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  #1  
Old 06-04-2005, 07:06 PM
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Misfire at low RPM under load

I feel like I've tried everything here, and I'm trying not to just throw money at this thing. I don't have any error codes indicating a misfire, but I've had the notorious water in the spark plug well problem in the past and it feels very similar. At various speeds mostly when the RPMs are low I get a chugging feeling like it's misfiring but it's sporatic. It only seems to happen when the car is hot. I recently had an EGR problem so I've already changed the DP sensor, the EGR regulator, and cleaned the EGR ports on the throttle body to correct the problem, so I'm pretty sure it's not EGR related. I've change all 8 plugs and coil boots also. I also bought a coil and have been one by one moving it to the next cylinder. I've been through 6 or the 8 cylinders so far, but I don't think this is the problem.

One of the guys at the Ford place said to check the IAC valve. Another guy there said that the idle would be off it that was the case and it's not. I'm running out of options. I could buy 8 coils and hope that fixes it but I'd be real upset if that didn't fix it.
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  #2  
Old 06-04-2005, 07:55 PM
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Hi gtpenc and welcome to the forum! what motor, trans, etc. do you have? I don't see anything for a signature.
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  #3  
Old 06-04-2005, 09:47 PM
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Sorry, I left all the important stuff out didn't I. It's a 2000 Expedition, 4.6L, 2WD, Trans. is either 4R100 or 4R70W (I don't know how to tell what trans I have).
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  #4  
Old 06-05-2005, 12:05 AM
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You could still have a bad coil. I chased after a problem like this pretty much the same way that you have. How many miles do you have on the Expy? When you put in the new DPFE & EGR valve did you reset the computer? Have you flushed out you trans and torque converter? Changed the fuel filter? Put dielectric grease on the new silicone boots? When you fixed plug #4 did you put extra clamps on the heater hoses right above it?
I hope some of this helps

Good Luck and keep us updated!
Sean
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  #5  
Old 06-05-2005, 12:25 AM
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My 5.4l 97 Exp missed as u describe for a few weeks (without code) , then the code for Pass side O2 sensor (upstream)came up. I think it has to fail often enough to show a code. What a pain to get to on the 5.4l. Replced it and code never came back . Now 10k Mi later I replace the Driver side (milage now about 90k). If they send false mixture voltage to CPU, it makes the engine run lean and miss. It was when towing at low rpm under load only.
And the system ignores them until warmed up, hence OK when cold. o2 Sensors do get old above 80k Mi, and can affect drivability. Got Bosh at Autozone for $50 Good Luck Mark
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  #6  
Old 06-05-2005, 07:11 AM
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Thanks a ton for the feedback. It feels good to not be alone with this problem. I can usually solve these things through persistance, but I'm really starting to loose convidense with myself.

I have about 85K miles. Yes, I put dielectric grease on the boots. What exactly do you mean about did I "reset" the computer? I erased the error, but should I have done something more? Transmission has been flushed at least 2 or 3 times at the shop (not Ford) and once originally at around 30+K miles at Ford. Fuel filter only has about 10-20K miles on it.

When my initial misfire happened due to water; I'm pretty sure it was from going through some deep puddles at high speed. I don't know that I actually have the notorious leak on the hose there, but I'll double check. Either way, I've already replaced those plugs and boots and cycled the coil through those cylinders.

The O2 sensor thing sounds interesting and possible I suppose. How would I know if it's bad (and which one) other than taking it to the Ford place for diagnostics?
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  #7  
Old 06-05-2005, 11:16 AM
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As "97xlt" has stated, it may be the o2 sensors since you do have 85K on it. You only have to worry about the front two and they sell them at AutoZone for $49 each and they're pretty easy to change. (reset computer afterwards)
When you replace any sensors (electrical / emission) you should reset the computer so that it can relearn the new parameters. Disconnect the negative battery cable (and turn on the headlights for a complete drain) for 15 - 20 mins then reconnect the cable. Keep in mind that it's going to shift funny for a few miles until it relearns.

Good Luck!
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  #8  
Old 06-05-2005, 11:50 AM
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I'm definately going to do the computer reset as soon as I get off the computer since I never did that after fixing my EGR system. As far as checking to see if the O2 sensor is at fault, can I disconnect them temporarily to force the computer to go into default mode? If the problem goes away, that's the most likely problem??? I don't want to drop another 50 bills on a sensor unless I know it has a pretty good chance at fixing it. Is the o2 failure a common problem as I've described it?
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  #9  
Old 06-05-2005, 12:30 PM
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OK, I reset the computer and took it for a drive. No difference.
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  #10  
Old 06-05-2005, 01:53 PM
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gtpenc,
Have you changed the plugs and wires or are they OEM? My experience has been, at 80K, the wires are used up and the plugs could use changing also. The symptom was a "stumble" or miss on acceleration and during the climb up a hill. I don't know if you have the COP or not; if not, no wires to change.

Make sure you use dielectric grease on the wires, both ends, and that the plug boot "snaps" into place on the plug. I have had 1 back-off twice and caused a "stumble." Hope this helps.

Curt
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  #11  
Old 06-05-2005, 04:47 PM
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Curt, I changed all the plugs to Autolite Platinum this weekend and changed all the coil boots and used dielectric grease. No difference. There aren't any plug wires to change, every cylinder has it's own coil.

I'm trying to rule out the oxygen sensors before I drop 50+ bones on one. Does anyone know if it would work to disconnect the sensors to force the engine to run in a 'default' mode? That way if the problem goes away I can say the oxygen sensors are to blame???
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  #12  
Old 06-05-2005, 10:02 PM
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No real good test that I know of. One could watch the voltage, but the output just moves a few tenths of a volt and should cycle up and down based on fuel input. (i.e. closed loop control) The o2 sensors are ignored on cold start up for some period. ( seconds, minutes ?) , they are the 4-wire heated type so thecomputer can count on reading them sooner. Does it not miss during the warm-up period ?
I've had a few Fords over the years, and consider the o2 sensors as tune up Items at 80k or so, seen the effects of "Lazy" response that stays within
Computers OK range, put messes with drivability/response . The Ideal situation is if the light comes on,as the system has 4 o2 sensors. Only the front (upstream) sensors are for loop feedback, the other 2 ensure you cats are working. (OBDii self test). I read of the perils of people throughing hundreds on not need sensors. But the o2's do wear out,some sooner than others. On early Expeditions, the passenger side ones fail due to A/C water dripping right on top of them. On a 5.4 it is a near impossible tight job.
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  #13  
Old 06-05-2005, 10:17 PM
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gtpenc,
Thinking waaaayyyy out there, have you put any gas dryer or gas treatment in your tank? Could if be condensation? If the weather has changed a lot lately and you tend to run on 1/4 tank frequently, it MIGHT be the problem. Just thinking out loud....

Curt
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  #14  
Old 06-06-2005, 07:56 AM
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97xlt, it doesn't seem to do it (at least not nearly as noticable) until it warms up. This is half the problem with trying to fix it; it takes a good drive to show the problem. One thing I thought about, when I was moving the COP from cylinder to cylinder, I changed spark plugs half way through. Maybe I had a bad coil and a bad plug, so I missed it first pass??? I wish it would just throw the code!! If by this weekend it hasn't I'll probably just change the front O2 sensors and maybe even have the trans flushed.

Curt, I put a bottle of "Tune-up in a bottle" in early during this problem thinking maybe a clogged fuel injector, but I've refilled the tank since then.
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  #15  
Old 06-06-2005, 12:02 PM
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I don't have your specific answer but the Ford system operation is very critical for some areas and for some conditions (to good).
During light load, the EGR system is called into operation.
What happens is the fuel is cut back (leaned out) and spark advanced.
This leaves a lean condition in the cylinders making ignition much harder.
If any part of the ignition system is marginal and/or the EGR is flowing to much, you can experience a missing condition. If the miss is very short term, a fault code will not be set because there are not hard conditions present for enough time to call it a hard fault as far as the PCM is concerned, because it has to adjust to varying condition as you drive.
If any additives were used that result in a coating on the plugs, a missfire condition could develope at the time of EGR operation with an already lean condition that normally occurrs besides inital plug condition and COP coil and or wire health. Could even be the new EGR parts are out of tolerence and stacked against proper operation of the system.
This area of fault is often a real hangup to get hold of so patence and thinking is the order of the day.
I tow with a 4.6L super crew at over 11,000 lbs and do at times hear a light ping during auto gear shifting under load only but that is more how the PCM handles things (not EGR operations) than a real fault so there is not much to be done about it or be concerned except try 91 octane gas.
Good luck.
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Old 06-06-2005, 12:02 PM


 
 
 
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