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4.0L OHV & SOHC V6 Tech General discussion of 4.0L OHV and SOHC V6 Ford Ranger engines.

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Old 01-03-2011
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factory spark plugs, now horrid mpg

I tried to search for the answer as to not waste anyones time, but i have come up on no answers. About a week ago i changed the what looked like factory plugs out of my 02 4.0. The plugs i took out looked like they served their useful life and looked completely normal, at that time i was getting 15 mpg city pretty reliably, truck was running smooth and everything. now that i have changed to the autolite, im getting barely 13 or 14mpg. I noticed that when i was buuying the new plugs, they said the factory plug numbers changed but they would give me the oem spec plugs from auto lite and they would be perfect. i went with the iridium as thats what the factory ones were. I noticed in the manual, as well as the autozone computer, that those autolite iridiums were set for a gap of .054, per factory specs. i made sure they were all gapped to that spec and went about taking the old ones out.

However, when i took out the ones that looked like original factory, the gaps on those plugs were massive, they were like.075 or more even. Now, my question is which are the correct gap numbers for the autolite iridiums? my truck now is running great, good idle, smooth, pulls normal, but the gas mileage is obsurd with the current .054 gap. should i regap those autolites to what my old plugs were set at? what am i missing here? can anyone help?
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Old 01-03-2011
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Well, not knowing the factory specs for your engine, I would try expanding the gap. Maybe not to the .075, but probably to .065 anyway, and see what happens. If your truck was running fine before, obviously the higher gap was working.
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Old 01-03-2011
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What engine specs do you need? 4.0 sohc, 2002 4wd 5 speed manual tranny 132k miles. i just dont understand why my factory manual should be wrong though. also, there are alot of ppl on here who say use platinums or fine wire plugs, not sure if iridiums are fine wire? they are the oem recomended which is why im using it....

should i stick with these plugs i have and just regap? or do u guys think i should get platinums?
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Old 01-03-2011
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There is no vehicle that I've ever seen with a .075 gap. The gap is due to the plug being worn out. Be careful with trying to mess with the gap on iridiums, you can break the tip off. .054 is about average for most Fords.
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Old 01-03-2011
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Yeah, .075 is high, and as mentioned, likely due to wear. However, you said your truck was running fine before, and getting better mileage, so a higher gap would most likely do fine for you, which is why I said raise the gap a little. A fine point plug would give you a hotter spark, but based on what you've said, I would try increasing the gap. As rcmustang said, careful. If you try to spread the gap the wrong way, you'll damage the plug.
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Old 01-03-2011
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yeah ill be really carefull gapping them, i was the first time around, i am going to check all my old plugs one more time when i get home, if they all have around the same gap, then i will try to raise my gap from .054 to about.065 and see what ahppens. are the autolite iridiums what i should be using per oem specs or should i go with the double plats? is it possible if i went to the plats with the same .054 gap, the heat difference could result in better mileage?
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Old 01-03-2011
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Typically a platinum will give you a better burn with less gap than a wider tip, but they have their own specs. The factory gap spec is for the factory plug. Try raising the gap to .060 before you go all the way to .065. That\'s still .006 wider than you are now, so it should make a difference. Good luck with it.

Last edited by ME00Stepside; 01-03-2011 at 04:21 PM.
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Old 01-03-2011
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thanks for the replies, i just noticed that the emission sticker in the engine bay said gap to .062-.068. That is MUCH closer to what the old plugs i took out were. im gonna try to do .064 and see what happens since its inmiddle of the range. should i notice any difference in engine proformace with the wider gap? or just better mileage?
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Old 01-03-2011
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You'll probably notice a slight difference, because you're making more power off your fuel, which is where the increase in MPG is from.
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Old 01-03-2011
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everyone with a 4.0 has seen the emission sticker i assume, the range is bigger than the .054 people are saying is what the factory spec is, i dunno if other rangers are different but if it says it on the sticker, im gonna go with that one. it even says in the manual, see emission sticker for gap. hope this helps
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Old 01-03-2011
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i just finished regapping all the plugs, and it still runs pretty smooth, feels like there MIGHT be a small increase in performance, hopefully i will notice better mileage off the bat. i ended up gapping the plugs to about .063, right in the range it says on the radiator sticker. ill keep u guys posted on whether this fixes the issue or not.
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Old 01-03-2011
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Good deal. Hope it makes the difference for you.
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Old 01-04-2011
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So, im not out of the woods yet. i drove around for like 20 miles with my buddy and i noticed that the truck does that jolting thing now, in gears 3 and 4 when i step on it around 2k rpm. im guessing that means that the .064 gap i did per the emission sticker specs is off? it said .062-.068 for the gap. this seems so ridiculous i cant get the correct gap info for one of the most popular engines for ford. should i go back to the .054 and suck up the bad mileage? i shouldnt have to do a compromise i dont think. any help is appreciated of course.
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Old 01-04-2011
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Did you reset the computer when you did the new plugs? Also you need to do new o2's when the plugs are changed at 100k they are both made from platinum and wear out about the same time.
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Old 01-04-2011
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Originally Posted by Blhde View Post
Did you reset the computer when you did the new plugs? Also you need to do new o2's when the plugs are changed at 100k they are both made from platinum and wear out about the same time.
yes, i reset the ecu for a good 5 min. i imagine i can check the o2 but why would it go out suddenly right when i change the plugs? should i go back to the .054 since there was no jolting before that?
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Old 01-04-2011
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I wouldn't go all the way back to the .054, since it was obviously not burning the fuel completely. Try in between.
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Old 01-04-2011
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How many miles are on the spark plug wires?
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Old 01-04-2011
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the wires have 132k miles on them as far as i know. they look stock and i have never changed them.
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Old 01-04-2011
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ok im redoing the plugs yet again, this time, im going to gap them to about .058-.060 i hope this works, let u guys know soon
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Old 01-04-2011
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You might as well replace the wires. I did just plugs, then just wires about 5k later. It idled much smoother, had more 5th gear pull. Put your truck in the pitch dark and look at the wires with it running, see anything?
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Old 01-04-2011
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ok i just went for a ride with the .058-.060 gap. the jolting has gone down considerably, but its still happening. My only concludion now is that the .054 was correct, and im gonna regap them one final time at .054-.056. i hope finally this fixes it.
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Old 01-04-2011
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Originally Posted by rolsmojave3 View Post
You might as well replace the wires. I did just plugs, then just wires about 5k later. It idled much smoother, had more 5th gear pull. Put your truck in the pitch dark and look at the wires with it running, see anything?
my truck idles like a dream, even with the wide gaps ive tried. and first and second gear feel strong, the jolting only seems to happen in lower rpms in 3rd and 4th. but i will try the wire thing tonight if this fails too.
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Old 01-04-2011
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Might be time for those as well.
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Old 01-04-2011
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Originally Posted by 02FX4U View Post
my truck idles like a dream, even with the wide gaps ive tried. and first and second gear feel strong, the jolting only seems to happen in lower rpms in 3rd and 4th. but i will try the wire thing tonight if this fails too.
You have higher loads in low rpm in higher gears. If the spark isn't strong enough (gap too big, etc..) it may be misfiring.....
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Old 01-04-2011
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you should always do a full minor tune-up. plugs, wires, and fuel filter. the real difference between what the plugs are made of is the life of the plugs. I wouldn't mess with the gap personally. I installed a set of E3 plugs in my '04 4.0l SOHC and it has the same stumble during acceleration and gets a flashing CEL for a misfire. And the E3s are non gapable. I think that the 4.0L SOHC Cologne is a piece of junk and it's about time they ditched it. It's been in manufacturing since 1968, in one form or another. When mine dies I'm doing a 5.0L swap.
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