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  #1  
Old 07-30-2014
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m.a.f. dilemma

The check engine light came on the other day, had it checked at a local Auto parts store, code said it truck was running lean...cleaned the m.a.f. with appropriate cleaner, check engine light went away for a few hours and came back on, went to junk yard and bought another m.a.f. ...long story short check engine light still on,plz help me
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Old 07-30-2014
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Is the engine running Lean?

Do you feel a lack of performance or is there pinging when accelerating or going up hill?

Most common cause of Lean mix is an air leak, vacuum leak.

With engine warmed up, idle should be 700-800rpms.
The IAC Valve is used by the computer to set the idle RPM.
With engine idling unplug the wire connector from the IAC valve, idle should drop to 500 RPMs or engine may even stall, either is OK and means no vacuum leak.
If idle stays high then you have a vacuum leak.

PCV valve hose would be the first thing to check.

To check for leaks you can pull one hose at a time from the vacuum manifold(where all the hoses hook up at the intake), pull the hose and plug the port with your finger, if idle drops to 500 or engine stall you found the hose with the leak
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Old 07-31-2014
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m.a.f. dilemma

Thanks I'll try that only other things that is seems like it's. Losing a lil power going up hills and Dosent always start on the first go. Let ya know how it goes, thanks for the response you All are life savers
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Old 08-03-2014
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m.a.f. dilemma

Unplugged the iav valve the rpm went to down to seemed to be it's norm,but did not cut out.should I bother messing with the pcv and hoses,sarcoma maybe.need it fixed asap since its due for emissions this week.
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Old 08-03-2014
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Idle needs to be BELOW, "it's norm" with IAC valve unplugged, or there is no point of having an IAC valve, computer has to be able to set idle at 700-1,200 depending on engine temp, if its lower limit is 700 then it will have no lower control.
With IAC valve unplugged on warmed up engine idle should be 500 or it can stall, either is fine.

So yes check for vacuum leak.
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Old 08-06-2014
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still struggling

Ok so I've, installed a new m.a.f., pcv valve,checked for vacuum leaks (none found), Unplugged iac and trucks rpm went down but it didn't cut out.went to emissions and still failed...same codes as last time, 171,174? Totally lost and frustrated I have 3 weeks till I have to go back to emissions. ..any help is appreciated. God bless

Last edited by jerseyman32; 08-06-2014 at 07:45 PM. Reason: k
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Old 08-06-2014
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Can you get/borrow/rent an OBD II scanner?

It will have a PID Menu, select Fuel Trim and use that to ID problem.

Just a quick run down on 171/174 codes
171 is lean on Bank 1
174 is lean on Bank 2
If you get both codes then this is most likely not an O2 sensor issue, for both to fail at the same time would be million to one shot.

So we assume whole engine is actually running lean.

What sets lean code?
The computer reads the data for the amount of air coming into engine via the MAF sensor, it then opens the injectors for a specific time for that amount of air volume.
It then watches data from O2 sensors and "trims' the open time for the injectors based on O2 sensors lean/rich data on exhaust's oxygen levels.

In simple terms the computer has a trim setting for best performance and MPG, that is 0 trim, "-" is less fuel added, "+" is more fuel added.
The computer constantly adjusts the open time of the injectors and watches O2 responses.
So if you monitor fuel trim on an OBD II scanner it can tell you alot.
Idle is usually -3 to +3 it will switch back and forth very fast that is normal, if you have a vacuum leak it will be switching between +5 and +10 or higher, as you increase RPM the trim will drop closer to normal(0) because there is less vacuum at higher RPMs, so less air being sucked in.

What sets the lean codes is when the computer is trimming fuel at +10 to +25 all the time and O2 sensors are still showing slightly lean or normal, and should be showing rich with those trim values, so computer is letting you know there is a problem.

Yes, MAF sensor is a common issue causing these codes, if MAF is under reporting the amount of air flowing in, then computer is not adding enough fuel at 0 trim, so it adds more, +20 and lets you know by setting codes 171/174.

Low Fuel Pressure can cause it as well, computer expects 65psi of pressure at the injectors on a 1999 Ranger, if pressure is only 40psi computer has to open injectors longer(above +10) to get a normal O2 reading.
When you rev engine to 2,500rpms and hold it there and you see fuel trims run up to +25 you probably have low fuel pressure that gets even lower when fuel demand increases.
You can rent fuel pressure testers.

If you have an EGR system then you will have a DPFE sensor.
DPFE sensors seem to be a problem on Fords, lol, (did I say that out loud?)
Anyway DPFE sensors tend to fail above 60,000 miles, these usually set a 401 code when they fail but not always.
DPFE sensor reports the flow of exhaust gas thru the EGR valve and into the intake manifold, so kind of like a MAF for exhaust gas.
If the DPFE under reports flow then too much exhaust gas is coming into the intake and leaning out the fuel/air mix, causing 171/174.
Non-Ford DPFE sensors are about $35-$45, unless yours is new I would just replace it, because it will fail.
Also make sure it's 2 hoses are clear and hold pressure, not cracked.
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Old 08-07-2014
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Thanks Ron I'll do all that you mentioned, I appreciate it very much! God bless
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Old 08-08-2014
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Your welcome.

Also I forgot to mention, engine needs to be warmed up when testing, computer runs the engine in "Choke mode" when cold, so higher idle, richer fuel mix and advanced spark timing.
O2 sensors are off-line at that time, and O2 sensors don't work in any case until they are heated up to 650+ degF.
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Old 08-16-2014
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Arrgghh, it's always something

Ok so had a buddy of mine hook up a obd 2 scanner last week and it showed the lean codes 171,174 he cleared the codes and they came back on. He also looked at the o2 sensors and they were fine,not sure what else he did,but when I went to the auto parts store they checked the codes again and now along with the po171. And po174, I'm got 1131 and 1151 which means both o2 sensors are bad. ..is this capable of ihappening or did he mistakingly do something with the obd sensor
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Old 08-16-2014
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1131 and 1151 don't specifically ID that O2 sensors are bad, the codes say the O2 sensors are not switching from lean to rich very fast, they are showing lean more than rich.

The Computer(PCM) constantly increases and decreases the "open time" for the fuel injectors and then watches the O2 sensor's responses.

For both O2 sensors to "fail" would be a long shot, so I doubt they are the problem.
If you can't find the reason then common factor is the PCM
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Old 08-16-2014
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As RonD stated could be PCM also the O2 sensor failure is highly unlikely for both as stated the extra codes can sometimes piggyback (what i call it) there may not actually be a problem with that system but the systems all work very closely together so if it was initially running lean then the O2 try to compensate (seeing readings outside spec) pushing them out of there normal operating range cause an additional code it doesn't necessarily mean something is wrong with them the PCM just thinks its broken because they repeatedly go out or normal operating range causing an additional code (caused by continual time driving with initial problem) Most sensors have a set number of times or duration that they can exceed spec before they set a code continuing to run/drive the truck with the lean could have made it exceed the set time before the codes set. Odds are if you find the original cause of the lean code the O2 codes will go away with it.

Last edited by pearlkid9988; 08-16-2014 at 01:57 PM.
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Old 08-16-2014
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thanks fellas

Can it be the dpfe? Or is my best bet to just bring it in for a diagnostic?
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Old 08-16-2014
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Disconnect EGR valves vacuum line and plug the line, don't want a vacuum leak.
EGR valve will no longer open "too much" if that is what is causing the Lean codes.

Clear the codes(unhook battery for 5 minutes), then drive until you get CEL again and see what it says, if it is just the EGR codes, then yes replace the DPFE sensor, and make sure its hoses are clear and connector is not corroded.
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