TSea99XJ

Quick rundown of what has lead to the issue before I get into the problem itself.

99xj 4.0 Auto

Had zero problems prior to this event.

Last winter, cold snap, Jeep wouldn't start for the girlfriend, cranked it intermittently until the battery drained. (I was in bed sick, this is all 2nd hand info to me). Guys from work came over, attempted to jump it, crank but no start. Towed it to our shop, let it warm up, changed the oil due to fuel smell (I believe it to be from over-cranking, not fuel in the oil), XJ eventually started. Problems have occurred since then.

Since then, I've had intermittent no start issues. (As well as...)
- Will almost always start first thing in the morning
- Wont always start after sitting off for short periods of time
- Wont start after sitting off for more than an hour
- Will usually start after sitting off for 2 or more hours
- Coolant temp not passing 150 degrees
- RPMs surging at idle or when coasting
- RPMs not dropping when off the throttle until about 5-10 seconds of no skinny peddle.

Sometimes it will start if I put the gas peddle to the floor, or cycle the ignition on/off a few times, or combinations of the two.

Now that the temp has gotten warmer and staying that way, it hasn't started for the last two days at all, with the exception of today when I got to work, where its been sitting since the last no start, and tried it right away while it was still cool outside, and it did fire so I could get it home.

Researched the problem on multiple sites including here, most everything pointed to the Crank Position Sensor, so I just replaced that tonight (Pain in the ****). It fired right up after the new sensor was in, but started with a rough/surging idle which i've been experiencing with the old CKS, and wouldn't refire immediately after turning it off.

More than open to any other opinions or suggestions!

Edit: If someone has the patience and is willing to take the time, I'm more than willing to do any testing/diagnosing myself and report back, just need more specific places to start than Fuel/Air/Spark.

Radi

Any vacuum leaks? Cracked/broken/disconnected lines? Intake manifold? That's often a cause of surging and slow return to idle. Connecting a vacuum gauge might be interesting. Might have a sticky/dirty IAC in the throttle body too.
Something has a heat soak problem too, it seems. CPS and Ignition coil are the common suspects. You can measure the coil resistance, but that's not always going to ferret out an intermittent. Swapping in a known good coil is a better test.

TSea99XJ

Appreciate the input!

I'll replace the ignition coil tomorrow assuming I can find one locally and see what happens, also find out about getting a vacuum gauge for testing, Where would I hook the gauge up for testing? Or is it pretty self explanatory.

I did remove and clean both the throttle body and IAC last fall, and as much of the inside of the intake manifold as can be reached without removing it, but I suppose that doesn't mean the IAC isn't faulty. I probably should have recognized that as the potential suspect since the whole reason for cleaning it in the first place was a fluctuating RPM at idle (about 750-1k).

Thanks again, will update with results as soon as I have them!

Radi

Vacuum gauge goes to one of the intake manifold vacuum ports. There should be one capped off that you can plug into.

• ENGINE STATE VACUUM GAUGE READING INDICATION
• Steady idle (800-1200rpm) Gauge steady, 17-22 Normal & healthy
• Steady idle (800-1200 rpm) Intermittently drops several needle divisions Sticking valve or broken valve spring
• Steady idle (800-1200 rpm) Steady, low reading, 8-14 Small vacuum leak or valve timing off; could have low compression/worn rings (verify with a compression or leakdown test).
• Steady idle (800-1200 rpm) Steady, low reading, under 8 Vacuum leak (check brake booster, vacuum lines, etc.)
• Idle (800-1200 rpm) Needle drops sharply on a regular rhythm Burnt valve, or a valve with clearance too tight
• Idle (800-1200 rpm) Needle drifts up & down, along with rpm drift Mixture off or small vacuum leak
• Idle (800-1200rpm) Vacuum gradually drops Excessive exhaust back pressure (plugged muffler or catalytic converter)
• Idle (800-1200rpm) Intermittent fluctuation Ignition miss; sticking valve
• Idle (800-1200rpm) Steady, above 22 Ignition timing may be too advanced
• Open & close throttle quickly Drops to about 2, jumps to about 25 Healthy engine
• Open & close throttle quickly Drops to 0, jumps to about 20 May confirm worn rings (especially if idle shows only about 15-20) Verify with a compression or leakdown test.