99 4.0 starts then stalls/dies

[Prom3theus]

Hey guys,

I have a very weird issue and I've tried everything I can think of but no luck. I have a 99 4.0, aw4, 135k. Sorry for this long description that follows...

The issue: every time I start the jeep, hot or cold engine, warm or cold ambient temperatures, the jeep fires up then stalls right away and dies. If I keep the acceleration pressed to maintain 800-1000 rpm, it won't stall/die, and after about 3-10 seconds, usually keeps the idle just fine even without accelerating (I say usually since few times I had to hold the acceleration more, for about maybe 20 seconds). It does leave a rich smell though, obviously due to apparently not having enough air. This issue started in July, after jeep sat for 2 weeks, but it didn't do it so often till maybe late August-September.

First thing I thought to do was to replace the iac and tps, after replacing them (echlin from Napa) the issue still persists. I also spot-cleaned the throttle body. I have dual batteries, 2 800 cca red top optimas almost 2 years old that seem to have 12.5 volt right before I start the engine. The alternator is a mean green 200 amp, and once I start the jeep it holds steady over 14v, no matter what I throw at him, except the winch. The engine ground is also just cleaned and I tried to feed the engine ground (for testing purposes) with a jumper cable, which made no difference. I turned the engine on/off while trying multiple things so many times, I am positive the batteries hold enough power, or at least they seem to(which leads me to believe this issue isn't caused by lack of power for the computer). What I have noticed was when I cranked, the voltage dropped to about 8.5 volts while cranking, but within couple seconds after engine start it went to 14.3. Releasing the acceleration right when the voltage went over 14v, would still cause the jeep to stall/die, holding it few more seconds and then releasing it would allow the jeep to idle normally.

I have also noticed that pulling out the iac electric plug would make the jeep start and idle without pressing the acceleration pedal. I connected the old iac when the engine was idling (while the new one was still in the throttle body disconnected) and I felt some very light, intermittent pulsation, like 2 per second, after about 30-40 seconds the metal cone and the small shaft (that goes through the spring) from iac was completely unscrewed (maybe came out about 1/2 inch on the top of its previous position), so I assume the computer wanted to cut the air by pushing the cone (again this was on the spare iac, not mounted in the throttle body). One of the questions I have is if this is how it normally works, I personally would've expected a faster "reaction" for setting the iac air flow. The second question is what the heck should I do next? The only thing that I didn't do is to clean the computer contacts, unplug the harness and clean it, but I am not sure how effective this would be since the jeep runs fine once it can keep the idle. I haven't noticed any increased mileage either. And btw, forgot to mention, O2 sensors were both replaced about 10k ago, in 2013, with new ntk from Napa. Scanning for codes provided none, and no check engine light on either. Please chip in if you have any suggestions, I'm at the point that no matter how much I think, I have no other clue what to do to fix it...:confused1

Thanks a lot!

 

[Tim_MN]

The IAC is a normal maintenance item, if it does not function correctly, it should be replaced.

........every time I start the jeep, hot or cold engine, warm or cold ambient temperatures, the jeep fires up then stalls right away and dies. If I keep the acceleration pressed to maintain 800-1000 rpm, it won't stall/die, and after about 3-10 seconds, usually keeps the idle just fine even without accelerating (I say usually since few times I had to hold the acceleration more, for about maybe 20 seconds). It does leave a rich smell ....

For 1996+ Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator in not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM.


Your symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. The ECU will relearn the idle settings after a short period of driving and the engine will often start and idle normally.

Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem. Begin with basic trouble shooting 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 and replace them as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body.

If the symptoms reoccur, you need to have your battery and/or the alternator load tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Even with low battery voltage, the engine will start, but the ECU can re-boot multiple times and a new battery may be required.


Cleaning the Idle Air Controller (IAC), regular tune-ups, and using fuel injector cleaner should be part of your routine preventative maintenance.

 

[RoyalXJ]

Try resetting ecu

 

[Prom3theus]

Guys, thanks a lot. Resetting the computer fixed the issue (fingers crossed). That was the only thing left and I am amazed I haven't thought about such an obvious thing earlier on... I reset it by disconnecting the battery plus, ground it to the negative terminal for 30 seconds to discharge any capacitor, put it back on the battery plus, turn ignition on without start, turn on headlights, turn off headlights, ignition off. Then crank and so far, it looks good, it doesn't smell rich on cold as it did before and it smells/sounds normal. Effing computer gremlins... So the pointers here were very good, thanks a lot for your help!

 

[mechbullxj]

CPS?....

 

[RoyalXJ]

No problem glad it worked for you. After replacing or cleaning any sensor or doing any engine mods it's best to reset the computer so it can recognize what you did and relearn the normal operation.

 

[Prom3theus]

No problem glad it worked for you. After replacing or cleaning any sensor or doing any engine mods it's best to reset the computer so it can recognize what you did and relearn the normal operation.

Correct, that's what I believe the fsm also states... Well, I'll definitely remember this next time, lol.

Mechbull, I didn't consider cps, once it started and idled for a bit, the engine never stalled till next start, usually cps throws fits when gets warmer. Also, mine was replaced last year, more as preventive maintenance since I had a long trip for few weeks with the jeep and I didn't want to risk messing with it at the wrong time.