P0171 & P0174 Codes on '99 Ranger 4.0L V6 (OHV) - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 10-22-2015
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P0171 & P0174 Codes on '99 Ranger 4.0L V6 (OHV)

Hi Guys (and Gals),
I recently (~ 1 wk ago) bought a '99 Ford Ranger XLT 4x4 4.0L V6 for $2400. The truck runs well and the 4x4 works great. On the way home the check engine light came on (hooray for people who clear the codes right before showing vehicles to someone). Anyway, I borrowed a friends OBDII scanner and pulled the codes which are P0171 & P0174 (Bank 1 & Bank 2 Lean codes). On a side note, I'm also trying to find an oil leak culprit and monitoring my levels to see just how bad the leak is but as long as my levels don't drop rapidly this is a lower priority issue.


My plan for troubleshooting the codes is as follows:
1. I am told that there is a rubber elbow going into the PCM that often leaks so I'm going to check that first.
2. Buy an OBD II scanner with Real Time Data capability (don't really want to do this but I assume it will come in handy in the future since I'm now the proud owner of a high-mileage pickup).
3. Check O2 Sensors with scanner by monitoring O2 readings as I spray Carb starter fluid into intake vacuum port (readings should spike to indicate rich mixture). If O2 sensors don't pick up the rich mixture, I'll replace.
4. Next I was going to do a carb starter fluid manifold leak check by spraying carb starter fluid around the intake manifold to see if the engine runs rich at any point.


What do you guys think of this plan? I'm basically following combined steps from several online posts. Are there any better (and safer) ways to do an easy intake manifold vacuum integrity check? I'm thinking if I can shroud the area to get nice still air, perhaps I can use smoke from a candle to see if it is being "sucked" into the manifold anywhere.


Any thoughts/suggestions/comments are greatly appreciated. Thanks!


Also, I will update the thread with photos and comments as I progress through this. I need to smog the truck to complete my title and registration transfer so I'll hopefully be working this out very soon...or maybe I'll clear the code just before going and see if I can pass a smog to get it over with.
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Old 10-22-2015
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Having both banks with Lean code means it won't be O2 sensors, i.e. two O2 sensors failing at the same time would be so rare that i wouldn't even bother to test them.

Also lower intake manifold leak would almost always effect only 1 bank, so long odds on that as well.

Upper intake leak, i.e. PCV valve hose or other vacuum hose connection could effect both banks.

You need to understand what those or other codes mean, and that means a quick course in how engine computers work.
1.The MAF(mass air flow) sensor measures air flowing into the engine

2.The computer takes that info and calculates the 14:1 air:fuel mix needed

3.Computer then opens fuel injectors for the specific time it will take that size injector with 35psi fuel pressure to release the calculated amount of fuel.

4. Computer then gets Oxygen data from O2 sensors, too much oxygen means Lean burn, too little oxygen means Rich burn.

5. If Lean burn the computer opens injectors longer, this is called Short term fuel trim(STFT), and it will be a + or - number.
0 = computers calculation based on MAF sensor data
+ means fuel injectors are being opened longer
- means shorter open time

Normally the STFT will change from -5 to +5 and do that very fast, computer is constantly adjusting injector open time(pulse width) based on feed back from O2 sensors

If STFT should get up to +15 to +25 for any length of time, the computer will set Lean code for that bank, or both banks if both STFT are in that range.
Engine is never actually running Lean, the code means the computers calculations based on MAF sensor data is incorrect, it is having to add more fuel than expected, so it is giving the driver a "heads up" that something is wrong.

Knowing how this works now, it could be MAF sensor data is incorrect, clean the MAF sensor, should be done every 3 to 5 years
Could be extra air is sneaking in from somewhere, not going thru the MAF sensor, yes could be vacuum leak(upper intake) or a leak in the large tube running from MAF sensor to intake, these can crack or gasket at either end can leak.

Could even be low fuel pressure, lower pressure means less fuel flows thru injector when it is open, dirty fuel filter?



You can get OBDII Bluetooth units, these display the info on your smart phone or laptop(with bluetooth), not expensive and work on any vehicle made from 1995 and up
Check the price of the smartphone app before buying the OBDII unit, sometimes it is worth spending $10 more on the OBDII connector if the app is less money or free.

Last edited by RonD; 10-22-2015 at 10:59 AM.
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Old 10-22-2015
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RonD,
Great post, and thanks for the explanations. I reset the codes yesterday and after an hour of driving around they still haven't returned. I'm wondering if a vacuum leak (at least a significant one) would trip the code faster than that. Anyway, I have a three day weekend so I'm probably going to start by checking the MAF to see if it's dirty, then check lines going into MAF, PCV, etc.; then I'll probably go ahead and do the carb starter fluid leak check on gasket. I was thinking the same thing about the O2 sensors, both banks should trip if one is bad. I guess if I don't find either a leak or a dirty MAF, I'll but the real time data scanner and see what's going on with the sensors. Thanks Again!
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Old 10-22-2015
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Thanks

And just a correction, your '99 will have 65psi fuel pressure.
Ford switched to Returnless fuel system in the late '90's, which used 65psi previous years used 35psi
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Old 10-25-2015
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Update:
1) I ordered a scanner with real time data capability. Think this will hopefully help me figure out what reading is kicking the codes.


2) I removed and inspected the MAF sensor. Nothing looked out of sorts. Maybe I could imagine a layer of grime but in general it looked pretty clean. I sprayed it with MAF cleaner regardless.

3) I took some very basic excitation voltage reading of the MAF to see if it was at least active. With key on, engine off I saw minimal voltage (maybe .1 VDC). Then I turned the truck on. At idle, it jumped up. Oddly enough, I found that with my multimeter on the MAF connector, the engine seemed to bog out when I pressed the gas pedal. Figured maybe the resistance of the multimeter setup was putting load on the MAF and throwing off the signal??? Heck I don't know. But when I removed the leads from the connector it was back to normal. The fact that it jumped up at idle made me think the MAF was probably working. Once I get real time data i'll verify this.

4) Today I decided to isolate different portions of the intake and did a cigar smoke leak check. First I just removed the air filter housing and taped around a hose that I put into the inlet duct. I then blew smoke in toward the intake manifold. The only smoke I observed was coming from the IAC valve. I don't think that is abnormal or unexpected (please correct me if I'm wrong).


5) Next, I taped off the inlet at the throttle body inlet. I also removed the IAC valve and taped it off. I then blew smoke in the manifold via the brake booster line. Mostly I did this because I was worried that smoke was not getting past the IAC and all the way into the manifold when I performed step 3 above. I observed no smoke coming from anywhere around the manifold and I could feel back pressure as I was trying to continue blowing smoke into the booster line (indicating it was leak tight).


6) I did go ahead and spray starter fluid around the manifold and some areas I thought could be leaking. I was a bit nervous about starting a fire so I didn't spray very liberally.


At this point, I'm thinking of looking into the fuel injectors. I'm thinking maybe clogged injectors? Although it would be odd that both banks show lean. Or mayabe fuel pump? But then again, I would expect another engine code if that were the case.


Once I get my scanner, I'm hoping the real time data will help shed some light! I gotta get this dang thing smogged so I can register it under my name!


Any ideas/input would be greatly appreciated. Thanks!

Last edited by glreynol; 10-25-2015 at 11:58 PM.
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Old 10-26-2015
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The IAC valve is a sealed plunger you shouldn't see smoke coming out of it's vent if that is what you mean.
IAC valve has a vent on it's case that allows air to get out and in from behind the plunger, this allows the plunger to move in and out of its tube without being effect by air pressure behind it.
I can see a little air passing by the seal but not much.

Start engine and run until warmed up, then let it idle and unplug the IAC valve, idle should drop to 500rpm or engine may even stall either is good, it means no vacuum leak.
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Old 10-26-2015
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Thanks Ron, I'll definitely try that hot disconnect test and see what happens. The smoke coming from the IAC was fairly significant. It took a couple exhales of smoke but once the intake was full of smoke, it would come out of the IAC housing any time I put pressure (blew into the tube). I read several mixed review online that claimed this was normal, but I was struggling with understanding how that is acceptable (unless there's a check valve, leaks aren't usually one way!). I guess I was thinking that the plunger seal unseats on shutdown (i.e. vacuum seats it). Maybe this will turn out to be the issue.


I will probably try this test when I get home tonight and let you know how it goes. Thanks again Ron! I really appreciate you taking the time to distribute your knowledge!
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Old 10-26-2015
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I went ahead and disconnected the IAC. Engine RPMs dropped to a 500 RPM purr and sprung a new code (P1504) which I believe is just an IAC open circuit (expect it to clear).


I'm wondering if I shouldn't start looking into the fuel system (filter, injectors, etc.). Since I'm throwing both lean codes I'm thinking clogged injectors seem unlikely. Maybe filter?
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Old 10-27-2015
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Yes, if there are no air leaks, and MAF sensor seems fine then low fuel pressure would be next on the list.

You can get pressure gauges for $20-$30.

Thing with fuel pressure is that it can test OK with no load, but in actual driving conditions where fuel is being consumed in larger volumes it can fall to the point of getting Lean code.
What I would do is to test pressure by leaving engine off, but turn key on and off a few times to prime system pressure, should be 65-75psi, then let it sit for an hour or so and see if pressure holds above 55psi.
If it is dropping leave it longer and see if it continues to drop.

If it stays at 55psi or higher then start engine and raise RPMs to 2,500(approx.) and hold it there, watch if pressure starts to drop, if so then there is most likely a fuel pump problem, it can't maintain pressure.
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Old 10-27-2015
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Thanks Ron. Would you expect a P0087 code to pop up if fuel pressure was low? Or can it be low enough to kick a lean code without being low enough to kick a fuel syst pressure code?
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Old 10-27-2015
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Newer models(I think around 2004 and up) are suppose to cycle power to the fuel pump based on a pressure sensor but..........never seen that code come up before, diesels will, never seen it on gas models.....yet.

Your 1999 won't have a fuel pressure sensor so couldn't generate that code in any case.
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Old 10-31-2015
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I'll chime in here, I have a 98 b4000, (4.0l ranger) and I had same codes after I did my rebuild (both cylinder heads cracked) monitored o2 sensors with work scan tools and uppers weren't responsive at all, likley from all the coolant going by them.

It's always worth a look to see if o2s are pooched as it may be an indicator of bigger issues.
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Old 11-03-2015
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Quick Update:
My buddy who used to be a master ford technician helped me troubleshoot it. He's been saying from the start that it's probably the mid intake manifold gasket. Turns out, he was most likely correct. While spraying starter fluid at that gasket we had his real time scanner plugged in and saw the STFT rising from -15 ish to positive. I just went and bought intake gasket kit. Have everything pulled off intake and plan to pull manifold and replace gaskets this weekend.
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Old 11-06-2015
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Good work, let us know if that does it.

Not sure what "mid"-intake gasket is, there is the lower and upper and then the throttle body gasket.
There is a Fuel Rail gasket as well, which is between upper and lower gaskets, is that the one?
Although i would expect a fuel leak more than an air leak in that one.
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Old 11-15-2015
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Hi Guys,
Progress is going pretty well...a bit slow because I'm very **** about cleaning everything and am painting valve covers and waiting on fuel injectors to come in because I figured I'd change out the injectors while I'm down here. Anyway, I just put the lower intake manifold back on with the new gasket and realized today that I didn't put a bead of RTV around the coolant ports, I only put RTV at the transition from valve cover to intake valley...Now, a day later, I'm wondering if I should loosen the lower intake and prop up the front enough to run a quick bead of RTV around the coolant port also...do you think that's a bad idea? Will the gasket resettle fine? I don't want to create leaks but I also don't want coolant leak. Any thoughts are greatly appreciated. Thanks!
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Old 11-15-2015
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Tough call, it should be fine, I have done 4.0l OHV with and without sealant on intake coolant passages, never had a problem either way.

What generally causes that type of leak is the intake bolts backing out which is common on the 4.0l OHV, it also causes a vacuum leak.
I might use locktite or similar on bolt threads but not remove intake
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Old 11-22-2015
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Update:


I got my truck back together. Overall, started right up. I had forgotten to tighten down the fuel supply line on the fuel rail (had only hand tightened) so that was a small mess but otherwise it went fine.


Now noticing that it runs a bit rough when in gear (drive or reverse) and sometimes even stalls out. Check engine light is back on after about 15-20 miles of driving. Ran the codes and I'm now getting P1131, P1151, P0171 (P) and P0174....So I'm stoked to have caused more problems than I fixed.


My only issue with all the repairs I did is that I sheared the torx stud head off one of the fuel rail stud bolts. This is an obsolete part but I was abe to reuse it and get it torqued down by using two nuts and torqueing the top nut (it seemed to be torqueing down the bolt fine...).


I decided to troubleshoot the MAF sensor further. I hot disconnected the MAF and the idle seemed to drop to a really nice 800-900 clean idle. Then I put the truck in gear (both drive and reverse) and the slightly rough condition seems to go away....Does this sound like a bad MAF sensor????
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Old 11-22-2015
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When you unhook the MAF sensor the computer switches to Open Loop so it is running pre-set fuel mix based on RPM, Speed and throttle position.

So not really a definitive test that MAF sensor is bad.

After engine is warmed up it is in Closed Loop and is using the O2 sensors, your code say the O2 sensors on both banks are showing low voltage, lean.

O2 sensors generate there own voltage, 0.1v to 0.9v, low voltage means too much oxygen in exhaust(lean) and high voltage means not enough oxygen in exhaust(rich).

Your new codes means O2 voltage is low and not switching(getting higher) when computer adds more fuel
The O2 voltage changes a few times each second, so very fast, because it is not changing means wires are unhooked(0.0volts) or exhaust manifold leaks on both banks is sucking in air.
The 171/174 codes are a result of the O2 sensors having low voltage at this time.

I would use real time scan and see what MAF sensor is reporting, make sure it is set to metric scale, grams per second, gm/s, 4.0l engine should read about 4 gm/s, 5.0l engine 5 gm/s, at idle.

And after engine is warmed up, O2 sensors need to be 600degF or higher to work, look at the O2 voltage , sweet spot computer tries to maintain is 0.4v but it is normal for voltage to range from .3v to .6v.
It reads like they are stuck down at low voltage.
Spray some fuel into engine and see if voltage jumps up.

Last edited by RonD; 11-22-2015 at 01:49 PM.
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Old 11-22-2015
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Hi Ron,


So I checked the MAF sensor with my scanner...looks good (as usual you are right again!). It was 4 gm/s and increases proportionally with increased throttle/engine speed.


I also took a look at both bank1 02 sensor 1 and bank2 02 sensor 1. Both basically sporadically read between 0.1 & 0.9 V. When I remove the brake booster vacuum line and spray starter fluid into that intake port both 02 sensors pretty much hold 0.8-0.9V readings.


Any thoughts? Maybe it's a fuel filter (still haven't replaced that)? I guess it could still be a leak but that would sure be frustrating!


Thanks Ron D!
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Old 11-22-2015
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What do the STFTs show, short term fuel trims.

0 = MAF data and computers calculation at 14: 1 ratio
+ number means computer is having to open injectors longer than calculation says, so O2 on that bank is showing lower voltage.

- number is the opposite, O2 had higher voltage so computer is closing injectors sooner than calculation would call for


After warm up what is the engine idle RPM, and what is it with IAC Valve unplugged?

If idle drops down to 500rpms or engine stalls with IAC Valve unplugged then no vacuum leak, if it stays at or above 700 then there is a leak somewhere
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Old 11-22-2015
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My STFTs are high...+ 19 to 29 range. When I disconnect the IAC, the engine RPMs drop down to 500 and hold there.
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Old 11-22-2015
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Ron,
I also looked at O2 Bank1 Sensor2 (think that's downstream of the cat) and it appears to be dead (holding at 0.0V and not moving). Could this be the issue?
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Old 11-22-2015
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No, rear O2 should work but it's primary purpose is to check if Cat is cleaning exhaust, it also contributes to LTFT, long term fuel trim, which is only used when driving cold engine and in other Open Loop situations.
It couldn't cause your problem or codes.

Those are high STFTs, and with no vacuum leak you are down to low fuel pressure or Computer problem.
Both upstream O2 sensors failing at the same time would be, well..................a very long shot.
BUT(big but) the rear O2 sensor is not working at all could be sign of wiring or computer O2 circuit issue.
If O2 sensors were wrong and computer is adding too much fuel then you would see rough running and more smoke out the tail pipe because engine is really running Rich at +20
You could pull one out and see if it shows rich running, black carbon build up, don't think spark plugs have been in long enough to see that.

Last edited by RonD; 11-23-2015 at 08:39 AM.
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Old 11-22-2015
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Well, here's to hoping this isn't a computer problem...I don't even know where to start troubleshooting that. Since I've already done injectors sounds like I need to look at filter and pump...
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Old 11-22-2015
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I have an 02' Edge 4.0 with a similar issue, following!

Mine run great above 1k RPM, but has the tick below that number.

Maybe your solution will be useful in my scenario.
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