outside your house) as the breather just behind the grill. Requires a bit of dremel work, but works nonetheless. I can send some pics of my own mod if you want.
All of the above has typically "felt" like it gave me an extra 20HP give or take on both 4.3 chevys I have owned.
BTW, removing the catalytic converter is... illegal. In fact, defeating any emissions equiptment is afaik. I only recommend doing this for ehh.. "testing purposes".
