Intermittent Sputtering Engine Barely Able to Cold Start - 2024 JLU 3.6L Manual

[One Eyed Willys]

Hey guys, looking for opinions on what could be causing this rough start/no start.
I have a 2024 JLU Willys 3.6L, manual trans. Got ~27.5k miles on it.

Today I got in and tried to start it up (~32F out, cold start). The engine turned over initially but then could not maintain idle, sputtered, and shut down. I tried once more with the same result. The third time, I tried to again to start it as normal and it sputtered but then smoothed out and held normal idle and appeared to work as normal.
Attached is the video of this last 'successful' start; the video starts just after I pressed the start button. My hand was off the start button and feet off the throttle, clutch, and brakes with the truck in neutral.
I checked the info in the gauge cluster, all seemed normal. Normal starting temps, 31-34 psi oil pressure, 14.6-15.0V on the battery.

I did have this issue twice before, once a couple weeks ago and once more like 8 months ago. Each time was a cold start (though not necessarily cold outside) and it sputtered out on the first start but then started right up no issue the second time. At the time I blew it off as me not pressing the start button long enough, though that seems stupid in hindsight. Intermittent, can't replicate it consistently at all.
Kind of acts like a snowblower/atv with closed choke on a cold start. Maybe something wrong with fuel mixture like an issue with the fuel pump? Gas mileage has been low at 14-15 mpg recently but I chalked that up to winter gas and cold weather with lots of idling.

I dropped it off today at the dealer so they can look it under warranty, but still curious what y'all think it could be. Want to have confidence any repairs will actually fix the problem and that there's no permanent damage. This issue seems different to me than previous posts on here about rough idle or sputtering engines.

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[JLRyder]

No ck eng light? No codes? Did I miss that? Pretty strange in that regard alone.

 

[One Eyed Willys]

No ck eng light? No codes? Did I miss that? Pretty strange in that regard alone.

No check engine light, other idiot lights, or anything. Don't have a code reader (I really should get one) and it's stuck at the dealer for now so couldn't check that.

 

[BCDube]

I had a similar issue the last two weeks but not often enough that I could troubleshoot. I did check for error codes but nothing, Since filling up the tank a couple days ago it has not happened again. Don't want to bring it to dealer just yet.

 

[One Eyed Willys]

I had a similar issue the last two weeks but not often enough that I could troubleshoot. I did check for error codes but nothing, Since filling up the tank a couple days ago it has not happened again. Don't want to bring it to dealer just yet.

It first happened to me when I had a a very low tank, like less than 1/8" full and ~40 miles range on the dash. Immediately went to the station to fill up thinking that would help and it was fine for the next 8 months.

Haven't let it get that low since but wonder if that was part of the problem? I know it's hard on the fuel pump running the tank low but I can't imagine one time not even to empty would be enough to cause real issues.

 

[BCDube]

The same here with the tank about 1/8 full and I park on the driveway which has a 5 degree slope to it. I also park face out if that makes any difference to the fuel pickup. And since filling up a few days ago I have not experienced an issue with starting. So maybe were on to something here?

 

[azwjowner]

I'm no expert mechanic, but upon reading the first post, my first thought was insufficient fuel pressure for some reason (it builds on multiple attempts). So your experiences with low tanks and slopes makes sense...

 

[AUXITO]

Possible causes of this problem:
1. intermittent failure of fuel pump or fuel pressure regulator, insufficient pressure when cold.
2. reduced performance of ignition coil or spark plugs at low temperatures.
3. unstable signal from crankshaft/camshaft position sensor.
4. carbon buildup on throttle body or idle speed control valve.
5. Injector nozzle partially clogged, under-injection on cold start. 6.
6. Fuel quality or additive problem.
7. Faulty engine coolant temperature sensor, causing ECU to incorrectly calculate the mixture.

My advice would be to first check for fault codes, even if there is no light on, there may be a stored PENDING code. Then check the fuel pressure, especially the holding pressure when cold. Test the fuel pump's ability to hold pressure, if the pressure drops quickly, there may be a problem with the fuel pump or regulator. Check the condition of the spark plugs for carbon buildup or gapping problems. Clean the throttle body and check the idle control valve. Clean the injectors with a fuel additive. If the problem persists, you may need to check sensor signals, especially the crankshaft position sensor, or replace the fuel pump.

It's best to have it checked directly at the dealer.

 

[alphawolff]

This is most certainly a fuel issue. Either a failing pump or contaminated fuel.


If it's only happening on steep grades at extremely low fuel level then you probably have your answer. It's odd, though, as the fuel pump module's design has a bowl that collects a decent amount of fuel specifically for hill starts like this. If it ran fine when you parked it then started like this the next day I'd strongly suspect the fuel pumps bowl isn't sealed correctly allowing the fuel to drain back into the tank. The pickup then struggles to refill it on the first start due to the low fuel level.

It's good practice to not let your fuel level drop below 1/8th of a tank in regular use in general. I'm not sure if this would warrant a fuel pump replacement unless you really wanted it done.

 

[One Eyed Willys]

Thanks for all the thoughts so far everyone. Good advice, will definitely be buying a code reader.

Small update after talking to the service guy. He said it did throw some codes and that the tech found a software update that he said matched the description of issues I was seeing. He couldn't tell me any more detail over the phone, so I'm hoping I can get a lot more on what the codes were and what this software update addresses specifically later today.
Pretty suspect answer in my opinion since it's so intermittent and it appears to be a rare problem.

Will update later as I get more info.

 

[One Eyed Willys]

I'm no expert mechanic, but upon reading the first post, my first thought was insufficient fuel pressure for some reason (it builds on multiple attempts). So your experiences with low tanks and slopes makes sense...

Low fuel and slopes are not a good combo, however this most recent occurrence was at half a tank. Could be that the pump was degraded already, but I really don't see how running a fairly new truck at low fuel for 1-2 days total should be causing a permanent issues. If that were the case, this issue would be extremely widespread. Plenty of people on this forum talking about running it all the way to empty.

 

[alphawolff]

Thanks for all the thoughts so far everyone. Good advice, will definitely be buying a code reader.

Small update after talking to the service guy. He said it did throw some codes and that the tech found a software update that he said matched the description of issues I was seeing. He couldn't tell me any more detail over the phone, so I'm hoping I can get a lot more on what the codes were and what this software update addresses specifically later today.
Pretty suspect answer in my opinion since it's so intermittent and it appears to be a rare problem.

Will update later as I get more info.

I'm not aware of any such TSB with the 3.6L. The last JL 3.6L PCM update we got was for the rampant o2 sensor failures we saw last summer. He's probably confusing the o2 run rough concern for this one.

 

[azwjowner]

Low fuel and slopes are not a good combo, however this most recent occurrence was at half a tank. Could be that the pump was degraded already, but I really don't see how running a fairly new truck at low fuel for 1-2 days total should be causing a permanent issues. If that were the case, this issue would be extremely widespread. Plenty of people on this forum talking about running it all the way to empty.

Testing the fuel pressure would probably be a good step here. I agree that low fuel shouldn't cause the problem, but a weak pump could certainly be exposed, perhaps.

 

[One Eyed Willys]

I'm not aware of any such TSB with the 3.6L. The last JL 3.6L PCM update we got was for the rampant o2 sensor failures we saw last summer. He's probably confusing the o2 run rough concern for this one.

Another update talking to them again. He didn't give me a TSB number but 99% sure the tech was referencing this TSB you posted as the fix. Mentioned the "vehicle runs rough" as the condition that led them to it.
He said there actually was not many codes, he mentioned one about a out of range battery voltage (maybe that's P0563 or P2504?).

Asked if they could hold onto it another day and check it out again tomorrow. I'm sure it will run fine when they test regardless of if its truly fixed. Not much else I think I can get them to do at this point.

Hoping that this software is somehow a fix but will probably try to do some of the trouble shooting steps listed in the previous posts on my own such as checking fuel pressure. Maybe throw a can of Sea Foam into the tank.

 

[alphawolff]

Testing the fuel pressure would probably be a good step here. I agree that low fuel shouldn't cause the problem, but a weak pump could certainly be exposed, perhaps.

Seconding the pump. I suspect the bowl isn't holding fuel so the pump struggles starting on inclines when low on fuel.

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