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Rough idle at cold startup and PO302

 
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Old 24 Dec 2017, 11:34 am
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Default Rough idle at cold startup and PO302

Hi All,

I'm posting this in hopes that it can help others, and maybe provide me some feedback for my PO302 error code. As I was chasing the cure for my PO302 code, I decide to replace the lower air plenum seals (also known as intake manifold gasket). Well, when I got the manifold off I found a lot of air intake matter buildup (some call this carbon) inside the lower plenum and inside the ports of the head. I vigorously cleaned all of the areas with a can of carb cleaner and a toothbrush, there was a of cleaning to do on my 2004 base model, equipped with a 5 speed manual, and 140K on the odometer. Well long story short, the early morning rough idle problem disappeared completely. I am relatively sure it was the cleaning out of the muck that cure the problem, as I had performed an earlier test on the intake seals that proved negative for a leak. FYI, the PO302 code, intermittent idle misfire, and low idle speed continue to plaque me. I have already tried all new gaskets on the plenum and valve cover, new plugs, wire, coil pack, and cam shaft positioning sensor (not a Mopar one though). Cleaned IAC, throttle positioning sensor, and the throttle body, cleaned and ohms tested all the injectors. My cold compression test show no more than 3 psi difference between cylinders, with the low being 145 and the high being 148. The car starts and runs like a top otherwise.
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Old 24 Dec 2017, 06:14 pm
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Default Re: Rough idle at cold startup and PO302

Seems u you covered a lot of ground. Have u did a fuel pressure check to see how much output is coming from the fuel pump? You mentioned low idle speed and P0302 cylinder #2 misfire. If that injector is weakest of the 4 it might manifest at that cylinder first.
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Old 24 Dec 2017, 06:51 pm
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Default Re: Rough idle at cold startup and PO302

When you clear the code, how long does it take to reappear?
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Old 25 Dec 2017, 12:56 am
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Default Re: Rough idle at cold startup and PO302

replace the computer next
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Old 25 Dec 2017, 08:03 am
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Default Re: Rough idle at cold startup and PO302

If the throttle body hasn't been cleaned regularly, I'd try cleaning it again, but off the car.

I'd remove it and take off the TPS. Then I'd use some throttle body cleaner and a tooth brush to carefully scrub everything clean. Work over a catch pan and note if / how much gunk is removed. The throttle body deposits can be very stubborn. Check the throttle plate shaft for excessive play or looseness.

Also, I'd check for leaks in the vacuum system.

Low cost stuff to try, you might get lucky.
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Old 25 Dec 2017, 08:15 am
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Default Re: Rough idle at cold startup and PO302

Did ya check the magnet behind the cam sensor to make sure it's not cracked? It can happen if the cover has been over torqued. Torque on it is very light. I don't know it, but THINK it's around 8 (?) INCH pounds.
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Old 25 Dec 2017, 09:01 am
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Default Re: Rough idle at cold startup and PO302

I've been chasing down this pesky PO302 for nearly half a year
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Old 02 Jan 2018, 11:49 am
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Default Re: Rough idle at cold startup and PO302

HI All,

Thanks for all the inputs. I have found my problem (THE FIX). It was a bad connection in the wiring harness going to the injectors. It was the large connector that is attached to the block under the lower air plenum (intake manifold). I cleaned it out with parts cleaner and applied some non-oxidation type grease and the code is gone and the car idles perfect. I hate to admit this this, but I think I caused the problem by pressure washing the engine. Anyway, if you are getting the PO302 code and have a regular idle misfire, try cleaning and drying out this connector. I would also recommend wrapping this connector in something to prevent future water intrusion, if you are a neat freak like me and have to wash your engine. Thanks for your ideas and time.
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Old 02 Jan 2018, 03:51 pm
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Default Re: Rough idle at cold startup and PO302

it still makes me laugh how much time I spent re protecting the dang harness in the engine compartment but very glad I did, even 2 wire sensor leads got wrapped in 3/16" fuel line (slit open w/ a razor) and die electric grease in a gazillion connectors.
great thread- we need more like this! btw, pressure/steam washing is great but its a really good idea to disconnect the ol battery! and don't destroy the radiator fins...
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Old 03 Jan 2018, 03:06 am
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Location: Grass Valley, CA
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Default Re: Rough idle at cold startup and PO302

Using fuel line to wrap the wires is a good idea. On this large connector, I am going to cut a piece of inner tube, separate the connector, slide the tube over it and then reconnect it. I thought I could just zip-tie the ends. It won't be waterproof, but highly water resistant I believe.
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