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Discussion Starter #1
Truck has 75k on it, new fuel pump 55psi @ idle, MAP @ idle 10.3 (19 inches of vacuum). Tight Distributor shaft, newer cap, rotor wires, fresh plugs, filters, cleaned MAF ... runs like crap at anything above idle but is smooth when accelerating.
If i unplug the MAP she runs/drives fine sept a slight flat spot off idle but generally drives fine and doesn't smell rich ???
Im thinking its got a vacuum leak (not hearing any obvious vac leaks) somewhere making it go lean.

Anybody else encounter this kind of problem with their 4.3 where when you unplug the MAP she runs fine, would like to know what others found as the problem here ? thks.
Any ideas ?
 

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Could be a bad map or vacuum leak. Spray brake clean-flamable type- all over your intake, vacuum lines and around the brake booster especially where the master connects. A common vacuum leak is the line that passes in front of or above the brake booster that connects to the vacuum resivior in the drivers fender area.
 

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I thought it was the MAP too but a new one didnt fix it, someone suggested propane, attach a hose to the bottle and take that around the intake and hoses ......
 

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Discussion Starter #5
I think the ECM's response to unplugging the MAP is to richen up the mix as it sees low MAP signale, the ECM pulls back the rich mixture that the O2 now senses and the motor runs okay. Fuel is probably richer than normal but not stinky rich, Unplugging the MAP is similar to driving the truck with your foot in the throttle to smooth it out.
 

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It goes into open loop and ignores alot of the sensors and operates on a set table. Yeah you can use propane i use brake clean. Listen for a vacuum leak while its running especially by the fender/brake booster.
 
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