4.0 93 motor in a 4.0 91 issues

ok i got my 93 motor in my 91. started it up and it runs. just after you start it it runs very good for the first second then the check engine light comes on and it takes a crap. i changed the crank position sensor, the oil pressure sensor, the sensors on the intake. what have i not changed that would make it run like crap?

Make sure all the connectors are connected to the proper sensors; such as CPS, MAP, IAC and TPS....
Verify fuel pressure to fuel rail and injectors along with spark to plugs...

a guy on jeep forum says to check the balast resisitor what affect would a balast resistor have on a motor?

Well, the ballast resistor (eliminated in 94) is a white ceramic piece mounted to the driver's side fenderwell. It reduces the power from the fuel pump relay to make the pump run quieter. It can be a problem causing an open circuit. It can be bypassed without any issues other than possibly a noisy pump.

There are multiple paths of power to the fuel pump. When you turn the key to ON, power to the pump is 12 volts through the ASD (or B Latch on a Renix) relay bypassing the ballast resistor, and this lasts for 3-seconds and then the PCM/ECU cuts the ground to the relay if it hasn't sensed a crank sensor signal. Next, when you turn the key to START, the fuel pump gets 12 volts from the starter relay, bypassing the ballast resistor. Third, when the engine is running, from idle to just short of WOT, the fuel pump gets power from the fuel pump relay, through the ballast resistor where the power is reduced to approx. 6 volts. And last, when the engine is at WOT, the fuel pump gets 12 volts from the oxygen heater relay, bypassing the ballast resistor.

So, pull the wires from the ballast resistor and jumper them together.

Post up the results.

wouldint that cause a problem with it not starting instead of just running crappy

Well, as it starts off the ASD relay and the Starter Relay, both bypassing the ballast resistor, then switches to the fuel pump relay through the ballast resistor to run, then it could be causing a problem with rough running if it is intermittently providing power to the fuel pump. Now, it would take the average DIY guy about 30 seconds to temporarily bypass the ballast resistor, so it isn't that hard to try. In fact, it probably took more time for your reply than it would to have done it.

Maybe we should hit the RESET button--fully describe CRAPPY.

joe_peters said:

Well, as it starts off the ASD relay and the Starter Relay, both bypassing the ballast resistor, then switches to the fuel pump relay through the ballast resistor to run, then it could be causing a problem with rough running if it is intermittently providing power to the fuel pump. Now, it would take the average DIY guy about 30 seconds to temporarily bypass the ballast resistor, so it isn't that hard to try. In fact, it probably took more time for your reply than it would to have done it.

Maybe we should hit the RESET button--fully describe CRAPPY.

Click to expand...

well i would have went out and done it if it wasnt twenty miles away. unlike a lot of people i dont hava a garage on my house so i have to use my grandpas but thanks for your replys

definition of crappy: strats idles at about 800rpms after a second idle drops to about 300rpms

first thing is to use the key trick to get the stored codes, then figure out what's complaining

http://www.allpar.com/fix/80s-codes.html

studstill said:

well i would have went out and done it if it wasnt twenty miles away. unlike a lot of people i dont hava a garage on my house so i have to use my grandpas but thanks for your replys

definition of crappy: strats idles at about 800rpms after a second idle drops to about 300rpms

Click to expand...

Sounds like you may have an IAC (idle air controller) issue. Get some throttle body safe aerosol cleaner, remove the intake tube and pull the IAC from the throttle body. Clean the throttle body and really clean the IAC port. Clean the IAC itself, be careful, the pintle can break, use the cleaner, toothbrush, cotton swabs.

Put it back together and see were the idle ends up.