[bstwilliams]

I have a 94 jeep grand cherokee 5.2 and when it sits for the weekend it won't start on Monday when its time to go to work. I am also having problems with the engine studdering at an idle. The check engine light will come on and its all I can do to keep it running. It smells like it is fluding when this occurs. I have replaced all filters that have to do with the fuel system and the fuel pump is about to take a dump but other than these problems it is still as strong as the day I got it in 94 and now 300,000 miles on the same stock engine. Please help.

[TxGreaseMonkey]

. Check for codes.

. Check or replace the Engine Coolant Temperature (ECT) Sensor, which affects fuel injector pulse width. The ECT Sensor is a temperature dependent resistor (thermistor), whose resistance decreases as the engine coolant temperature increases. It provides a richer mixture when cold, leaner mixture when hot. They become corroded, due to not changing the coolant frequently enough, flood the engine, and waste a tremendous amount of fuel.

Afterwards, clear all codes.

[bstwilliams]

I replaced the ECT and the jeep started but it is running like crap and won't idol still. Could this be an O2 sensor problem?

[TxGreaseMonkey]

What codes were thrown? You are smelling fuel, due to incomplete combustion. You need to go over the ignition system. Only use the same Champion spark plugs that originally came in the vehicle. It wouldn't surprise me if the Ignition Control Module (ICM) and/or coil is shot, after 300,000 miles. Bottom line: You have an ignition system, not fuel, problem.

[bstwilliams]

Well, I fired the jeep up tonight and it was idoling smoothly until I gave it some gas. Then it wouldn't stay running after that. Chamipon spark plugs are in it. I don't know the codes because no one around here has the reader except the dealership.

[bstwilliams]

I really don't want to pay their prices but I may have to. Ill see what they can offer me. Not to question you, because this far you have been right on the money, but do the O2 sensors have anything to do with the excessive fuel smell?

[TxGreaseMonkey]

Since you have an OBD-I vehicle, code readers won't work. If you checked a Hayne's Manual, you would likely find out how to jump a service check connector and obtain codes.

Until the engine warms up and comes out of "open loop" mode, I don't think the oxygen sensor is a real problem. My bet is that you have classic Ignition Control Module (or Jeep equivalent) failure. This would be like points on old engines not opening and closing when they are supposed to. Hence, you have erratic spark and the engine won't stay running.

Conceptually, the ICM is a large output transistor (switch) controlling current through the primary coil. ICMs replace points and condenser in older vehicles. In the primary coil, current builds to 5 to 12 amps, generating lots of heat (due to inductance). Then, the ECM controls when the ICM "switches off" current to the primary coil, causing amperage to go to zero. When this happens, 12 to 14 volts in the primary coil is "stepped-up" to 30,000 volts in the secondary coil. This process creates a "type" of alternating current required for the coil (step-up transformer) to work. In a properly working ICM, timing is precisely when needed to fire each spark plug.

My hunch is that your ICM (or Jeep equivalent) is not firing when it's supposed to. This results in lots of excess fuel and flooding.

[bstwilliams]

I will get that checked out. One more question. When the check engine light is on, the jeep won't shift into overdrive. It never does this when the light is off. The correct fluid and amount is in the trany, I just want to know what is going on.