Hello I have a 1997 Ranger 4wd 4.0 and it’s currently throwing the P0171 (bank 1) and P0174 (bank 2) lean codes. This happened to me last year and my buddy and I tried to read the PID fuel trims to diagnose the problem further. Sprayed the manifold for leaks - nothing. Everything looked OK so we reset the code. Just popped up again, but I believe there’s actually an issue that was misdiagnosed in the past.

I know those codes can mean a few different things, however when I floor the gas my truck shuts off which I’ve read may be a fuel issue. It’s also idling rough. I did clean the MAF sensor (did not replace though) I’m hoping that narrows down what it could be and someone could point me the right direction.

Thank you for any help

 

Lean code means computer is opening fuel injectors longer than it has calculated, usually 20% longer before it sets a code

Computer "knows" its running a 4 LITER engine so it "knows" how much air will be coming in at any RPM, its just math
It then calculates the 14.7:1 air:fuel ratio based on RPMs then modifies that based on MAF data and air temp, and throttle position
Computer "assumes" correct fuel pressure, because there is no "fuel pressure sensor"

1997 Ranger will run with 30psi fuel pressure
If there is a fuel filter or fuel pump issue then pressure will be less and that causes Lean code
Test pressure if you can, if not change fuel filter, not expensive and should be changed every 5 years or so


An air(vacuum) leak is also a common cause for Lean code, computers calculations are thrown off because ALL the air coming in is not going thru MAF sensor
If you get 2 Lean codes on a "V" engine then it won't be a lower intake leak, that only effects 1 side/bank of a "V" engine
Also wouldn't be O2 sensor issue because to have BOTH fail at the same time would be really really long odds, lol


If an engine is actually running Lean it will ping/knock, if running Rich it would make gray smoke
These codes are set because computer has noticed a problem with its calculations

 

Great information. Thank you

No ping/knock and no smoke. It does idle rough and my truck will stall around 500-1500 RPMs. What would cause this? Since it wouldn’t be a lower intake leak where should I be looking?

Today or tomorrow I’m going to replace my fuel filter and clean the MAF sensor again. Will try to get more information from the computer hookup this week.

 

Have cleaner and will clean MAF tomorrow. Fuel filter has one quick connect on one end and a simple clip on the other. Kind of odd. I took the clip off and I could simply remove the line by pulling. To no surprise gas shoots out. Is there a way to stop the flow of gas while changing the filter?

 

No, it is siphoning out of the tank or draining back out of the engine end, shouldn't be too much though

 

Would it be a bad idea to take out fuel pump relay and let it stall?

 

Yes, or the fuel pump fuse

 

Fuel filter changed and MAF cleaned. No leaks i manifold. Codes still present and stalling occasionally still occurs around 1000 RPMs. Could it really be both O2 sensors? Any other recommendation?

 

I would rent a Fuel Pressure gauge, there is a port on the engine to hook it up, just to take that off the table

You can get a Bluetooth OBD reader for $15-$25, works on any vehicle sold in the US or Canada since 1996, it was "the Law"
So not a bad investment, they connect to a smart phone or tablet wirelessly(bluetooth) and the APPS are free or $5 for a full meal deal, which most don't need

This allows you to see Live Data while driving, same stuff the Computer is basing its air/fuel mix on, so you should see "why" computer is opening injectors longer
If you get one get an extension cable with it, some OBD plugs can be hard to get at, so the cable is worth the $5

If the engine had blown head gasket or was burning oil then yes BOTH upstream O2 sensors could be effected at the same time, longshot

 

Wouldn’t those just show the codes I posted about? I know someone who has one that shows fuel trims, etc. I will do that this weekend as well as looking into a fuel pressure gauge and report back.

I appreciate your help. Thank you

 

No, the bluetooth readers do read codes but you can also watch Live Data like fuel trims, MAF voltage, O2 voltage, coolant temp, air temp, while you are driving, in real time
You see what the computer sees, the Lean codes come from "what the computer sees"

Google: torque lite app
Its the free one

Torque pro costs $5

 

Sorry, but I wanted to post this short video of the surging of the engine: https://streamable.com/02ypf

While it’s surging like that the RPMs are jumping back and forth between 500 and 1000. About 20% of the time when I slightly give it gas it stalls. It almost sounds like it’s not getting enough fuel. However, if I let it surge like that for a minute or so it will stop and RPMs will rest at a normal (I think it’s normal) 600. Hmm

edit: came across this in my research - fuel rail gasket https://www.ranger-forums.com/4-0l-o...6-4-0-a-40394/

will try and replace and report back

 

Well...I went and did all manifold gasket including fuel rail. Good news is I fixed my coolant leak. Bad news is that I’m still idling funky despite the lean codes disappearing. When I have it in drive with brakes on it’s perfect. When I put it in park it idles rough and sometimes it idles smooth, but at a constant 1200 RPMs. Can you think of any small hoses that I may have missed that people tend to forget? Possible I need to drive it more to reset the computer?

Any other recommendations? Thanks

 

Plug PCV hose and see how it runs, PCV valve should be closed at idle

Throttle body gasket can leak, did you replace that one?

 

Dumb question - what’s the best way to plug it? Plug it where it connects to intake or on top of valve covers?

I did not change throttle body gasket

 

Just take the hose off PCV valve and stick a bolt in it or ??, what ever fits

Not a fix, its a test

PCV valve is a bit of an oddball valve
It opens and closes based on engine vacuum
At idle the high vacuum Sucks it closed, when driving the vacuum is lower so it opens and sucks in "blow-by"(exhaust gases) that are in the crankcase

It has a weight inside, the thing you feel clunking when you shake one
IF vacuum is high enough then it can suck that weight UP to close the valve, when vacuum goes down weight falls and valve is open

So if there is low vacuum because of engine issue or leaks or just a failing PCV valve then you can get that up and down idle

 

Will do. Would idling normal while in drive with brake on be an indicator of a certain problem?

 

Nothing comes to mind

 

Changed PCV. Not fixed. Decided to spray some carb cleaner around and I finally found the issue.

There’s a leak in the EGR pipe that goes into the upper manifold. I sprayed the cloth sheathing around that pipe and idle changed. Problem is is that the screws on the valve are rusted to the point where I can’t unscrew it. I guess everything needs to come out if possible

 

Ron, I appreciate all your help. There was a crack in the EGR pipe leading into the upper manifold behind the the cloth sheathing. Instead of replacing the whole pipe, as it’s rusted behind belief, I put a little JB Weld on it for now which seems to have fixed the problem. I’ll edit my original post with the fix for future reads in this thread. Thanks again

edit: does not look like I can edit my original post

 

Good work and good find

Thanks for the follow up, it will help someone else down the road

 

I really hate reviving this post, but I'm just having a hell of a time figuring out what's going on.

Check engine light came back on throwing P0171 (lean) and now the new code P0173 (fuel trim malfunction). I noticed my truck is acting as if there is vacuum leak once again. I believe it's my EGR valve. The pipe from the valve to the upper had a tiny crack in it. I JB Welded it for now, but after a week the weld is starting to break down and the crack is visible once again. Take a look at what is preventing me from changing the valve and that upper pipe: http://imgur.com/a/4uYYTVk . What the hell is going on there? How does one remove that.

As far as the P0173. Fuel pump?

 

What engine?

If a 4cyl then Lean can be fuel pump or filter issue

If a V6 engine then no because only one side of engine is throwing a Lean code, BOTH sides would need to be effected if its a fuel pressure issue

 

V6. I really think the issue is the EGR. https://imgur.com/a/4uYYTVk . If you look at the picture the nut is...not there? How do I tackle getting that off?

 

Sorry to dredge this thread up yet again. I'm getting the p0171 & p0174 (via Torque pro), unlike other the previous poster, I am not having any rough idle. However, my truck does die periodically after applying the brakes hard.
Truck: 99, 4L OHV, 4x4