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Discussion Starter #1
You can check my previous post history as this is a reoccurring problem but here’s the skinny.
2010 3.8 JKU w/67k on the odometer. The vehicle will sputter occasionally when stopped. I had this problem last year and after running sea foam they I take and fuel I thought I had it beat.

Now I am thinking since it’s returned it might be an exterior temp issue. It only happens when the temp is below 60. And it was around winter last year when I had the problem. I live in TX btw. Regular mX. Plugs and wires changed less then 18 months ago. Temp inside on info center reflects actual temp outside and no codes.

So I am leaning towards possibly a bad coil. Temp drops resistance changes and I get my problem. Any other suggestions? I have to change my valve cover gasket as it’s leaking and will tackle this at the same time.


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The bad thing about seafoam is it cleans too well sometimes. Its possible that you knocked loose some gunk and it entered your fuel injectors. There are filters in each injector, if they are full of trash no amount of seafoam is going to work.

The only good way to tell for sure is pull them and put them in a test stand. I would pull them and have them professionally cleaned.
 

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Same exact problem. Engine idles very oddly when it is cool out. Smooth as butter when it's warm out. Odd.


I would guess it’s a small vacuum leak. When warm the parts expand and seal better, when cold....


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Discussion Starter #6
Cats at 67k??? Not a lot of highway miles but still isn’t that a little early? And why only when cold?


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Cats at 67k??? Not a lot of highway miles but still isn’t that a little early? And why only when cold?


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The main difference between cold and warm is open vs closed loop in the ECU. When it’s cold it ignores data from the various sensors (most notable O2 sensors) and just uses internal defaults for fuel/air and ignition timing. When it’s warm it adjusts based on data from the sensors. Hence my suspicion around a vacuum leak. It can adjust once it’s warm.

It could also be a bad connection to a plug or coil that tightens as it warms, but that’s unlikely and it would also deteriorate very quickly over time. You can check this by opening the hood and looking at the motor right after a cold start. There would probably be arcing all over the place if this were what was happening.


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Discussion Starter #8
The main difference between cold and warm is open vs closed loop in the ECU. When it’s cold it ignores data from the various sensors (most notable O2 sensors) and just uses internal defaults for fuel/air and ignition timing. When it’s warm it adjusts based on data from the sensors. Hence my suspicion around a vacuum leak. It can adjust once it’s warm.

It could also be a bad connection to a plug or coil that tightens as it warms, but that’s unlikely and it would also deteriorate very quickly over time. You can check this by opening the hood and looking at the motor right after a cold start. There would probably be arcing all over the place if this were what was happening.


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I understand the difference between open and closed loop. Would this apply though based on exterior temperature and not engine operating temperature?

For instance the other day @ 650 am in the in the low 50’s. Drove my daughter to school and two mins after leaving my house w/ operating temp 1/4 - 1/2 of the way to normal it sputtered at the light. On my way back 15 min later now w it at normal operating temp it happened again.

However since temps have come up no issues. They are dropping back down again and I have another run to drop my daughter off and I am curious if it will happen again.


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I understand the difference between open and closed loop. Would this apply though based on exterior temperature and not engine operating temperature?

For instance the other day @ 650 am in the in the low 50’s. Drove my daughter to school and two mins after leaving my house w/ operating temp 1/4 - 1/2 of the way to normal it sputtered at the light. On my way back 15 min later now w it at normal operating temp it happened again.

However since temps have come up no issues. They are dropping back down again and I have another run to drop my daughter off and I am curious if it will happen again.


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Sorry, missed that in the original post. No, I don’t think this is a closed/open loop issue. I’m afraid I’m running out of ideas.

Maybe there’s a problem with your IAT sensor. Do you have an OBD II data logger app? Or even just a scanner that can show real time data.


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