Ok here's my situation. I've got a '92 Explorer. It's the backup vehicle for my '70 F100. Anyway, it runs great, drives great, and generally works well. It has 208,500 miles on it. There's just one little problem.

14mpg highway, 11mpg city.

These should NOT get that low of mileage. Here's what I've done:
Plugs
Plug wires
O2 sensor
Clean Mass Airflow sensor - code indicated
Replace Mass Airflow - code indicated, replaced after cleaning didn't help
Inflate tires to 35psi
Check for brakes pulling, nope
Replace air filter with K&N - checked today, absolutely no oil contamination further up the line (no oil contamination of Mass Airflow sensor)
Check computer codes - everything OK
Muffler & pipes surrounding rusted out bad, replaced muffler with cheap-o glasspack
Plugs read ok, very light tan

None of the above made ONE BIT of difference.

Now this thing has gotten bad mileage since I started driving it at about 160,000 miles. My dad drove the thing since 80,000 miles, and it had a replacement set of heads put on due to a crack leaking water into the combustion chamber, fouling a plug. So it's been in the family for a while. But it's ALWAYS got bad mileage.

So a challenge - tell me what's wrong.

 

i am just thowing this out but i heard that a bad computer that is on the way out can cuase bad gas millage i had a 91 explorer and bought it at 209000 miles and sold it at 238000 and got a good 17-18 city 20-22 highway

 

Try some seafoam

 

When my Explorer (91 E.b. 2wd) started getting bad mileage it turned out to be the fuel pressure regulator.It is vacuum operated and sits on the fuel rail with a vacuum line and the fuel line attached to it.Pull the vacuum line off and see if theres gas in it,is so its bad.However mine showed no signs of gas in the line but still wasnt functioning properly.After changing it im getting 18 or 19 mpg in mixed driving conditions.By the way Ive got close to 270,000 miles on mine and still with the original motor and auto. tranny.Hope this helps.

 

I tried the Seafoam last year. Didn't help anything. Did make a cool smoke cloud though!

I already looked at the fuel pressure regulator. Pulled the line, no gas coming out. Also, pulling the line at idle speeds up the idle. I think it's working fine, but if you guys say I really need to change it I will.

Wouldn't the thing not run at all or be down on power if the computer was bad?

 

Computers don't go bad slowly. It's an Intel chip inside your ECC-IV - not a pentium 4 but still a fairly modern piece of electronic equipment. If it was bad, nothing would work.

You said you cleaned/changed the MAF due to a code. What code? I am not aware of any code that says "you need to clean or replace your MAF". You likely had a left or right bank lean code or soemthing else that lead you to believe the A/F mixture is off - as demonstrated by reduced mileage. (Funny too that when I finally needed to clean mine, I got no ECC codes but a boat load of ping told me it was too lean.)

Your milage isn't too far off from my 93 with 168K miles on it. I'd say it's something just a little off due to the age.

I support the other comment on the fuel regulator, from other threads I've followed and the work you've already done to date. You don't need to get fuel passing through the vacuum lines to have a bad regulator. If you get no blow by back to the fuel tank, you can get too high a pressure at the injectors and run a little rich. This might not be rich enough to get a O2 rich error code, but enough to knock 2-3 MPG off as it appears you are at.

Tan plugs are typically OK - you are right. I have always noticed my plugs on this 4.0 to be much more white than tan - and thats at 15-18 MPG. So maybe the fuel curves are a little leaner in these PCM's than traditional thinking.

My suggestion is to pressure test the fuel system - it's an easy check with the shrader valve already installed. More than 45 PSI and that's your problem.

And be careful interpretting PCM codes. I recently had a left bank lean 176 code that was false. It was a bad O2 sensor that read lean so the PCM juiced up the left side and it actually ran rich. Dropped fuel milage a bit too. Left plugs were all black.

Good luck.

 

I dont remember the code. That was over a year ago. It was something about incorrect voltage from the mass airflow sensor.

Intel 8086 chip in the computer if I remember correctly.

My brother has a pressure tester. When we tested it, it wasn't above 45psi, but after shutoff, the pressure leaked down to about 5psi in 10 minutes. When running, it's about 40psi if I remember correctly.

 

Intel 8061 is the processor in my information. Close enough I guess.

Sounds like you're pretty thorough so far. Also sounds like the MAF issue is still present. This sensor is the primary A/F mixture concentration determinating factor. Secondary is O2 sensors.

Does the code still exist? (Unclear by you first thread.)

Are you running in closed or open loop once the engine is all warmed up? All PCM diagnostic code readers will tell you this. If there is an issue with the MAF sensor, the PCM will run open loop. This runs off a different A/F map which is richer (to be safe) - to the tune of a few MPG as you have noted.

I am curious that since you (family) have had this car for a while and mileage has always sucked, and since post 80K miles you were beyond any available Ford warranties and thus probably not in to a Ford dealer for service, then this MAF error may have been there a long time and no Ford dealers ever caught the code.

I followed a thread in here a couple weeks back where a rich problem and MAF code of low return voltage to PCM. It ended up that a wire in the harness was smashed and had to be resrtipped and soldered. Codes went away and mileage went back, assuming it went closed loop.

Since the MAF works on voltage return to the PCM, all connections and circuit integrity is highly critical.

At this point I would focus back on the MAF circuit. Check for loose connections, corroded wires, smashed harness...

Some info here http://fordfuelinjection.com/index.php?p=26

I would think if you pinned out "C" terminal to ground and got good continuity, this output to the PCM is grounded - bad circuit.

Another way to test is with a wiggle test. Hook up the diagnostic testor, get the engine warm so the O2 sensors work and look at open/close loop mode. Wiggle the MAF wires and see if you can get a shift from open to closed.

There is more info on the internet as to how to test a MAF as well.

 

I replaced the MAF, with a stock replacement. The code cleared, and has not returned. The computer currently has no codes.

OBTW, how many O2 sensors does a '92 explorer have? I only found the one, down under the passenger side after the y-pipe comes together. And I replaced it, with no benefit.

 

Right. 92's only have 1. My 93 has 2, right and left side. Later ones have 2 for OBD-II support - one before and one after the CAT.

Still sounds like you are running in open loop.Though this would usually give you a CEL and code. Might be worth checking into the open/closed loop status to verify that. Otherwise, if all checks out OK, you got a car that get's crappy mileage.

You can alwaysget a A/F readout from a dyno shop. They can tell you if the actual emmisions look too rich. Real time data like that doesn't lie.

 

Could I just get a voltmeter and read the output of the O2 sensor? If so, where's a good place to attach said voltmeter?

Edit: Isn't there an air temperature or coolant temperature sensor that could be causing problems?

 

Don't think that will tell you what you need. If you were getting greater than .45 V, rich, all the time, the PCM would give you an error code. Since you're not getting a code, O2 sensor is probably switching OK.

The point is forget about the electronics on the car for a minute and look at the real evidence - kind of like CSI on TV. This is what an independent A/F map will tell you.

On the other hand, if you are assuming then that the O2 sensor is bad and is telling the PCM car is lean and thereofre the PCM richens it up, you could loos gas mileage. But I thought you were happy with the plug color?

Anyway, here's a description to help you test it on the car. Connector should be right above the bell housing between the engine and firewall.

Testing O2 sensors that are installed

The engine must first be fully warm. If you have a defective
thermostat, this test may not be possible due to a minimum
temperature required for closed loop operation. Attach the
positive lead of a high impedence DC voltmeter to the Oxygen
sensor output wire. This wire should remain attached to the
computer. You will have to back probe the connection or use
a jumper wire to get access. The negative lead should be
attached to a good clean ground on the engine block or
accessory bracket. Cheap voltmeters will not give accurate
results because they load down the circuit and absorb the
voltage that they are attempting to measure. A acceptable
value is 1,000,000 ohms/volt or more on the DC voltage.
Most (if not all) digital voltmeters meet this need. Few
(if any) non-powered analog (needle style) voltmeters do.
Check the specs for your meter to find out. Set your meter
to look for 1 volt DC. Many late model cars use a heated
O2 sensor. These have either two or three wires instead of
one. Heated sensors will have 12 volts on one lead, ground
on the other, and the sensor signal on the third. If you have
two or three wires, use a 15 or higher volt scale on the meter
until you know which is the sensor output wire.

When you turn the key on, do not start the engine. You should
see a change in voltage on the meter in most late model cars. If
not, check your connections. Next, check your leads to make sure
you won't wrap up any wires in the belts, etc. then start the
engine. You should run the engine above 2000 rpm for two
minutes to warm the O2 sensor and try to get into closed loop.
Closed loop operation is indicated by the sensor showing several
cross counts per second. It may help to rev the engine between
idle and about 3000 rpm several times. The computer recognizes
the sensor as hot and active once there are several cross counts.

You are looking for voltage to go above and below 0.45 volts.
If you see less than 0.2 and more than 0.7 volts and the value
changes rapidly, you are through, your sensor is good. If not,
is it steady high (> 0.45) near 0.45 or steady low (< 0.45).
If the voltage is near the middle, you may not be hot yet. Run
the engine above 2000 rpm again. If the reading is steady low,
add richness by partially closing the choke or adding some propane
through the air intake. Be very careful if you work with any
extra gasoline, you can easily be burned or have an explosion.
If the voltage now rises above 0.7 to 0.9, and you can change it
at will by changing the extra fuel, the O2 sensor is usually good.

If the voltage is steady high, create a vacuum leak. Try pulling
the PCV valve out of it's hose and letting air enter. You can
also use the power brake vacuum supply hose. If this drives the
voltage to 0.2 to 0.3 or less and you can control it at will by
opening and closing the vacuum leak, the sensor is usually good.

If you are not able to make a change either way, stop the engine,
unhook the sensor wire from the computer harness, and reattach
your voltmeter to the sensor output wire. Repeat the rich and
lean steps. If you can't get the sensor voltage to change, and
you have a good sensor and ground connection, try heating it once
more. Repeat the rich and lean steps. If still no voltage or
fixed voltage, you have a bad sensor.

If you are not getting a voltage and the car has been running
rich lately, the sensor may be carbon fouled. It is sometimes
possible to clean a sensor in the car. Do this by unplugging
the sensor harness, warming up the engine, and creating a lean
condition at about 2000 rpm for 1 or 2 minutes. Create a big
enough vacuum leak so that the engine begins to slow down.
The extra heat will clean it off if possible. If not, it
was dead anyway, no loss. In either case, fix the cause of the
rich mixture and retest. If you don't, the new sensor will
fail.

 

Yes. Air and coolant temp enter into reference table in the PCM and help determine loading. Loading vs throtle positon and MAF give the primary A/F ratio, then tweaked by O2. But from what I've read so far, and I've yet to pull my PCM table (working on that now), the air and coolant temps have only a minor contribution. Maybe enough for you problem. Certainly cooler engine and more condensed air will have more oxygen and vice/versa. A big function of coolant temp is turning on and off the open'closed loop mode. This is where a PCM diagnostice reader comes in handy. Mine is from Autoxray (.com) and is a little pricy but Autozone has cheaper ones ~$60-$70.