inconsistent start after motor swap

hey guys, will do my best to explain in detail my problem/symptoms

popped the motor in my 00 xj sport with 4.0, would still start after it threw a rod thru oil pan and
it's started since the swap but very inconsistently and it needed to be jumped since the battery is on its way out. im going to try jumping with thicker cables.

i used an 01 xj with matching 4.0 (both coil pack) and it went in without splicing anything. its started but i haven't been able to get it going consistently. it'll just crank and crank.

no codes, had thrown a cps code but reset it and started with same CPS, still switched it after to be on safe side, never thru a code. i believe the 1st one was just dirty
changed CPS with spare (notching trick is a life saver)
changed fuel tank completely so also pump, filter, regulator, and that pia piece that leaks back fuel pressure- it's started with either tank at least 1x
swapped around relays for fuel pump and can audibly hear it
cleaned grounds and made sure i wasn't missing any wires off harness.

i haven't checked actual fuel pressure but i'm getting a decent squirt off the shraeder and when i do get it to start it'll stay running. will also have to check spark but not sure the best way.



i don't need the 01 harness or computer, do I? everything clicked right together with no modification or headache, and it's started multiple times since the swap. i have them but can't see why i would need either

what should i try next? i have a whole parts jeep still but i think it's something dumb im forgetting or not checking?

i'm not a pro mechanic, just a shade tree trying to get their jeep going again and am running out of ideas. any guidance would be rad!

If the battery is junk I'd start with that. Low battery voltage causes the PCM to lose it's memory, particularly the idle and can cause hard or no starts. If it's low enough it may crank but not have enough voltage to even turn on the PCM.

Fix the known problems before you suppose any other problems.

trippled said:

If the battery is junk I'd start with that. Low battery voltage causes the PCM to lose it's memory, particularly the idle and can cause hard or no starts. If it's low enough it may crank but not have enough voltage to even turn on the PCM.

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my ford has a great battery so i'll switch em and see if that resolves it.

Tim_MN said:

Fix the known problems before you suppose any other problems.

Click to expand...

can't argue with you on that.

switched the battery out and still same issue.

Start with the basics. Determine if you have battery charging issues or connection issues. Jeeps do not tolerate low voltage, bad wire connections, or poor grounds.

Perform routine maintenance of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, wire brush, and clean until shiny the cable/wire ends and whatever they bolt to.

Place your DVOM (Digital Volt Ohm Multi-Meter) on the 20 volt scale. First check battery voltage by placing your multi-meter's positive lead on the battery's positive post ( the actual post, not the clamp ) and the negative lead on the negative post. You need a minimum of 12 volts to continue testing. Next, leave your meter connected and take a reading while the engine is cranking. Record this voltage reading. Now connect your positive lead to the battery terminal stud on the starter and the negative lead to the starter housing. Again, crank the engine and record the voltage reading. If the voltage reading at the starter is not within 1 volt of battery voltage then you have excessive voltage drop in the starter circuit.

Typical voltage drop maximums:
• starter circuit (including starter solenoid) = 0.60 volt
• battery post to battery terminal end = zero volts
• battery main cable (measured end to end) 0.20 volt
• starter solenoid = 0.20 volt
• battery negative post to alternator metal frame = 0.20 volt
• negative main cable to engine block = 0.20 volt
• negative battery post to starter metal frame = 0.30
• battery positive post to alternator b+stud = 0.5 volt with maximum charging load applied (all accessories turned on)


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went back to clean all the grounds and there was an issue with one of them. repaired, made sure everything was extra clean and she fired right up! i knew it was something stupid, because i'm stupid. then i wasn't seeing oil pressure, threw in the other one i had and bam, reading oil pressure! now just need to finish hooking up the header to the rest of the exhaust and i'll have a jeep again! oh and refrigerant ._.

thanks for the input, tim!