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Old 08-19-2011, 07:18 PM   #1
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P 0300?

Ok when i first bought my jeep less than a month ago, i got two miles away from dealership when it started running badly and spit out p0300 codes. Brought back to dealer and they did a tune up and it ran well. Now everytime i get it above 70mph the engine light starts blinking. If i let off the gas it would go away. So when it did it today i punched it. Engine light is constantly on and p0300 codes. Rode like $hit at first but now runs normally. What is going on?

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Old 08-19-2011, 07:44 PM   #2
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300 is multiple misfires, multiple cylinders. Back to the dealer, it could be a slew of things, and since it was just worked on for the same code, they should take care of it.

Could be bad coil, cap/rotor, plug wires, valve springs, and so on.

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Old 08-19-2011, 08:42 PM   #3
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Shouldnt that have been addressed when they did the "tune-up"?
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Old 08-19-2011, 10:30 PM   #4
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It also running fine now. But engine light remains on.
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Old 08-20-2011, 11:19 PM   #5
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Any thoughts?
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Old 08-21-2011, 05:26 AM   #6
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It also running fine now. But engine light remains on.
have dealer scan it-take note of codes then clear it-see if light comes back on and if it does, same codes?
how "complete" of a tune up did they do? just change plugs or a complete tune up? pop the dist cap and chk out the contacts/rotor. also do wires look new?
personally i'd check everything they did-pop a plug, make sure new and correct one and correct gap. process of elimination-least you'll find out what it isn't and sooner/later should find out what it is.
is this a jeep dealer?
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Old 08-21-2011, 10:46 AM   #7
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Yes it is a jeep dealer.
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Old 11-26-2011, 07:34 PM   #8
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Ok old thread but it happened again. It seems to act up when i go 70+. Check engine light comes on, runs horrible, codes are multiple misfires. Since the engine doesnt have regular wires and cap and ect. what do I do?
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Old 11-26-2011, 07:41 PM   #9
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Take it to a quality repair facility.

Any good scanner (dealership quality, not home style) should be able to be able to plug and drive. This will give the tech live data, so they can actually diagnose the issue at 70 MPH, when the issue exists.

For example, I had a fuel pump that was *juuuustt* starting to go bad. It would run fine at low speed, but, as time would go on, the pump would start to heat up, eventually starting to lower the volume of fuel. I had a pressure guage hooked up, and from appearances, the fuel pressure looked fine. When we hooked up the live data scanner, you could see the injector pulses kept getting longer and longer and longer. It was obvious, my runability problem was lack of fuel. Replaced the pump, and all issues were solved.

A good scanner is a lot more than "pulling codes". A good scanner in the hands of a good driveability tech is a good thing.

Edit: and since this is in the TJ threads, you have a 5 to 15 year old rig. That means the coils, fuel pumps and almost all the electronic sensors are getting older.

A lot of times, a tune up is just done "just because". Most of the times, it's a good thing. But that doesn't mean it fixed your problem. But it was still probably a good thing, due to age.

Another thing to consider is carbon deposits. Some engines, especially if the prior owner uses crappy gas, will develop carbon deposits on the combustion chamber valves. Sometimes, these deposits will fall into the valve, causing it to remain partially open. By 'revving' the piss out of the motor, you can sometimes blow that deposit out, thereby clearing the missfire. Good fuels like Chevron have Techron, a top engine cleaner, as an additive. You can also buy it at your local auto parts store. Or you can use a Top Engine Cleaner if you think this was a culprit.
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Old 11-26-2011, 07:43 PM   #10
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Ok old thread but it happened again. It seems to act up when i go 70+. Check engine light comes on, runs horrible, codes are multiple misfires. Since the engine doesnt have regular wires and cap and ect. what do I do?
P0300 random misfire. I just encountered this very same condition on a 97 4.0. After quality tune up I ended up having to pull the head and grind the valves. It seems the two of the exhaust valves were burnt and it only really showed up after we would run it hard on the hiway. Cruising around at low rpm there was never a misfire detected. Keep us up to speed on the resolution.
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Old 11-26-2011, 07:55 PM   #11
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P0300 random misfire. I just encountered this very same condition on a 97 4.0. After quality tune up I ended up having to pull the head and grind the valves. It seems the two of the exhaust valves were burnt and it only really showed up after we would run it hard on the hiway. Cruising around at low rpm there was never a misfire detected. Keep us up to speed on the resolution.
Easy to diagnose with a basic compression test.
I haven't heard many of these.
Much more common for the 6 cyl to lose a crank sensor. But that's definately NOT the case here, as it wouldn't run at all if that happened.
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Old 11-26-2011, 09:20 PM   #12
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Easy to diagnose with a basic compression test.
I haven't heard many of these.
Much more common for the 6 cyl to lose a crank sensor. But that's definately NOT the case here, as it wouldn't run at all if that happened.
Right on, I was summing it up. There was a ton of diags before going to mech problems.Mines off and running without issue now.

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