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  #1  
Old 05-29-2015, 05:42 PM
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mis-fire codes but does not mis-fire

The symptom started when I was traveling at ~60 in a 70 mph zone and pushed the gas to the floor in order to pass a slow moving truck going up a steep incline and get back up to speed. Jeep down shifted, rpm's increased but engine lost power and forward speed slowed, check engine light came on (misfire). I let up on the gas, the trans shifted and the jeep came back up to speed.

I dropped it off at the jeep dealer and they keep it for two days. They serviced (pan drop) and checked the trans - good, the fuel pressure - good, the exhaust backpressure - good, new spark plugs. They wanted to keep it but I needed it over the weekend so no resolution.

So for now, I feather the gas (let up on the gas when I feel the trans need to shift) and have got up to ~90 mph. The jeep runs fine, no hesitation, trans shifts fine, no problems. But under load and gas press to the floor, lose power, CEL flashes and then lights up steady with misfire codes (various cylinders). I've repeated this sever time.

I look through the forum but can't find a solution. Lots of good suggestion and thanks but unfortunately folks don't seem to reply with a "that worked!"

I'm thinking about replacing the coil pack and TPS but am seeking advice on whether this might solve the problem.

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  #2  
Old 05-29-2015, 07:19 PM
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Re: mis-fire codes but does not mis-fire

Welcome to the garage.
This seems to be a fuel supply problem. Did the stealer check the fuel pressure at the moment that the problem is there? I wouldn't believe for now that a new TPS will solve the problem and each cylinder has its own coil, so i would skip that also for now. Eventhough the fuel pump can deliver enough pressure without flow, its still the question if it does when the fuel starts flowing, esspecially during high fuel flow at WOT. I would start putting a new fuel filter/regulator on it.
How is the mpg? If it is lower then before, then its also pointing towards a lower then normal fuel pressure.
That you say that you didnt feel the misfires, also makes me think that it is something for all cylinders. A single or some cylinders misfiring, while the rest is still ok, makes the engine run rough, while if they all are starving from fuel, you simply loose power, but it still runs smooth.
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  #3  
Old 05-30-2015, 02:10 PM
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Re: mis-fire codes but does not mis-fire

Thank You! Your reply describes the loss of power exactly but given that it seams like a fuel problem and the stealer OKed the fuel pressure, How would you test the pressure or fuel flow when the engine is under extreme load and again, why a mis-fire on cylinders x&y&z instead of a low fuel pressure code. I had my doubts when the CEL come on the second or third time with different cylinder mis-fire codes. Under normal driving conditions the jeep runs perfect, no complaints. Oh one other symptom you nailed, the gas mileage has dropped slightly but I thought that might have to do more with larger AT tires.

How can I narrow the problem down to the fuel filter/regulator before dumping more money on the problem?

Thanks again for your effort to help.
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Old 05-30-2015, 02:57 PM
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Re: mis-fire codes but does not mis-fire

There is no fuel pressure sensor, so you will not get any fault code for low fuel pressure. The fuel filter/pressure regulator is quite cheap, around USD 30 i believe, so would be my first try. Changing it can be a challenge, since the "quick disconnect" connectors are not anymore so quick to disconnect after so many years of dirt, salt etc be working on them.
That the misfire codes change to other cylinders each time tells you that there most probably is nothing wrong with all those cylinders/valves/plugs/injectors, otherwise the fault should return each time at the same cylinder(s).
Eventhough the problem can be the filter/regulator, it can still be the fuel pump itself as well.
The stealer tested the fuel pressure, but did they tell you how they did it. Just checking the static pressure is one test, but checking it with the engine running and at WOT is what could indicate a too low pressure when the filter/regulator doesnīt allow enough fuel to pass by or if the pump canīt deliver enough flow.
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Old 05-30-2015, 03:31 PM
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Re: mis-fire codes but does not mis-fire

Quote:
Originally Posted by Frango100 View Post
There is no fuel pressure sensor, so you will not get any fault code for low fuel pressure. The fuel filter/pressure regulator is quite cheap, around USD 30 i believe, so would be my first try. Changing it can be a challenge, since the "quick disconnect" connectors are not anymore so quick to disconnect after so many years of dirt, salt etc be working on them.
That the misfire codes change to other cylinders each time tells you that there most probably is nothing wrong with all those cylinders/valves/plugs/injectors, otherwise the fault should return each time at the same cylinder(s).
Eventhough the problem can be the filter/regulator, it can still be the fuel pump itself as well.
The stealer tested the fuel pressure, but did they tell you how they did it. Just checking the static pressure is one test, but checking it with the engine running and at WOT is what could indicate a too low pressure when the filter/regulator doesnīt allow enough fuel to pass by or if the pump canīt deliver enough flow.
That was quick, thanks. I'm OK with the price (~$50.00) but having to drop the gas tank is a problem. Some of the info has it out side the tank and some have it in the tank. I have a 2002 GCL 4.7 V8, do you know if it is in the tank or does it have 2 filters?
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Old 05-30-2015, 03:36 PM
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Re: mis-fire codes but does not mis-fire

The filter/pressure regulator sits outside of the tank, just in front of the rear axle left side. There is a filter screen in the pump inlet, but that one is normally not the problem.
Filter/regulator is item 7 on the drawing.
On Rockauto.com they sell the filter from USD 14 to USD 36.
Attached Files
File Type: pdf Fuel filter-regulator.pdf (116.2 KB, 7 views)
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Old 05-30-2015, 05:34 PM
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Re: mis-fire codes but does not mis-fire

Thanks! I'll give it a shot but before I do, I will have another pressure test to narrow it down to the filter/regulator or fuel pump or possibly neither.

Will post the results.
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Old 05-30-2015, 07:13 PM
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Re: mis-fire codes but does not mis-fire

Ok, but then they need to check the fuel pressure while driving and when the problem is there.
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Old 06-10-2015, 10:25 PM
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Re: mis-fire codes but does not mis-fire

The fuel filter made no difference but I tested it 4 or 5 more times. When I hold the gas to the floor and keep it there even after it starts to lose power, it seems to lurch forward a bit like its running out of gas? I don't know that this is important but will also do this at slower speeds. Also how would I test the fuel pressure while this is occurring?
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Old 06-10-2015, 11:36 PM
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Re: mis-fire codes but does not mis-fire

You need to connect a fuel pressure indicator on the fuel rail and keep it in a viseable place during driving. Maybe stick it to the outside windscreen with tape.
Do you have a scanner to check the sensor outputs? Not getting enough fuel for acceleration can be caused by several sensors. Observing them could give a better idea what is wrong. I would check the MAP sensor output, eventhough it is difficult to determine what the pressure should be at WOT.
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Old 06-11-2015, 11:23 AM
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Re: mis-fire codes but does not mis-fire

One other fact, the check engine light flashed several times during the test but went out without a code. The 3 turns of the key showed "done."

Although the stealer said they checked the fuel pressure, I will test the fuel pressure and flow rate and see if that gets me closer to a solution. Thanks again!
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Old 06-11-2015, 11:40 AM
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Re: mis-fire codes but does not mis-fire

If you have a smartphone, you could buy a cheap bluetooth dongle and get the free Torque app. With this app you can read and clear fault codes, but also monitor many engine parameters. The dongle costs around USD 15 from e-bay and is a cheap but very usefull tool.
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