Long Cranking Start - Intermittent (3.6L)

[jeepfam]

Has anyone dealt with an intermittent long cranking start on their JL? I cannot for the life of me replicate it. It happens at random times. It's happened on a cold start and after running for a bit and then restarting after 15min - 1 hour of sitting. I happened in Aug, then about a month or so later, and last Saturday. I cannot pinpoint anything common that is triggering it which makes it hard to chase down...totally random and intermittent.
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[Jebiruph]

Have you checked that the battery cables are tight?

 

[jeepfam]

Have you checked that the battery cables are tight?

I have checked battery cables and made sure all fuses were pushed in tight. I even started using a different gas station to see if maybe it was bad fuel. Still pops up randomly.

 

[jeepoch]

Jeff,

An engine start is rather sophisticated in today's age of electronic controls. Lots of things all have to function and line up in their proper sequence. However, the basic theory is easy to understand.

You need fuel, air, spark, sufficient piston compression, a non-blocked exhaust and enough rotational energy to crank.

If all of these are available and the controller delivers the correct stoichiometric fuel/air mixture (typically via a pulsewidth of fuel at a specific manifold pressure from the fuel injectors) at the piston's intake cycle. Followed then by adequate spark delivered near the piston's top-dead-center position. With enough compression to ensure no lost combustion energy when the fuel/air mixture ignites. And finally only if enough pistons are all contributing to overcome rotational engine drag will you have a successful 'start'.

So lot's of variables and moving parts. The crankshaft position sensors are also very critical. This is used to track each pistons position (intake / output cycles as well as their top-dead-center spark firing determinations).

For a random problematic starting condition, we can rule out certain things. Such as compression, crankshaft timing, and exhaust blockage. These would all lead to way more frequent problems, like never being able to start.

This leaves then fuel, air and spark. On this list could also be anything that the engine controller depends on for calculating the amount or duration of any of the above. This can include O2 sensors (both upstream or downstream of the catalytic converter), the Manifold Air Pressure (MAP) sensor, the fuel pump as well as the evaporative purge system. And yes, even the quality of the fuel.

If the problem is somewhat frequent, then any of the circuits that deliver fuel, air or spark, such as wiring or their connections, or even the various components themselves, fuel injectors, spark plugs, O2 Sensors etc would be suspect. However, the modern engine controller is pretty darn good at detecting faults in these circuits and should blow fault codes. Of which should easily be discernable via your cluster's EVIC display.

Lastly, I would suspect a clogged fuel filter.

Unfortunately, a lot of people don't realize that not all underground fuel station tanks are clean and contaminant free. Sediment can and will seap into these tanks and build up over time. NEVER FILL UP WHEN THE TANKER IS THERE! When the fuel station tanks are being refilled, the new fuel stirs the sediment up and it then takes time for everything to settle back down to the bottom. Fuel is typically lighter than dirt or other contaminants so the fuel will become pure enough again over time. How much time is anyone's best guess.

So my unscientific (based on uneducated assumptions due to lack of detail), would be:

1. Change gas stations.
2. Inspect or replace your fuel filter.
3. Look for any blown fault codes.
4. Inspect all spark cables.
5. Inspect / replace your spark plugs.
6. Inspect / replace your fuel injectors.
7. Inspect / replace your O2 sensors.
8. Have the dealership run a diagnostic on your engine controller.
9. Verify your starter is cranking.
10. Sell your vehicle hoping the next one runs better.

Jay

 

[Ruby Clatterbox]

My JK V6 gas did that once in a while. It would crank for like 5 seconds before starting, then finally fire. Don't know why, never took it in cause it never had any rhyme or reason to it.

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