Hey, on my 97 Ranger 2.3, I'm getting a p0171 check engine code. Bank 1 running lean. I've replaced my fuel pressure regulator(it was running low psi, this did fix that, but not the P0171 code), PCV valve, and spark plugs. I didn't see any obvious vacuum hoses that looked cracked or anything, they all look like they're in fairly good shape. If anyone has some idea on what else I could try, that would be appreciated.

 

Welcome to the forum

MAF(mass air flow) sensor should be cleaned every few years, I would do that.
If MAF is dirty it will under report air flow, so computer adds less fuel, and you get a Lean Code.


After engine is warmed up it should be idling at about 750-800RPMs
While engine is still idling unplug the IAC(idle air control) Valves 2 wire connector
RPMs should drop down to 500 or so, engine may even stall, either is good it means no vacuum leaks
If RPMs stay high, there is a leak somewhere.


Upstream O2 sensor(near exhaust manifold) is what tells computer if exhaust is Lean or Rich.
These use a chemical reaction to get Oxygen levels in exhaust, so do wear out, 100k miles is recommended replacement time for upstream O2
These tend to wear out Rich, causing false Lean codes, so MPG goes down as they get older.
If close to 150k miles since it was last changed I would just change it to get back best MPG, that will pay for cost of O2 over the next 150k miles.

O2 could cause P0171 but a long shot, usually you would get a "slow/no O2 switching" code along with Lean code if O2 was having a problem.


And the P0171 could go away on its own.
If fuel pressure had been low for while computer could have set a high Long Term Fuel Trim to compensate for that, and it will take a week or two for computer to adjust for the higher pressure

 

Thanks for the quick and detailed response. I just got done cleaning my MAF, so I'll have to update you on that in a couple days. I don't have a tachometer, but I did unplug the IAC and the RPMs noticeably dropped a bit, and after plugging it back in they went back to normal. I'm assuming that that's working fine, and I have no air leaks. Just to reiterate, cleaned MAF, and IAC dropped RPMs. So what I'm gathering, it's probably not an O2 sensor? I'm not getting the no O2 switching code, just the Bank 1 running lean.

 

Good work

Yes, if RPMs dropped with IAC Valve unplugged then computer is setting the idle level not a vacuum leak :)

Now just a heads up about a code like P0171:
Computer calculates the amount of fuel to add to the intake based on the air flow data from MAF sensor AND the RPM table for the 2.3LITER engine it has in memory.
Computer "knows" how much air a 2.3liter engine will suck in at any RPM and throttle position already, it is just math, and it uses the MAF data to fine tune that air flow because higher altitude air is thinner than at sea level, and colder air in winter is thicker than warmer air in summer, these are the variables the MAF is there to deal with.

Once the computer calculates the amount of fuel required it opens the fuel injectors for 100ms(milliseconds), as an example, then it waits for O2 sensor to report Lean/Rich.
If Lean computer opens injectors 105ms, then waits for O2 report
If Rich computer opens injectors 95ms, then waits for O2 report

This happens in micro-seconds in real time, and computer is constantly adjusting open time for injectors, based on RPM, throttle position and MAF data, with feedback from O2 sensor.

This adjustment is call STFT(short term fuel trim)
0 STFT is what computer calculated
+5 STFT means computer is adding open time to injectors
-5 STFT means computer is shortening open time for injectors

The average over time of the STFT is called LTFT(Long term fuel trim)
LTFT is used mostly at cold start, or if MAF or other main sensor should fail, allows computer to run engine better without all data coming in.

-10 to +10 STFT is fairly normal for STFT and LTFT on older engines

When either gets to 15-20 above or below 0 is when computer will set a code
+20 would be Lean code for LTFT
-20 would be Rich code for LTFT

+20 doesn't mean engine is running Lean, it means computer is adding more fuel than it calculated to make O2 sensor data "normal"
So engine is running fine, computer is just letting driver know there is a problem with the calculation, i.e. data from MAF or O2, air leak, fuel pressure, ??

 

Ok, that was a ton of cool info, thanks! So if this code keeps happening, would it be a good idea to replace my O2 sensors? Or just the one before the catalytic converter? I'm currently at a hair above 97k miles, and I've read online it's good to replace those every 100k miles. I don't think that's the issue I'm having currently, but I have no idea.

 

Well, bad news. About 60 miles after cleaning the MAF, the check engine light came back on. Still the same code. Should I replace the fuel filter, and see if that does it? Or just go straight for the O2 sensors? I don't think my MAF is bad, because my Ranger idles just fine, with normal RPMs and stuff. I do have the ohm values of the fuel injectors, and I'll check those later, but I seriously doubt that's it. Mainly because my truck sounds normal, but the computer says it's running lean.

 

Remember it is not actually "running" lean.
Code means computers calculation is Lean in the "real world"
So too much air or not enough fuel is the issue, and computer is compensating for it and letting your know about it

Yes, fuel filter should be changed every 5 years or so.
Also run a can of Seafoam or similar injector cleaner in the gas tank, I do this once a year, really smooths out my idle on the 4.0l

yes, changing upstream O2 at 97k would be good even if it is not the problem, your MPG will get better

Not sure checking ohms on injectors would ID a problem, you would have a misfire if injector was out of spec.

Checking fuel pressure would be helpful
Pressure at idle, 30-40psi is spec for 1997 and earlier Rangers

Pressure when holding RPMs high for 30 seconds or so, slowly dropping pressure means filter or pump is clogged or failing

Pressure after 5 minutes engine/key off, Pressure should hold for MONTHS with key off, drop to 25psi or so is fine but it should stay there

Autoparts stores often have tool rentals or loaners, so you can rent a pressure gauge kit, deposit is often full price of the gauge, $50, then you get it all back on return(loaner) or 90% back, so $5 rental

 

Ok, I gotcha now. I can't change my fuel filter till tomorrow, so I'll have to update again. I actually have a fuel pressure gauge, and have tested it. My truck specifically, according to the Hanes manual, needs 35-45 with the engine off, key forward, and 25-35 with the engine running. I have 40, and 30 respectively. It sounds to me that the fuel filter and O2 sensor are the last things that I can try. You're right about the injectors. I don't know what I was thinking there. Is there anything else I can test for?

 

Run engine at higher RPMs for 30 seconds while watching fuel pressure, it should stay steady, if it is slowing dropping then at highway speeds(high fuel demand) you will get lean code because of low fuel pressure

 

Tested, and it holds a steady 35-37 PSI. From what my Hanes manual said, 25-35 is good. So I'm guessing that fuel pressure isn't an issue, and it's all O2 related? Because I think that's the last piece to the puzzle. It's the only thing that I haven't changed.

 

Fuel pressure is fine.

No real test for a "working" O2 sensor, so swap away.

usually you would get a "slow switching" code along with Lean or Rich code if it was O2 related, but.............

One of the benefits, besides extra power, in having a V6/V8 engine is that each bank(side) has its own O2 sensor, so O2 issues are easier to pin point, or you can just swap O2 locations.

You could swap your O2s front to back and visa versa, they are the same sensor models