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1998 Ford Ranger Misfire

I have a 1998 Ford Ranger 4.0L 4x4

I have had a check engine light on and off for the past year or so. Using my cheap-o scanner, I determined that it was for a lean mixture. No big deal, nothing really noticeable about the way it was performing or driving.

A few days ago my truck started idling a bit rough and doing some bucking at cruise. By “cruise” I mean, only when I try to maintain a speed. When accelerating or decelerating everything is normal. In park when the rpm is below 2500, it is a bit rough. Anything above that is 100% normal as far as I can tell. It also triggered a new check engine light code, that said that the #6 cylinder had misfired.

After taking it to my local shop I was informed that I would need upper intake gaskets and a new set of plug wires. I was also told that it was probably a good idea to put in a new set of lower intake gaskets due to some oil in the intake manifold.

I was feeling pretty frisky, so I went and bought the tools and parts to replace the gaskets and wires. I also got a new set of plugs.

Even though I was told it was a good idea to put in the new lower intake gaskets, I chose not to replace them at this time due to the fact that the mechanic I talked to said that it was not necessary to fix my current problem but was just a good idea. After slaving in the garage for what seemed like days. Not only did I have a new set of upper intake gaskets, new set of plug wires and a new set of plugs installed. But also a wealth of information on all kinds of gaskets and valves I had never heard of before. All that with minimal damage to myself and my truck!

The intake gaskets that I replaced are the three little rubber guys in the bottom of the plastic intake manifold. After replacing those items. I checked that the leaks had been repaired by running the engine and spraying some WD-40 on every possible hose, connection, nook, cranny, crevasse and pathway that anything could take to get into the intake system. Looking for any change in rpm or smoke from the tailpipe. There was none. After that, I was satisfied that there were no leaks.

Although the truck does have a bit more pep too it while accelerating, it still has all the same symptoms that appeared a few days ago. Rough idle, missing while trying to maintain speed, etc. Since I cleared the check engine light while working on the truck, the lean mixture codes are gone, but the misfire code has moved to cylinder #4.

After hooking up my cheep-o scanner to the truck. It appears that the #2 injector bank is trimmed much richer than the #1 bank. Running the truck at 2000 rpm for a few minutes gets these indications:

LT FTRM1: -2%
LT FTRM2: +10%
ish on both of those.

Now the way I understand it is that those numbers are telling me how much the computer has adjusted the pulse width for that bank of injectors. So what I don’t understand there is how exactly it decides to add or subtract fuel to one bank or the other. I assume that this is done by the 02s11 and 02s12, and that it’s looking for some magic ratio between the fuel and 02. Which would mean that for whatever reason the #2 bank is getting more air, and the computer is compensating by adding more fuel. Indicating that I still have a vacuum leak?

Now at this point I spent a good bit of time looking around the internet for someone who had a similar situation occur that might be able to give me some insight on what’s going on. I’ve found 2 or 3 people with the EXACT same symptoms as me and they all have stories that go something like this “I’ve tried replacing the plus, plug wires, ignition coils, MAF, IAC, fuel filter, fuel pump, fuel injectors. And now I’m at a complete loss because nothing has changed!”.

I’d kind of like to avoid randomly replacing parts, and actually figure out what’s wrong before proceeding any farther. Now, I’m not a mechanic so I would like someone in the know to double check my logic. Here are my thoughts are so far:
The truck performs well under load. Which tells me that the plugs, plug wires and the entire ignition system is doing it’s job.
I can press the accelerator all the way down and get a normal response from the engine. Which tells me that the fuel filter and fuel pump are doing their jobs.
Unplugging the MAF does not do anything to the symptoms.
Unplugging the IAC causes the engine to die unless you hold some throttle in, but does nothing to the symptoms.
Also the fact that there is an asymmetry in the fuel trim tells me that those sensors probably aren’t the problem.
As far as I can tell the computer has no idea how much fuel the injectors are actually putting into the cylinders. So if there was a clog / leak in the injectors, it would never know it. Which would mean that it wouldn’t be adjusting the fuel trim and would just run lean in one of the cylinders.
The fact that this only occurs at a lower throttle setting (which is where you are going to be getting the most vacuum), strongly indicates a vacuum leak.
If the computer actually works the way I think it does this could also be a sensor problem.

Is there some way to know for sure that I need to replace the lower intake manifold gaskets?
Are the 02 sensors interchangeable? Can I just swap them to check them?
Is there some other option that I’ve overlooked?

This is the first time I’ve done anything more then change the battery on any car. It’s entirely possible that I have some how screwed this up. So any advice or knowledge would be greatly appreciated!


I know this is from years ago but I have the same model truck with the same engine and I have the same exact symptoms. I just want to know has anyone figured it out ? You can add a TPS to the not the fix to the list of other parts. I thought it might be the Cam Sensor but that usually only acts up at higher RPM’s and I had that replaced 2 years ago.

But what year ?
2000 and older Ranger’s the 4.0 was an OHV (Over Head Valve)
2001 and newer Ranger’s the 4.0 was a SOHC (Single Over Head Cam)

This ended up being a spark plug that had the electrode bent during installation.


Thanks for letting us know :+1: