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95 4.0 No Spark No Fire

CJXJPIT

NAXJA Forum User
Was up in The hills taking my dogs for a run and jeep just quit. Was down hill all the way back to the house so was able to cost all the way home except the last couple hundred yards. Got into my shop and started looking around. Found wires on o2sensor harness had been chewed up by little critter. Soldered those all back together still crank no start. Noticed insulation on wires from crank sensor leed looked cooked from being to close to exhaust. Replaced crank sensor still crank but no start and no spark. Fuel pump is coming on when you turn key and seems to have a good spray at the fuel rail. Napa guy says it must be your cam sensor not sending signal to computer if you have no spark and you have already replaced CPS. So new cam sensor installed and still crank but no fire. When back probing the coil I have power on both wires without the key on. Not sure if that is right that you would have power on the signal wire with out turning key on. Just for a laugh I replaced the coil and you guessed it, same deal cranks but wont fire and when checking for spark by the old method of putting screwdriver in coil wire and having some one crank I still have no spark. Is it possible the computer is shot and is there any way of checking the ECM?
Just wondering if any one mite have any suggestions. I am embarrassed that all I have done is thrown parts at it and still have a dead jeep. Any help will be appreciated. Thank's, Frustrated in Colorado.:bawl:
 
You said you had missing insulation....did you check all the fuses?
 
A couple of things that you can look at that might be helpful for troubleshooting.

Is your check engine light on when you turn the key from off to run? If you have a BUS fault they check engine light won't light with the key in the run position.

Fuel rail primes when you turn the key to (through) run, but really doesn't build any (enough) pressure until the PCM gets a good pulse, while cranking and the fuel pump relay closes. No spark or fuel while cranking is usually some sort of sensor fault, CPS and Cam sensor are the more common but not the only possibilities.

Just a hunch, but try swapping a couple of relays, ASD relay for one. They don't often go bad, not likely it is your issue, but easy to check.

If your check engine light isn't coming on with the key in the run position, unplug your sensors one at a time until the check engine light comes back. Your issue may be in the wiring which is a whole other can of worms.

It pays to check the PCM inputs well before opting for a new PCM. A whole lot of guys have swapped out a PCM on a hunch and wasted a lot of money. PCM has a fairly rare failure rate.
 
What I did not add was this is a 95 4.0 I bought from a wrecked XJ. I installed into my 84 CJ-7 six years ago. Bought a wiring harness designed for this swap. This jeep has run perfect for over six years. Would always start with just a touch of the key. A few months ago I noticed it would have to crank for about a second or two before it fired. In back of my mind I was thinking maybe it's time for plugs,cap & rotor. Then this happened. Did check this morning and no engine light comes on when key is in run position. Have not checked any relays but the only thing that looks like relays to me are two mounted on the firewall that were part of the new wiring harness. Plan to try and swap those around today or unplug one @ a time like was suggested and see if I can get engine light to come on.Any more thoughts or things to try would be appreciated. Thanks in advance.
 
Having power to the coil without the key on means something is seriously wrong. The more likely reason is your ASD relay is welded closed or maybe the guts are fried. Possible (remotely) that you have crossed a wire and are supplying power to the dark green and orange circuit (for lack of a better name, yours may be a different color). It might be pulling the ASD relay and seeing if the power goes away to the coil (or injectors or alternator) may be helpful.

That circuit in an XJ is the dark green orange wire. I have no idea what it is in your harness. Supplies power to the coil, alternator and injectors.

The coil and injectors trigger on the ground side of the circuit. The alternator regulates on the ground side of the circuit.

The ASD relay is normally closed when the PCM receives a pulse signal from the CPS. No check engine light in a normal XJ can indicate the PCM has shut down the power supply to various sensors as a protection against overheating the drivers.

Unplugging the sensors one at a time can reset the BUS (allow the drivers to cool and reset). An easy way to spot a faulty sensor (shorted sensor).

I honestly don't know what exactly your issue might be. Re soldering wires back together raises some alarm. You may have wired the O2 sensor heater circuit into the O2 sensor signal circuit. Or the O2 sensor is internally shorted causing a crossover. Just a shot in the dark. You really can't count on the harness O2 sensor wires being the same color as the O2 sensor pigtail wire colors.

Could be the critter was busy and you have another wire some place with no insulation laying against another wire with no insulation. Been there done that, a Pine Martin made a major portion of one harness (on the wifes XJ) look like bare copper spaghetti. :(
 
What 8MUD said...I agree this is not right...

"When back probing the coil I have power on both wires without the key on. Not sure if that is right that you would have power on the signal wire with out turning key on."
 
Just a final follow up for those who made some suggestions for me. After replacing CPS,cam sensor, and coil, will still no results I bought a used ECM and yes that is what it turned out to be. I checked continuity on all my sensors first and came up with no shorts before I tried the new ECM. Not sure what made the ECM fail. It was not fried because it was still able to send codes but something was not right with it.
Thank's to all that chimed in.
 
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