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Old 06-04-2014
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Join Date: May 2014
Location: Richmond
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Icon4 Rough Idle 3 Codes

I finally got a chance to run the codes on my 98 4.0 and these are the 3 that I have:

P0122 (Throttle Position Sensor)
p1451 (EVAP)
p0420 (Catalytic Converter)

So, the truck idles rough at speed around 1500rpm. The P0420 code is from the cat being gone, and I'm assuming the same for the P1451. My question is, if I replace the TPS and reset the PCM, what are the chances of this fixing my rough idle? I checked my EGR valve and it looks like the worst part on the engine, so could that or the DPFE be my problem? I'm really hoping it's just the TPS and I can fix it and be on my way. Will replacing the TPS fix the idle even with the other 2 codes being constant? It's driving me crazy.
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Old 06-05-2014
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EVAP system is not part of Cat converter system.

EVAP system could have a vacuum leak and that is the cause of the poor performance.
EVAP system will have a vacuum hose connected to the intake manifold, find it and remove it then cap that intake vacuum port, see if engine idles better, if so EVAP has an air leak

First start engine and then remove the EGR valves vacuum hose, check if there is any suction(vacuum) on that hose, there shouldn't be, if there is then replace EGR modulator, it is leaking or stuck open.
Remove EGR valve and make sure it can open and close all the way, it can get carbon built up in it and then won't close all the way so exhaust gas leaks in and cause poor idle.

I wouldn't replace the TPS sensor just yet.
P0122 = low voltage from TPS not bad TPS
So I would spend that money on a Volt/Ohm meter.

Remove the TPS connector and turn on the key.
Set Meter to read 20vDC, or just vDC
One of the wires on that connector should have 5 volts, if not then that wire has come loose somewhere or broken, could also be bad computer since the 5 volts comes from the computer.

If you have the 5volts then reconnect connector to TPS.
Use a sewing needle and pierce one of the other two wires, usually the center wire, and read the voltage, should be .6-1.0v.
Below .5v will usually set the P0122 code.
Slowly open throttle and watch the voltage, it should steadily climb until throttle is open all the way(WOT) and at that point you should see 4.5v to 4.9v.
If voltage jumps around at any point then TPS is bad, well it has a bad spot, which could be at closed throttle point which is where it sits most of the time.

TPS can be adjusted and if it is not tight it could have slipped.
It is rotated to set "throttle closed" voltage between .6v and 1.0v, .9v is best
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