4.0 bad mileage, stumped.

InsaneChris7 · Oct 2, 2014

Hey guys, bought a 97 explorer 2 dr sport (I know, not a ranger) command trac 4wd, 4.0 OHV, 5 speed for CHEAP. Thing is mint but a few minor problems. Ran like crap when I got it, MAF, IAC and TB were filthy so I cleaned them up and its running a lot better. New fuel filter, EGR/DPFE is good, new plugs, wires/coil is good, fuel pressure is at 40 on the rail and injectors are good. Has a slight stumble at idle with ac on after a few minutes of idling (no big deal because a "stumble" is like 760rpm to 730). HORRIBLE gas mileage, decent power, no stumbles at any rpm except idle after a few minutes. No CEL at all, O2's seem to be good. Only mod on vehicle was a previous owner installed K&N filter and box kit. 192k miles. Runs at about 180-185 degrees and doesn't take long to get there, never seen it go over 189 on scan tool. Mostly new charging system so no voltage problems, no noticeable vacuum leaks. Idle smells slightly rich but not bad. Getting 7-9 mpg around town driving it easy. No visible smoke from exhaust and FPR has been replaced.

RonD · Oct 2, 2014

If you haven't disconnected battery yet then do it, after cleaning or changing sensors you need to tell the computer to reset "learned" values, so disconnect battery or use scanner to reboot.

Should be running 195-200degF for best MPG and lubrication, someone put in a 180deg t-stat, factory is 192-195deg

The lower temp will cause a slightly rich mix but that would only be part of the issue.

Can you see the fuel trims on your reader?
If they are in the +12 range it could be O2s
They should be -5 to +5
Computer reads MAF air flow data and air temp data and then sets an expected 0 fuel trim.
Computer normally adds more and less fuel to make sure O2s are responding quickly, the -5 and +5.
If your trims are always in the + than means O2 is reporting lean so computer is adding more fuel, if - numbers then O2 is reporting rich so computer is adding less fuel.

Idle will always be +, run engine up to 2,000 rpms and watch trims

Upstream O2s generally last 100,000 miles.
Downstream last longer since they see cleaner exhaust after Cat

Quick easy test for leaking injectors:
Turn key ON(RUN)
Press gas pedal to the floor and hold it there
Turn key to START
Engine should crank but not start or fire, fuel injectors are shut off
Release gas pedal at any time and engine should start normally, injectors back on

If engine fires with gas pedal to the floor then it is getting fuel from somewhere.

If you did a static fuel pressure test and pressure stayed up around 30psi, key off, and didn't continue to drop then there is no leak in the pressure side, i.e. injectors, FPR or fuel pump check valve.

But return lines can leak, no easy test for those, just visual inspection, and run your hand over the line where ever you can and then smell for gas, small leaks won't show on the ground easily but can lower that MPG alot.

InsaneChris7 · Oct 2, 2014

Ill check trims in the AM. Ive had the cables off the battery probably 4 times, all overnight and runs fine for the drive to work, afternoon it returns to the same. Thought temperature, so I unhooked it at work, ran fine in the afternoon, back to same in the AM. Good 40 miles one way to work everyday. Ill check temps again tomorrow, if its 180ish I'll swap tstat just to make sure. Injector test went well, no start. static test sits at 32#, had the gauge on for half a hour, lines all look good and don't seem to find anything wrong. Also I'll monitor the o2 readings on the way to work, its been a week or so since i checked them and no CEL so I wasn't paying too much attention to them. Also forgot to mention its the OHV motor, not the SOHC.

AgPete139 · Oct 2, 2014

I'd get rid of the K&N. If it's improperly oiled, you're just mucking up your MAF again.

What are you pulling for vacuum?

.054" spark plug gap. If you haven't regapped or changed these yet, I would for a 200K mile vehicle.

I'd also check the resistance in the ignition wires with a multimeter. Look for around 7000 Ohms. This also causes a weak spark and a possibly rich mixture. At this age, it's worth the quick check.

I wouldn't quite put it as a fully dumping injector just yet. But, if not caught, this will wash away your cylinder with gas and wear your rings up in no time flat.

RangerSVT · Oct 2, 2014

Do a cylinder compression test along with the above suggestions. ..

SVT

Dune Runner · Oct 2, 2014

Have you looked at the plugs? And I agree with getting rid of the K&N

4.0 bad mileage, stumped.

InsaneChris7

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