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Discussion Starter · #1 ·
Could the hard start I'm experiencing with rough idle and P0300 CEL be related to the heat soak issue on some of the older TJs?

I figure I'm going to do the heat shield and heat shield injector sleeves either way.. but wondering if I should expect to keep having the issue afterwards?

If I'm likely to keep having the issue where would you check next?

I've done all the O2 sensors and the plugs already.
 

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Fuels system - injectors , pump, filter. Send the injectors out to be cleaned, check the fuel pressure and the pump.

Hard start can be fuel system, injectors leaking, low fuel pressure. High speed misfire can be low fuel pressure at rpm. 300 is generic misfire code.

Trouble shoot the fuel system.
 
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Discussion Starter · #5 ·
Fuels system - injectors , pump, filter. Send the injectors out to be cleaned, check the fuel pressure and the pump.

Hard start can be fuel system, injectors leaking, low fuel pressure. High speed misfire can be low fuel pressure at rpm. 300 is generic misfire code.

Trouble shoot the fuel system.
SteveVB,

I've always understood that. What I'm trying to learn is how cold start is rock solid everytime... vs a warm start being inconsistently bad.

It's also odd to me that after a warm hard start, rough idle, and CEL, I can turn off the engine, start it again and it fires right up and runs fine.
 

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SteveVB,

I've always understood that. What I'm trying to learn is how cold start is rock solid everytime... vs a warm start being inconsistently bad.

It's also odd to me that after a warm hard start, rough idle, and CEL, I can turn off the engine, start it again and it fires right up and runs fine.

Have you checked fuel pressure? After sitting and running?

I would guess that PCM cold start injector pulses may be long enough to overcome injector/fuel problems, and restarts don't get the same treatment from PCM . But Im just making a WAG.

I would test fuel system. Injector o rings, pump etc... its a 10plus year old vehicle, likely that multiple parts of the system are at the margin and together they fail for certain operating conditions.

If it were my vehicle I would clean or replace the injectors , new o rings for either. Put a gauge on fuel line see where I was pressure wise during starts, running, and over night. See where that takes me.
 

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Discussion Starter · #9 ·
Have you checked fuel pressure? After sitting and running?

I would guess that PCM cold start injector pulses may be long enough to overcome injector/fuel problems, and restarts don't get the same treatment from PCM . But Im just making a WAG.

I would test fuel system. Injector o rings, pump etc... its a 10plus year old vehicle, likely that multiple parts of the system are at the margin and together they fail for certain operating conditions.

If it were my vehicle I would clean or replace the injectors , new o rings for either. Put a gauge on fuel line see where I was pressure wise during starts, running, and over night. See where that takes me.
That's really helpful info! Thanks SteveVB. I didn't think about the cold start injector not kicking in on subsequent starts. Makes a lot of sense.

I'm out of town right now, but when I get home on Monday I'll get an update.
 

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Discussion Starter · #11 ·
The dealer's DRB scan tool can go much deeper into the PCM than the run-of-the-mill scan tool.
I can pull a lot of info from the one I have. It's the BlueDriver one. Last time I asked the dealer to dig deeper they provided no additional information. Perhaps your dealer is a gem.. but the 3 near me really suck. Never tell me anything new, I generally have to explain to them how it works.
 

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Discussion Starter · #14 ·
For those interested... The wiring harness connector pins were loose. I grabbed one from the junk yard cut off the old one and soldered in the one from the junkyard and we're back in business.

I could cause the code to throw while messing with the wires near the connector. That's how I discovered it.
 

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For those interested... The wiring harness connector pins were loose. I grabbed one from the junk yard cut off the old one and soldered in the one from the junkyard and we're back in business.

I could cause the code to throw while messing with the wires near the connector. That's how I discovered it.
Which wiring harness did you change?
 

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Pretty sure he was talking about the IAC harness.

I replaced the IAT already as the old one was throwing a different code. So that's hopefully not the issue.

But I had the same thought. Double checked the wiring to it too. It was a little frayed so I re-wrapped the wire just in case.
- DAA
 
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