You may have a clogged up egr valve. When the egr valve gets clogged up with carbon, the truck will run rough, surge stall, and be very anemic on power. Is the service engine soon(check engine) light on? I've dealt with this issue before, and those 4.3 v6's tend to get egr issues. Try this: when it bogs and runs rough, stop truck, open hood, and tap on egr valve base( where it bolts to engine) if rough running clears up, there's your problem. Egr valve is located on front of engine just to the rt of alternator bracket.
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You may have a clogged up egr valve. When the egr valve gets clogged up with carbon, the truck will run rough, surge stall, and be very anemic on power. Is the service engine soon(check engine) light on? I've dealt with this issue before, and those 4.3 v6's tend to get egr issues. Try this: when it bogs and runs rough, stop truck, open hood, and tap on egr valve base( where it bolts to engine) if rough running clears up, there's your problem. Egr valve is located on front of engine just to the rt of alternator bracket.