Code 41 System Lean, Fuel Control 4.0 OHV

[Elutheros]

It's a 1991 Ranger with the 4.0 OHV. Before I started the recent maintenance it had the same code. Because it had zero compression on #4 I got new heads(the previous remanufactured heads were a mess), all new intake gaskets, new pushrods and rockers and de-coked the intakes, rebuilt the injectors, installed a new fuel filter, and a new FPR. I tested the fuel pressure previously and it was 30 at idle so I didn't suspect the fuel pump. It's all back together and still running rough and throws a Code 41. I used propane to search for vacuum leaks and found none. Looking for next steps to diagnose this before I trash my new heads and valve job. And, I almost forgot--new O2 sensor also.

 

[alwaysFlOoReD]

30 psi at idle is low. You should be 40 +-

 

[RonD]

Code 41 is System lean - Fuel control

No one knows all the codes by heart so good to post definition with the exact code

Lean code means, after full warm up, the computer is opening the injectors at least 15% longer than it calculated from sensor data

Gasoline's air/fuel ratio is 14.7:1, and that is a WEIGHT ratio
14.7 POUNDS of air to 1 POUND of gasoline
14.7 grams of air to 1 gram of gasoline
a WEIGHT ratio not volume, which is why 200mpg carbs and "pre-vaporizers" were and are HOOEY

Computer "knows" it is operating a 4 liter engine, so it knows how much air will be sucked in at any RPM, its just math
And it "knows" how much air WEIGHS at sea level
Computer also "knows" fuel pressure should be 35psi(approx.) and it "knows" fuel injector size, and it knows how much gasoline weighs, so it knows how much fuel(its weight) will flow from that size injector when its opened for say 100milliseconds(ms)

It then uses throttle sensor, MAF sensor and air temp sensor data to off-set the RPM calculations
MAF sensor tells computer the WEIGHT of the incoming air, higher elevations means lighter air, warmer air is also lighter than cold air(hot air rises, because it's lighter)
Air temp sensor(IAT) is also helpful for air temp vs elevation detection
Throttle sensor is more for smoother acceleration because air flow thru MAF data is slower to change on quick acceleration.

So say the computer calculated the open time for the injectors at 100ms and sends that fuel in
It then looks at the O2 sensors voltage, .1v is lean(high oxygen), .9v is rich(low oxygen), O2 sensors see Oxygen not fuel

If O2 shows .1v then computer opens injectors 102ms, then 104ms, then 106ms, ect.....until O2 shows .3-.4v
If final open time was 118ms then thats an 18% difference so computer sets a code to let driver know that something is wrong with its base calculations

All this happens in under 2 seconds, just FYI
So engine is never actually running Lean, if you hear pinging/knocking THEN engine is actually running Lean

Alwaysfloored is right, expected pressre is 35-40psi
But I think 30psi pressure should be OK, unless it is dropping below 20psi when engine is under a load it should be fine, but you should hear pinging if that's the case

An exhaust manifold leak sucks IN air, O2 sensor sees that air so causes a "false Lean"
Any misfires means the oxygen was NOT burned/used up, so its dumped into the exhaust and O2 sees that as Lean

MAF sensors need to be cleaned because as they get dirty they report lower air flow/weight, so it throws computers calculations off, so computer has to add more fuel than it calculated, above 15% more and you get the code

And of course the Vacuum leak, its unreported air coming in, so MAF sensor data is off which makes computers calculations off
Any air leak does that, like in the large air tube from MAF sensor to intake manifold, if its letting air in then same as vacuum leak, unreported air, so check it for cracks

 

[Elutheros]

Thanks RonD. I'm going to clean the MAF and see what happens, make another round of vacuum leak checks, and put a stethoscope to all the injectors to make sure they are working. It idles rock steady--just a little rough.

 

[Elutheros]

Well, I cleaned the MAF--no change. Bought a stethoscope and I can hear all the injectors working. Did another round of vacuum leak checks and nothing. Checked spark on every cylinder and they are all good. Took the cover off of the ECU and checked the capacitors--everything looks brand new. Finally checked the fuel pressure: 30 @ idle and never goes above 30 when the engine is at higher RPM. Let is sit for 5 minutes and the pressure sat at 29. Sooooo, is it really the fuel pump? I guess that's the most likely answer but I'm not 100% convinced.

 

[Elutheros]

After all that it turned out to be the crummy Autolite sparkplugs. New Motorcraft plugs installed and all is well.

 

[alwaysFlOoReD]

Glad you figured it out.
I'm trying to figure out why it threw the code 41?

 

[RonD]

Code 41 can be from misfires, no oxygen is burn in that cylinder and O2 only sees oxygen so...............
O2 shows high oxygen(low volts) which is "lean" as far as computer is concerned

Good work on the spark plugs, never had an issue with Autolite, but usually get Motorcraft for my 4.0l in any case