Strange Warm Start 4.0 - JeepForum.com
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post #1 of 16 Old 02-05-2019, 06:16 PM Thread Starter
JonS
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It starts right up cold and restarts fine while still in open loop. However once at operating temperature in closed loop if I shut it off and restart it 5 minutes later it has a definite miss until I rev it in Park. I checked the injectors warm and they all read fine. It feels like a flooded engine which clears with reving. Plugs, cap wires fine. If I don't stop it will run forever nice and smooth. Ideas? No codes come up.



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post #2 of 16 Old 02-06-2019, 12:33 AM
Mr. Puddles
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Does it studder and kick when you drive it like that? Does shutting it down and restaring again fix the issue? Does it do this till about 3k rpm?

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post #3 of 16 Old 02-06-2019, 08:25 AM
Some_Anchovies
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It starts right up cold and restarts fine while still in open loop. However once at operating temperature in closed loop if I shut it off and restart it 5 minutes later it has a definite miss until I rev it in Park. I checked the injectors warm and they all read fine. It feels like a flooded engine which clears with reving. Plugs, cap wires fine. If I don't stop it will run forever nice and smooth. Ideas? No codes come up.
I have this issue every once in a while. Stumbles for a bit and then once it clears it runs smoothly. Reving up a tad seems to clear it immediately. I chalked it up to an injector sticking open. Ran some PEA through the tank a couple 400 mile trips and I don't have the issue anymore. IDK if that will fix your issue though.

95' ZJ 4.0L, 5-Speed Swap, NP231, 275k
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post #4 of 16 Old 02-06-2019, 08:40 AM
zjosh93
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That's not uncommon for the 4.0. Couple things:

On the later 4.0s (not sure if 98 counts) they had an issue with injector heat soak. Mopar and aftermarket places make insulation kits.

Some people have had issues with air trapped in the rail. If you hit the stem of the fuel pressure test port when it's hot and vapor comes out you might fix it. This happened to me two or three times on my 93 when I had the injectors out.
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post #5 of 16 Old 02-06-2019, 08:43 AM
jtec
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try a fuel system cleaner for a likely inj issue a good step 1, my choice 'seafoam' or 'techron'

You could do a fuel pressure test turn key on engine off observe and post PSI then watch to see if pressure drops, post drops Xpsi in 5minutes.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #6 of 16 Old 02-06-2019, 03:00 PM Thread Starter
JonS
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I've run 3 bottles of Techron (1/tank full) in the past month. Thought that fixed it but it returned. I'm thinking a leaking injector too. Got a Check Engine light a few weeks ago diagnosed as Engine Misfire Cylinder 5, but the light hasn't come on since.

It's just an annoyance but I'm living with it so far.


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post #7 of 16 Old 02-07-2019, 02:07 AM
Some_Anchovies
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I've run 3 bottles of Techron (1/tank full) in the past month. Thought that fixed it but it returned. I'm thinking a leaking injector too. Got a Check Engine light a few weeks ago diagnosed as Engine Misfire Cylinder 5, but the light hasn't come on since.

It's just an annoyance but I'm living with it so far.
Hook up a fuel gauge to the injector rail and see if you have a leakdown problem after the engine is shut off. You can also pull the injector rail off with the injectors attached and see if that one is leaking fluid. Or pull plug #5 and see if you notice a strong fuel smell coming from that cylinder.

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post #8 of 16 Old 02-07-2019, 03:11 PM Thread Starter
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Will have to borrow a pressure gauge but did pull plug 5 and plug 4. Both look totally normal, no smell of gas and no carbon or oil.


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post #9 of 16 Old 02-07-2019, 10:51 PM
ZeeJay1997
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Fuel pressure could be the problem but Have you thought about the front O2 sensor? It can cause a rich condition and can cause misfire codes without the PCM coding the sensor.


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post #10 of 16 Old 02-08-2019, 07:28 AM Thread Starter
JonS
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Quote:
Originally Posted by ZeeJay1997
Fuel pressure could be the problem but Have you thought about the front O2 sensor? It can cause a rich condition and can cause misfire codes without the PCM coding the sensor.
Since it only happens upon warm re-start and disappears with a rev or two, no. It runs like a top once cleared out. Could that be a faulty O2 sensor?


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post #11 of 16 Old 02-08-2019, 07:37 AM
jtec
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DO THE SUGGESTED FUEL PRESSURE TESTING...

Guessing gets expensive....

I see No reason to suspect o2 at this time.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #12 of 16 Old 02-08-2019, 07:53 AM
ZeeJay1997
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If the fuel pressure testing is ok, unplug it and see what happens. Doesnt cost anything.


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post #13 of 16 Old 02-08-2019, 08:35 AM
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, unplug it and see what happens. Doesnt cost anything.
I think ZeeJays means "unplug o2 sensor" and advise on drivability -

Easy free my kind of test...

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #14 of 16 Old 02-08-2019, 08:47 AM
ZeeJay1997
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Unplugging the O2 sensor will keep it in the open loop mode, running with default fuel, and may mask the real problem. But at least you'll know whether or not the problem is caused by closed loop.


If you decide it is actually something in closed loop causing it, here's a link to the mode description.


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post #15 of 16 Old 02-08-2019, 04:20 PM Thread Starter
JonS
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Quote:
Originally Posted by ZeeJay1997
Unplugging the O2 sensor will keep it in the open loop mode, running with default fuel, and may mask the real problem. But at least you'll know whether or not the problem is caused by closed loop. If you decide it is actually something in closed loop causing it, here's a link to the mode description.
ZJ - That link is FANTASTIC. Thanks!


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