Recently ive noticed hard starting occasionally, sometimes it will backfire out the intake when i try to start it. Today on my way back to college it would randomly start missing really bad and was backfiring out the intake badly. Thats leading me to think that it is going out of time so im thinking my infamous cps has finally taken a crap. This is somewhat important to get it fixed since i need my jeep every day this week so any help would be greatly appreciated. Thanks, and its a '91 4.0 5 speed

 

When my crank shaft position sensor started going out I was getting random stalling, and my gauges would act weird when this happened. Jeep started fine other than when it would stall, it would have to cool off for about 10 minutes before it would start again. Never had backfiring. Just my personal experience, who's to say there isn't different ones out there. The only way I diagnosed it was by doing my own research on the net.

 

You might un-do the cap and check for condensation.
I'd gap my plugs, clean the cap and rotor in the kitchen sink, and see what that does. (dry the high temp plastic).(or you could toss it in the garbage can and buy all new).

Does your check engine light work? No harm in trying this;

1984-1990 w/AMC engine are pre-OBD.

1984-1986 w/GM V6-173 are OBD-I (GM control setup.)

1991-1995 are OBD-I (Chrysler)

1996-2001 are OBD-II (Chrysler)

The "Key Tricks"
Older (OBDI) models - Start with the ignition off. Within five seconds, switch the key on, off, on, off, on. ("On" is NOT "Start" and "Off" is NOT "Lock"). The "check engine" light will flash. Count the flashes. Each code is a two digit code, so a (for example) 23 would be FLASH FLASH (pause) FLASH FLASH FLASH (long pause). It will never flash more than 9 times, so watch for those pauses!

Newer (OBDII) models - Put the key into the ignition, push down the odometer reset button, then turn the key to RUN with your finger on the button - and then release the button. The odometer will go

 

Backfiring in the intake is often related to a "lean" fuel air mixture.

Here are a few recommendations.

1. Fuel pressure. Must be done with a gauge; no shortcuts. You should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator (be sure this line is pulling good vacuum) and the pressure should jump to approximately 39 psi

2. Test your MAP sensor. Directly involved with fuel air mixture

3. Be sure all tuneup hardware is fresh. Secondary ignition is critical. Plugs, wires, cap, rotor and be sure it has a fresh fuel filter; that can affect fuel delivery.

 

Thanks for all the help, almost the entire ignition is new (cap, rotor, wires, plugs). I dont have any check engine light and i know it does work. Ill see what i can figure out with it

 

what is the process of testing the map sensor? i know where its located and how remove it, just not how to test it.

 

I recommend getting yourself a maintenance manual where all of the testing procedures are detailed. Factory service manual is best, followed by Chiltons and Haynes.

Ebay sometimes has used FSMs. Here is a link to a licensed, legal digital copy of the FSM. And it's cheap. I don't see one for the 1991 year, but there is one for the 1993 year and that is going to be almost identical to your 1991.

http://www.pacificcoastmanuals.com/F..._cherokee.html

 

Ok thanks for the link. I just went out and tested for a cel and its not working, bulb must be burnt out. Ill replace the bulb tomorrow and see what happens.

 

In my experiece, backfiring like that was due to the timing chain slipping a tooth or 5. How old/many miles are on the chain?

 

214k on everything but the head was replaced about 80k ago

 

So i got my cel working, only code thrown that would cause an issue was the map sensor, i replaced that but now the fuel pump will not run. i tried reinstalling the old map sensor to be sure that was not the issue, no luck. Now my next issue is, im getting 6.7 volts to the fuel pump, is this normal or should i have 12?

 

I might suspect low voltage is a bad connection or a problem with the relay. Guys have posted problems with the relay socket/plug. Might try swapping it with the AC clutch relay.