I am a newbie to th forum and to vehicle mantainence (so go easy on me).
I have a 1999 4.0L with 138K that is throwing the P0171 and P0174 codes - left and right bank lean codes. This has happened before and I had a shop take care of it, but I was hoping to tackle it on my own this time. Based upon what I have read in the forums and assorted advice I have tried the following:

1. Cleaned the MAF Sensor
2. Tried a Gumout fuel additive
3. Replace the PCV Valve
4. Checked the PCV Value hose (put my mightyVac on the small hose from the valve to the engine and it seemed to hold a vacuum)
5. Used Seafoam in the tank
6. Replaced the fuel filter
7. Cleaned the "flapper" valve on the air intake with Carb cleaner

After many of these steps I borrowed a code reader and cleared the codes. The check engine light usually returns after 60 miles of driving.

Other items that may or may not be significant:

1. The truck only gets driven once or twice a month for short trips
2. It seems to idle just fine (many have mentioned rough idle with this problem - I don't have that)
3. Fuel pump was replaced about 2 years ago
4. This has happened twice before in recent years and this is the at least the second time in January (a cold month)

Could I really be unfortunate enough to have had both O2 sensors fail at the same time?

Is it time to take it to the shop or is there something else a novice can do?

Any help would be appreciated

 

After 60 miles means it is not a computer issue.

O2 sensors are good, because "yes, it is very unlikely 2 would fail at the same time"

Lean means the O2 sensors are seeing too much oxygen in the exhaust.
The MAF tells the computer how much air is being pulled into the engine and at what temperature(colder air requires more fuel).
The computer gets the air volume and opens injectors to get a 14:1 air:fuel mix

So vacuum leak is the most popular thing to look for, because it means "more air" is coming in than MAF is reporting to computer so you get a 15:1 lean mix.
And this is ANY air, even if there is a leak in the air tube from MAF to throttle plate, thats air not coming past the MAF.
After warming engine up, unplug the IAC(idle air control) valve, the idle should drop to about 500-600rpms, if it doesn't then you do have an air leak somewhere.

And there is the EVAP system which could have a cracked vacuum hose, so it would be a come and go vacuum leak, hard to track down.

Also EGR system has an on and off vacuum demand.

The other side of the equation is fuel, if computer thinks fuel pressure is 65psi(which it should be on a '99) it opens the injectors for "xx" time to get 1 part fuel, if pressure is lower it might only get .8 part fuel.
Or if injectors are dirty they may only pass .8 parts fuel in "xx" time
Looks like you have addressed that .
And computer has parameters to open injectors longer if O2 reports lean, the reason you get the CEL is because computer has reach the edge of the parameter and is letting you know something is wrong.
i.e. based on the MAF data and the opening time for injectors, there should not be a lean condition reported by the O2s
So CEL comes on

The outward sign of the lean mix would be "pinging" when engine is under load, going up hill or even accelerating.

You could get one of those OBD II bluetooth adapters if you have a smartphone.
About $30-$40
These will let you watch the computer data on your phone, or laptop, while you drive, maybe you could catch a glimpse of exactly what is happening when the CEL comes on.
It could be a one off, or a rarely lean condition.
The 60 miles means things are fine for 50 miles
O2 sensors are not used until engine gets warm so I give that 10 miles, and then all is well for 50 miles more, so..............

 

A lot of good information to digest. I googled the IAC valve and will try that tomorrow. As a bonus, the IAC appears to be the source of another problem I have been having where I occassionally hear a noise from the air box that sounds like someone turned a vacuum cleaner on. Looks like I may need to clean it.

Thanks

 

I unplugged the IAC during idle after warmup and (based upon the tac) it looked liked it dropped from about 900 rpms to maybe 700ish. It was noticable but not significant. I then removed the IAC and sprayed some MAF cleaner in it. I replaced it and performed the same test. The drop was more significant this time, it went down to around 600 rpms. It was much more noticable.

CEL is still on. OBD2 tool should be here this weekend.

 

Unhook negative battery cable for 5 minutes to reset computer, most CELs won't reset on there own, i.e. if engine is now running normally the lean code CEL would still stay on until reset.
Resetting computer is always a good idea after working on a sensor or control(IAC), the computer relearns all sensors and controls after a reset.

 

I was wondering about that. I kept borrowing the device from the auto store to reset and the guy at the desk said it would reset on its own if the problem was fixed.

So far so good - no light. By tomorrow I should cross the 60 mile mark.

Thanks.

 

CEL returned at 35 miles. I guess I will wait for the OBD tool and hopefully whatever the problem is will stand out.

 

Well the first 5 minutes on a cold engine don't count as the O2 sensors are not on-line yet.
So while the mileage is relevant, the after warmup time/miles would be better

The O2 sensors are the only way the computer can read the lean/rich ratio(which is needed to set the CEL) and they need to be heated to 650+ degF in order to work, they are "heated" with battery power but still need a few minutes of exhaust temps to get up to operating temp.

Also Google: interpreting OBD II data

This will give you a head start when reader shows up
i.e.
STFT = short term fuel trim
LTFT = long term fuel trim

 

I tried a couple of things this morning. I may not have waited for the engine to warm up enough before trying the test with the IAC and now that I have the OBD reader I can get an exact reading on the RPMS. So this morning I unplugged the IAC and the RPMS dropped from about 910 to 830. Does this mean I have a vacuum leak?

Also noticed from the OBD reader that Bk 2 Sensor 2 O2 Voltage is a constant 1.27V (PID 0119) while the other three are fluctuating between 0.08 and about 0.88. I suspect this is a problem. I thought there were two sensors - a bank 1 and bank 2 sensor. The reader seems to indicate two sensors on each bank. And would this one bad sensor cause the lean condition (usually high voltage means rich according to what I have read). And shouldn't this cause only one bank to read lean and not both?

Any thoughts?

 

You have to wait till engine is warmed up to test RPM and IAC
Computer is still in Open Loop so running engine rich.

OBD II systems have O2 sensors behind the catalytic converters, these are referred to as sensor 2's
i.e. bank 1 sensor 2 is the O2 sensor behind the cat converter, if you have dual exhaust there will be two cats so two sensor 2's
These are there to make sure Cat convertor is working in lowering emissions.

I wouldn't put too much worry on their readings, they are not used by the computer for fuel/air, only as a comparison that the exhaust is cleaner from Sensor 1 to Sensor 2

So on a 4cyl OBD II vehicle you would have:
Bank 1 sensor 1 < near exhaust manifold, called "upstream sensor"
Bank 1 sensor 2 < after cat converter, called "downstream sensor"

V6/V8 OBD II vehicles will have:
Single exhaust
Bank 1 sensor 1 < near exhaust manifold, called "upstream sensor"
Bank 1 sensor 2 < after cat converter, called "downstream sensor"

Bank 2 sensor 1 < near exhaust manifold, called "upstream sensor"

Dual exhaust
Bank 1 sensor 1 < near exhaust manifold, called "upstream sensor"
Bank 1 sensor 2 < after cat converter, called "downstream sensor"

Bank 2 sensor 1 < near exhaust manifold, called "upstream sensor"
Bank 2 sensor 2 < after cat converter, called "downstream sensor"

To test upstream sensors you can unplug 1 spark wire on one bank, this will cause a rich condition on that bank, downstream sensor should also show rich change as the unburned fuel passes thru cat to be burned
Engine needs to be warmed up for this and idling when you unplug spark plug, if you shut off engine then restart O2 sensors will be off line for a minute or so.
Unplugging at the coil is best, so you are not holding a "live" wire after unplugging, lol, insulated gloves are nice.
Only run engine a minute or two like this as cat converter will start to heat up.

You can lean out both banks by unhooking a vacuum line, to test upstream O2 sensors, this effects both banks.

If sensor 2 is not changing at all then yes you may want to replace it, but sensor 2's usually last the life of the vehicle, unlike sensor 1's they don't get the rich or oil fouled exhaust directly, the cat protects them a bit.
Also check for exhaust leak, it can suck in air which will then show lean condition on O2 sensor

 

I have 4.0 V6 with single exhaust so I think I have located three sensors. Perhaps the Bank 2 Sensor 2 does not exist and that is why I am getting a constant voltage reading (but I would expect 0).

The forward sensors are moving around in voltage which I assume means they are makig constant adjustments and they do not seem to be maxing out in either direction.

Is there anything else I should look for in the OBD codes? My MAF is running about 3.6 g/s at idle. The short term fuel trims tend to bounce around going from -6% to +5%. Long term fuel stays pretty consistent around 25% on both banks. Unfortunately I cannot seem to get a fuel pressure reading from the device.

Thanks again for sticking with me on this...

 

Computer has no fuel pressure monitor, on a '99 it would assume 65psi at the rail.

O2 sensors don't make adjustments, they are just reporting oxygen content in exhaust.
The computer uses that information to change the "dwell"(how long open) on the injectors.

MAF voltage should go up with RPMs, MAF is the main sensor for fuel/air mix, O2 data is just for fine tuning.

LTFT at 25% at idle is fine, raise RPMs to 1,500 and watch it, then raise RPMs to 2,500 and watch it, should be at about 4% at these higher RPMs

Also check the IAT(intake air temperature), that should be almost the same as outside air temperature, if it is not even close then that could be your problem, colder air requires more fuel, so if IAT sensor is reporting 60degF and outside temp is 35degF mix would be lean.
On the '99 the IAT would be part of the MAF sensor.

Most likely the OBD II reader is injecting the Bank2 Sensor2 reading, if your vehicle doesn't have that sensor then there would be no data coming out of the computer for it.

 

I will check the IAT this weekend and also do more vacuum leak testing...

 

I did some vacuum testing by spraying MAF cleaner around the vacuum hoses I could find. I could detect no leaks. I checked the IAT and it was spot on. I checked the LTFT as perscribed and it dropped to around 20 and 17%. That does not sound close enough. What does it mean?

 

I'm willing to bet you have the oh so common intake gasket leak that the 4.0 OHV are known for.

Here's a how to
https://www.ranger-forums.com/engine...-engine-27084/

 

No, it should drop more, does the other bank drop as it should?

- number is rich
+ number is lean

Assuming no vacuum leak and the other bank is dropping down with rise in RPM:
Could be O2 sensor on that bank is failing, you could swap sensors and test that.
Could be one injector is dirty, this will lean out that bank

The computer tries to keep STFT at 0%(perfect mix), +/- 10% is fine as it is constantly changing mix to average 0%.

 

Both banks are high +20 on one +17 on the other.

 

There sure is a lot of material out there suggesting a vacuum leak. This may be a stupid question but are there some key areas underneath the vehicle I should try to check for a leak? I tried spraying all of the hoses from the top but haven't tried from below.

Regarding the intake gasket, I tried spraying around the gasket and didn't notice any significant change in RPMs. Then again I could not easily spray every end of it.

 

I had the same issue with mine, couldn't get to all the areas around the intake. Finally I said **** it and just did it as the gasket kit was only $25 and it only took 2 hours. After I got it apart I found the leak between cylinders 3 and 4.

 

Remember, newbie - novice. That procedure looks a little difficult, especially since I only know maybe what half the things in step 4 are. I might try it but only as a last resort.

 

This reads more like a general air leak, just a little higher on one side, so doubt it's an O2 sensor issue.

There is an old school method to track down an air leak in the intake manifold.
Warm up engine to normal operating temp.
Shut it off
Unplug IAC valve
Remove air plenum from intake
Start engine, check engine light will come on because MAF and IAC valve are off line.
Cup hand over intake to start limiting air flow, engine should start to struggle but it will also cause it to suck more air from any air leak.
Sometimes you can hear the whistle from an air leak.
You can also spray some carb cleaner or quick start around, but BECAREFULL, exhaust manifolds are HOT.


Another test is with engine off, warm or cold doesn't matter.
Remove air plenum from intake, block the intake hole, rag is fine.
Remove power brake vacuum line from the power booster, so it is still hooked up to intake.
Light a good cigar and pour a glass of scotch, some say the scotch is optional, they are idiots

Get a good puff of smoke in your mouth and blow it in the Vacuum line, repeat a few times, watch for wisps of smoke.
Cigarette smoke won't work, and you don't have to inhale the cigar smoke, that part is optional.

 

That smoke thing was a great idea. I don't know why I didn't think of that sooner. Unfortunately I didn't see anything and I had my wife double checking me. Maybe it was because it was old pipe tobacco and there was no scotch. Tried the other test too but to no avail. After I put everything back I also tried your old test of unplugging the IAC at temp and the rpms did drop down.

So I reset the codes because there were extra ones showing I am assuming due to all of the things I was unplugging. Then I watch the LTFT. Bank one seems considerably higher than bank 2 (12% to 4%). As I reved the engine, both trims would drop down to the 3-4 range and after 10 minutes or so of running and reving, I ended up at 15% and 5%. So I am guessing over the next couple hours or so of driving they will max out at 25% and the light will return.

Even though the fuel pump was replaced about 2 years ago, is it possible this is a fuel pressure problem? Is there an easy way to test that?

And one last item that I hadn't mentioned but I will just throw out there; after this happened last time and I got the truck back I noticed the fuel guage would not go higher than 3/4 after a fill up. Could this be a sign the fuel pump has got an issue?

Thanks

 

Bourbon or Rye is OK, even seen Rum used, although can't see that being a "good' test, any kind of whiskey was the way I was taught.


No, don't see fuel pressure causing a 1 bank issue.

So we are back to, "is bank really lean or is O2 just reporting lean in error".
I guess swapping O2s from one side to the other would test that.

A leak in the exhaust manifold can pull in air which will cause false lean condition on that sides O2.

You can test injectors on that bank with OHM meter, best test is with engine warm, not hot, just warm is fine.
Set OHM meter at lowest setting
Remove injector's wire
Put meter probes on injector's contacts
Should show approx. 14ohms, 11-18ohms is Ford's "within spec" range.
test all 3 and write down results, the ohms are like a compression test they should all be similar, if all 3 are 12ohms thats OK, but if two are 13ohms and one is 8ohms then replace the one injector.


Rangers did have that issue with fuel gauge not going to Full, no not a fuel pump issue, the sender in the tank is part of the fuel pump assembly but they do not share any wiring, they are just bolted together.
If I remember correctly you could disconnect the battery for 5 minutes, and that would reset it, not sure if it was GEM module related or the anti-slosh circuit or the voltage regulator.

1989 and later Fords use the 16-158 Ohms gauges and senders
Sender has 16ohms when float is at the bottom(Empty)
Sender has 158ohms when float is at the top(Full)

There should be a connector in the frame rail for the 4 wires going to the Pump and sender.
Black wire is ground for Fuel Pump
Pink/black is power for fuel pump

Orange wire is ground for sender
Yellow/white is for gauge

Connect OHM meter between orange and yellow/white wires, ohms should be close to what tank level is, then next time you fill the tank check it again, should be up near 150ohms

 

But I was getting left and right bank lean. After a reset it seems the bank 1 goes bad faster than bank two but ultimately both banks go lean. So you don't think fuel pressure could cause that?

 

Yes, if both banks go lean.

Your '99 should have 65psi at the rail.
You can rent/loan Ford fuel pressure testers at some auto parts stores, or tool rental places.
Or make your own with gauge that goes up to at least 100psi.
The test valve on the rail is a schrader valve, so the same as a tire air valve