Misfire Mystery

[01WHTRNGR]

A few days ago my SOHC 4.0 threw the code for random/multiple misfires. I replaced all the spark plugs and then it would throw the codes for specific cylinders or the random/multiple one. I unplugged the MAF and that fixed all the misfires. I then replaced the MAF with a new one I had laying around and that didn’t fix it. I then cleaned both MAFS and that accomplished nothing. I’m supposed to do a 3.5 hour drive Friday and I’m running out of ideas. I was hoping somebody could help me. Thanks.

Also, replacing the spark plugs appears to have sorted out the rough starts it’s had

 

[RonD]

Has it been the same fuel in the tank since this started?
May have some "bad gas" it happens, just add more gas

Unplugging the MAF disables Closed Loop on the computer, so it goes to Open Loop, it doesn't do air/fuel calculations, it uses tables in memory
I assume it didn't do the misfires until it warmed up a little, 3 or 4min, that's when its "normally" in Open Loop?

Upstream O2 sensors maybe they go lean when they are over 12 years old
Dirty injectors, throw off computers calculations, injector cleaner in tank

What were the specific cylinders?

 

[01WHTRNGR]

I put 70 miles on this tank of gas before it started misfiring. With the MAF unplugged it didn’t misfire at all. Otherwise misfires begin right away. Injector cleaner, I could use sea foam right? I believe the cylinders were 2,3,4, and 4. Not sure though as I forgot to write them down.

 

[RonD]

Yes, Seafoam in the tank

And its the same gas thru out the problem?
Could be you got a batch with extra water(bad gas), it takes a day or two to settle out and cause issues
But unplugging MAF sensor shouldn't matter for bad gas

MAF sensor can't cause "misfires" directly, neither can O2 sensors
Computer can because of miscalculated air/fuel mix, based on MAF/O2, but usually that's Bank related, i.e. 1 2 3 or 4 5 6
MAF and O2s are the two main sensors for fuel mix

No Cam sensor code?
Oh its a 4.0l SOHC, never mind about Cam sensor

 

[rusty ol ranger]

Hows the MAF plug and wiring look?

 

[01WHTRNGR]

Hows the MAF plug and wiring look?

All looks good. I can double check the wiring when I get off work

 

[North Idaho 5.oh]

How’s all the vacuum lines look? How about the rubber elbow at the left front of the engine below the throttle body? I have had experience where that elbow cracks on the under side and causes a whole mess of issues. Since it’s on the under side no one would ever be able to tell…. Just my 2 cents

 

[01WHTRNGR]

All wiring looks good. Bought a spark plug tester to see if it’s the coil pack but ran out of time to use it yesterday

 

[01WHTRNGR]

How’s all the vacuum lines look? How about the rubber elbow at the left front of the engine below the throttle body? I have had experience where that elbow cracks on the under side and causes a whole mess of issues. Since it’s on the under side no one would ever be able to tell…. Just my 2 cents

Started it and sprayed down vacuum line with carb cleaner and couldn’t find any leaks. I’m waiting for the engine to cool done before testing the elbow you mentioned

 

[01WHTRNGR]

Misfires are fixed. Now it's throwing the codes P0171 and P0174. I'm thinking its the MAF so going mess with that.

 

[alwaysFlOoReD]

I haven't read the whole thread. What fixed the misfires?

 

[01WHTRNGR]

I haven't read the whole thread. What fixed the misfires?

Replacing the spark plugs. I thought it was still misfiring, but when I took it for a drive the codes for it didn’t reappear and instead the codes I just mentioned appeared.

 

[North Idaho 5.oh]

Another thing to try would be to replace the
O-rings on the upper intake? I know that when I was presented with those codes wether it be from a 4.0 OHV or a SOHC … once I replaced the
o-rings, it cured my issue. Along with other obvious preventative maintenance like spark plugs and wires along with any other seals like the throttle body etc…. Sounds a lot more daunting than what it really is.

 

[RonD]

Have you abused your MAF sensor in some way?
lol

Sensors don't tend to fail, although often replaced, which can be a bad thing these days since "new" just means "NEVER EVER TESTED"
Only sensor that needs to be replaced is the O2 sensors, every 12 years or 100k miles

P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)

Codes don't mean engine is actually running Lean, it means computer calculation for the air/fuel mix was at least 20% Lean after warm up, so computer is adding 20% more fuel than it calculated to balance the O2 sensor's voltage

Since BOTH sides/banks are showing this then it has to be something in common to both sides
Yes, MAF sensor, lol
But also upper intake air leak
PCV leak
O-rings maybe but BOTH sides?

Other side of air/fuel mix is the fuel and fuel pressure
Computer bases its calculations on consistent fuel pressure
Dirty fuel filter?

 

[01WHTRNGR]

Have you abused your MAF sensor in some way?
lol

Sensors don't tend to fail, although often replaced, which can be a bad thing these days since "new" just means "NEVER EVER TESTED"
Only sensor that needs to be replaced is the O2 sensors, every 12 years or 100k miles

P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)

Codes don't mean engine is actually running Lean, it means computer calculation for the air/fuel mix was at least 20% Lean after warm up, so computer is adding 20% more fuel than it calculated to balance the O2 sensor's voltage

Since BOTH sides/banks are showing this then it has to be something in common to both sides
Yes, MAF sensor, lol
But also upper intake air leak
PCV leak
O-rings maybe but BOTH sides?

Other side of air/fuel mix is the fuel and fuel pressure
Computer bases its calculations on consistent fuel pressure
Dirty fuel filter?

What makes me think it’s the MAF is when I unplug it, the truck runs just fine. Fuel filter was replaced 25,000 miles ago. How would I check for an upper intake or PCV leak? I plan if the new MAF doesn’t work is too check O2 sensors. From reading I can do that with a multimeter