90 XJ only runs with starter cranking

heres my problem, the red wire for the oxygen sensor shorted against my new header. it wouldnt run untill i unshorted the the oxygen sensor. now it only will run with the ignition in the crank position. once it starts and i let the ignition return to the run position, the engine dies instantly. with the key in the crank positon it runs great, but the starter is spinning.

i think i messed up a fuse or relay for the oxygen sensor, but couldnt find anything.

BTW, a 4.0 should run without an oxygen sensor, not well, but it will run, i tested this with my friend's 89

any ideas would be great

thanks,
Jeff

Start problems

Hi Years back mopar used a resistor it was ceremiac & was mounted on the firewall. the resistor would fail & when you started it soon as you released the ignition key it shut-off There's a resistor wire in the start line. this is straight form chilton jeep manual. A WIRE OF 1.35OHMS. RESISTANCE IS SPLICED INTO THE IGNITION FEED TO REDUCE VOLTAGE TO THE COIL DURING RUNNING CONDITIONS. THE RESISTANCE WIRE IS BYPASSED WHEN THE ENGINE IS BEING STARTED SO THAT FULL BATTERY VOLTAGE MAY BE SUPPLIED TO THE COIL. BYPASS IS ACCOMPLISHED BY THE -- I- TERMINAL ON THE SOLENOID. THIS IS THE ROMOTE SOLENOID MOUNTED ON THE FENDER. A SHORTED RESISTANCE WIRE ON SOLENOID. DALE
[email protected]

wouldnt you want full voltage all the time??

massxj said:

wouldnt you want full voltage all the time??

Click to expand...

I just had a similar problem and all I did was used a 3" piece of wire and jumped the terminals with it on the fuel pump ballast resistor that is mounted near the front of the left fender well.

A bad fuel pump relay will also give you this symptom because it is bypassed when the ignition switch is in the start position. The ballast resistor is there to reduce the voltage when the engine is running so that there is less fuel pump noise.

PaulJ said:

A bad fuel pump relay will also give you this symptom because it is bypassed when the ignition switch is in the start position. The ballast resistor is there to reduce the voltage when the engine is running so that there is less fuel pump noise.

Click to expand...

That's what I thought too but after changing every relay, a new fuel filter, another fuel pump, another TPS I finally went to the "little wire" trick and that was the problem. Not saying I'm a mechanic but just trying to save someone from making the mistakes I have.

The ballast resistor for the fuel pump is bypassed during start, jumping it for a test may isolate the problem. The 30 pin at the 02 relay is powered at the same time as the, run (ignition) imput to the ECU, from the ignition switch. Be a good place to check for power, when the ignition switch is the on position. I´d guess a burnt set of run contacts in the ignition switch.
The ignition switch has a double set of contacts, one for start and one for run. The start contacts go to the starter relay and the start pin on the ECU, gives a little more ignition juice for the start (to counter act the drain from the starter). The other set of contacts go to the 02 relay and the run pin on the ECU (among others) for normal operation. Possible faulty CPU or burnt connector (unlikely).
To answer the statment above, about max voltage to the ignition coil, at all times. Enough spark for complete ignition of the fuel mixture in the cylinder, is enough. Extra voltage in and out of the ignition coil, will cause crossfire (the spark jumps from electrode to electrode inside the distributor cap), the spark plug electrodes burn and pit and any weak spots in the wires will arch. The distributor cap in the Jeep, is small, the electrodes are relatively closs together and corrosion, carbon dust and other conductors are present inside the cap. Extra igntion voltage, usually causes many more problems than it solves.

As stated start with the ballast resistor (small white ceramic block located on the driver's side fender wall near the air filter, it has two wires coming out of it.) It cuts down the voltage to the fuel pump after the vehicle starts to cut down on the noise (ever hear the noise the fuel pump makes when you turn the key to the on position). Apparently it was a problem for a while causing too much noise, and cutting it back silenced it but still provided adequate fuel pressure to run the vehicle. When I installed the shock portion of my lift I accidentally knocked off one of the wires on the resistor and had the same symptoms you are experiencing. As soon as I found the wire and re-connected it she ran great. Yours may have burned out. Jumper a wire between the two going into the resistor and try starting it. That will supply full voltage to the pump and it will probably be noisy. The ballast resistor probably also increases the life of the pump by reducing its workload.