1998 Explorer 4.0 misfire... | Ford Explorer Forums - Serious Explorations

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1998 Explorer 4.0 misfire...


New Member
March 14, 2016
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Year, Model & Trim Level
1998 explorer xlt
So I just picked up an 98 explorer 4.0 SOHC
Here's what I know.
PO installed an jet performance module then after awhile tried to run glasspacks and the whole system went crazy. He stopped messing with it after he reinstalled stock exhaust and the problem persisted. Now I have it.
Here's the codes thrown
P0301 cyl 1 misfire
P0302 cyl 2 misfire
P0304 cyl 4 misfire
P0171 system lean bank 1
I swapped plugs around and has hesititwtion at idle but feels to smooth out when revved. I haven't driven it far enough to get the CEL to light up. I'll do that later today and see what codes pull.
I pulled stock plugs and put them back in different location to see if the misfire followed. Will update with codes tonight

Can anyone give me slight clue of what to do now?
Doubt it's coil pack cause which coils are miss firing. Spark plugs look good.
Thanks for your time. Any help is appreciated

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Jet performance module? I'm not sure what that is or what part is replaces. Have you checked that the plug wires are installed in their proper seats in the coil? I would attempt to remove this module and replace it with the stock unit with whichever part this module was intended to replace.

Hey Mark. It's an chip/module. Yo unplug iring harness going to ecm. Plug this in and then plug the wiring back in. Oh it's coming off. I just got back from an trip. All coil wires are going where they need to go etc.

Aside from the module itself, there are multiple sensors that could cause misfires such as yours. Someone will come along and ask this question, is it the OHV or SOHC 4.0L V6?

A list of things to test/check:
-Clean MAF
-Test fuel pressure (60-65psi)
-Ensure o2 plugins weren't swapped from left to right during exhaust replacement (it happens)
-Lean condition can be a vacuum leak. The junk plastic intake manifolds usually leak from the gaskets at some point. I use a propane torch to test for leaks (without the flame of course)

Narrowing it down to spark or fuel issues makes things easier. I am leaning more towards vacuum leak or fuel delivery issue considering you are running lean. I am not that experienced with these Ford engines. From reading through this forum several times, there are multiple problematic sensors and vacuum leaks that even have mechanics guessing. The majority of lean running conditions appear to be the infamous intake gasket problem (about $18 for upper and lower set). The Jet module could also but at fault but I have no hands on experience with those.

Cleaned maf
Took out the jet module chip.
Fuel pressure
38psi key on
30psi running

It's still missing. Wants to kill itself on hills.
Won't trip an CEL agian. Trying to get it to. But it won't.
May take it to Ford tommorow.

30 psi? I am pretty sure that it is supposed to be 60-65psi. But there apparently is a returnless fuel system (which I have) and a typical return system with lower pressure. If you have the 2 piece fuel rail without a return line you probably have the same system as I do.

I'm taking it to Ford today. Paying for an used car inspection. They'll check pretty much everything on to it. Hook computer up and check parameters also having an power balance test done.
When we head in I'm stopping at Oriellys and swapping fuel filter in parking lot.
Fill in more info today

Dont do that. Leave the filter be if your taking it in

....Let us know the damage. If they find it.