Disconnecting map sensor will put the truck in open loop, permanently. It will run the truck off of tables, and ignore O2 sensor input. How does the little short vacuum line for the map sensor look?

I am pretty sure all of these trucks were speed/density systems. (map sensor, as opposed to MAF.)

 

its a 97 5.9l gas. original map sensor, when unplugged stopped misfire. replaced map sensor thinking it was the cause. new map sensor installed and still misfires. somewhere in 5 volt system is telling pcm to adjust for timming. new distributor, cam and crank sensors, plug wires, cap and rotor. sync was set and wires routed properly.

 

vac tube is good. no dry rot or cracking. scan shows map working.

 

New distributor? Then fuel synch is probably off. Has to be set with computer. Cam sensor is in disty.


At the risk of not reading from beginning of thread, plug wire routing cold also cause misfire. See TSB.

 

Is it just random misfire? Or, specific cylinders?

What brand crank sensor?

 

Plug a GOOD scanner in and see what cylinders show misfire in stored codes. See if it is random or the same cylinder.

 

Do a vacuum test.

 

Using genisis scan tool. not showing cylinder misfire. timing jumping around. timing chain and gears were done two years ago. checked them and all is good. something is causing pcm to keep adjusting and freak out. still believe electrical short in 5 volt loop from cam, crank, map, tps, ect. going to do tracer test on it to locate short. hoping to find it. next step is replacing pcm or a bullet through the block.

 

Do a vacuum test.

 

random misfire, all cylinder misfire, also had 2 5 7 cylinder mis.
changed coil, wires, plugs, cap, rotor, coil sensor plate (under cap), o2 sensor. runs good one min. then misfires, then ok, then mis...
compression ok, 237k miles, fuel pressure & oil pressure ok. wtf?
reputable mechanic shop cant figure it out either. not the plenum gasget.
CAN ANYONE HELP ME PLEASE??? Before i put a bullet in the block!