engine misfire, cuts out only under load - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 11-17-2014
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engine misfire, cuts out only under load

ok, my brother got a 97 ranger, 4o auto. was complaing about gas mileage, 14mpg around town. figured to do tune. replaced plugs, air filter clean, looks new, went to replace 02 sensor just due to age. got drivers side and post cat changed, but pass side stuck in pipe and didn't have time to drop pipe to get it out. since that truck now gets 17-18 in town, 22 hiway. BUT, when accelerating at or above 2500-3000 there is a major bucking, feels like a misfire. or maybe ttrans lockup and out but very quick. no codes are stored. i thought we got the air filter/intake tube on crooked, and leaking but not that. engine idles fine, runs ok at light throttle, but accelerate hard or hit a grade on the freeway and this thing just stutters badly. you can keep your foot in it and it will clear out once revs climb. or back off and stutter goes away. was hoping someone may have an idea. i was thinking maybe tps? but i live 200 mile from my brother so hard to diagnose. will see him on turkey day, so.... any ideas out there? thanks for any advise/info -. mileage about 128k, previous owner had "lower intake gasket" or something like that replace according to records in glove box. ing wires, etc look ok, but did not meter test them
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Old 11-17-2014
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Lower intakes on the 4.0l OHV loosen up, they often need to be retightened, only takes a few lbs of torque so don't over tighten them, spec is 11-13ftlb

Clean MAF sensor, common issue for "bucking" on 4.0ls.

And yes that air tube from MAF to intake needs to be air tight, any leaks would be the same as a vacuum leak because all air going into the engine needs to come thru the MAF sensor.

Check EGR system, if it has one, could be opening EGR valve too much fouling up air/fuel mix until RPMs can catch up.
You can just pull off the vacuum hose from EGR valve, plug the hose, then drive truck to see if bucking is gone, if not then EGR is not the issue.
The CEL will come on after a few miles, don't worry, it will turn off after EGR valve is reconnected.
Computer uses a DPFE sensor to tell how far the EGR valve is open, it has 2 hoses going to exhaust manifold, make sure both are clean, don't reverse them.

If it was TPS related, i.e. a dead spot, then it would occur at same place on the gas pedal, and it wouldn't be through a range of RPMs, it would be sudden and then quickly recover if gas pedal position was changed, so TPS doesn't fit your description.
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Old 11-28-2014
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ok, cleaned maf, ck'd nuts at lower intake, all tight. disconnect egr and plug hose, still stuttering, the hoses at DPFE are clear. what else might we start looking at? any and all suggestions greatly appreciated. again, no codes are being stored. thanksmmjp
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Old 11-28-2014
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'97 was the last year of the 40psi return line fuel system with a fuel pressure regulator(FPR) on Rangers.
'98 and up use 65psi returnless system, no FPR

Have him pull off the vacuum line on the FPR and check it for fuel or fuel smell from leaking FPR diaphragm, although I would expect MPG to still be down if this was the case.

Only other thing I can think of is spark.
If he can check if it does the stuttering only after warm up it could be coil pack starting to fail or a spark plug, it could do it cold but after warm up would be more likely.
When under load the fuel mix gets richer so hotter spark is needed or you get misfires.
All misfires are not stored as codes, it can be hit and miss with these, pun intended.
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Old 11-28-2014
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thanks ron, ck'd fpr hose, dry and no smell. will see about r&r the coil pack and see what we get. i really appreciate the info. hope to get this running right soon. my bro has a little place in thailand he goes to every winter and is leaving in a week or so. hope to settle this before he goes. again, thanks much, jp
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Old 11-28-2014
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hi, just swapped out coil pack from jy. engine seems to idle better and slightly better acceleration at start from stop but my 'bucking' still there. so still looking. thanks again. jp
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Old 11-28-2014
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I hate to say this because I hate spending money on parts, lol, but.......

Because another coil improved the symptom, low spark voltage may be the issue, so I would look at putting on new spark plug wires.
Spark plug wires last a long time but............they do deteriorate, usually at the spark plug end, hard to test that since the issue would only be seen at high voltage
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Old 07-02-2015
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My 01 Ranger automatic 4.0 tends to be rough when just started. Then it smooth's out. Once on the road, I get a hard buck around 40 to 45 mph. Never bucks at high end while cruising or at low speeds. It was worse the other day when I was pulling a small trailer. On these models, it is in overdrive unless you physically remove it from that gear. As soon as I pushed in the button on the end of the column shifter which essentially took it out of over drive, the BUCKING stopped. Could it be a transmission issue rather then electrical, emissions or fuel. In every forum I have looked at where Ranger owners have complained about this problem, everything has been discussed except the overdrive issue. I think it is being overlooked since it is in overdrive every time you start off and drive. Each and every time you have to manually take it out of overdrive which is a unique function. Mine seems to be running much better if I can just remember to take it out of overdrive EVERY TIME I DRIVE!
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Old 07-03-2015
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Quote:
Originally Posted by 48bronco View Post
hi, just swapped out coil pack from jy. engine seems to idle better and slightly better acceleration at start from stop but my 'bucking' still there. so still looking. thanks again. jp
i was having the same problem , it was the coil pack

i replaced it with the screamin demon pack , problem solved

you definately need new spark plug wires though
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Old 07-03-2015
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Bucking may not be transmission related!

I think you are on the right track. I have the original plugs and wires in my truck to the best of my knowledge. Today I picked up some plugs but declined the clerks offer to go with new wires as well. Looks like another trip to the local Auto Zone. I guess the coil pack as well needs to be replaced if I'm no longer getting a strong spark. Is the coil pack difficult to replace or does it mean a trip to the local repair shop? I'll be a happy camper if I replace these 3 items and my truck gets back to running strong.
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Old 07-03-2015
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i would have the vehicles vin run through the dealership computer first .

to determine if the spark plugs have or have not been replaced ( if they have not ) you would be taking a awful risk trying to remove the spark plugs yourself ( they are probably thread welded to the cylinder head now ) and would incur cylinder head damage if you try

i replace spark plug wires every 2 years because of the engine heat and vibration just kills them over time

i replace ignition coils every 3 years because of eventual EMI leakage which happened again to me
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Old 07-04-2015
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Yikes...... Not sounding good!

Cheeseman.... Thanks for your input and or advice! I got the truck from my son who was the 2nd owner. How many young kids in college worry about swaping out their spark plugs in their truck? Other priorities at the time! So I picked it up from him with 110K on it. It's now up to 164K. Shame on me for waiting so long but I have several vehicles and have overlooked this one since it has been running fine. I'm betting those are the original plugs. Can they still be changed if they are original???? or do I just need to get new wires and coil? What brand coil would you recommend. I read on one thread that one Ranger owner put in the "Screamin Deamon" coil pack and it ran great. I checked Orielly's and they didn't offer that one but had 4 from $77 to $130. On plug wires, do you recommend a name brand like AC Delco or Motorcraft for $55 or am I safe with a little known brand for $37. I will check with the dealer where it came from for the spark plugs and check with my son to see if the plugs have been changed but I'm betting on them being original. Your in put would be appreciated. Thanks again!
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Old 07-04-2015
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i have the screamin demon coil pack installed and it works great , you can feel the extra OOOOMPH

i have my plugs gapped at .050 and no more SP412 copper core

i probably will get flamed for this comment BUT!! stay away from motorcraft plug wires , they are the worst on the market for electrical resistance

if i was you i would only purchase these wires

2001-2002 Ford Explorer 6 Cyl. 4.0L, 2001-2004 Ranger 4.0L: Granatelli Motorsports

although they are for the 2001 to 2011 4.0l engines ( email them for the proper wires set number )

they utilize a solid stainless steel wire inside and are the best wires you can buy
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Old 07-04-2015
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i almost forgot

spray a lot penetrating oil in and around the spark plugs against the cylinder head

you will be taking a great chance trying to remove those spark plugs now
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