Hey all, I have a 97 F150 with the 4.6 with 227k miles, and today it started running rough, like it was misfiring, but no code was showing up. I had gotten it home, found a dead spark plug wire (#1), changed it out, drove it around for a bit and then got a code p0171.

The only other problem I could find was the ignition coil on the passenger side isn't getting spark to the #1 cylinder. With the engine running, I lifted the plug off of the coil, and there was no spark.

I know this part needs to be replaced, but could it be the cause of the lean code? I would think that if there is no spark, then it would throw a rich code because the fuel isn't getting burnt up?

If anyone has any insight into how this works, and if the ignition coil could cause a p0171, I would greatly appreciate it. Especially because I don't want to go chasing possible vacuum leaks haha.

Thanks

 

No spark on #1 will set a code P0351.
The 171 code is excess oxygen detected on bank 1 and looks at all 4 cylinders.
They are on the same bank but no 'direct' relationship.
A lack of fuel would be closer to the cause if any of the 171.
There appears to be two issues present.
.
Here is how a misfire is detected;
Every time a cylinder come it's turn, the PCM know when that instant takes place and measures the rotation time using the crank sensor.
If the rotation time is slower than the 'average' of all the cylinders, it is deemed a misfire after it happens a set number of times to guard against setting a code for every small rotation time difference.
You can see this is not related or a cause of the 171 code.
Be sure your not missing spark by using a timing light to sample the spark.
The 171 code if looked at with a Scanner would show the fuel table for that bank, out of limits and shifted about 25% or more as the verification.
Sometimes you can't just go by looks and simple checks if the issue is not solved after a simple observation.
Good luck.

 

Spoken like true carburetor guy.



But these engines don't have fuel sensors. They have oxygen sensors!
So when there is a misfire, they don't know if there is fuel in the exhaust.
But the oxygen sensor sees the unused oxygen (because the oxygen was not consumed in the combustion process), and thinks more fuel is needed to correct for the excess oxygen.


As bluegrass stated, look at the fuel trims to see what is going on. Bank 2's fuel trim could be a whisker away from setting the P0174 code.

 

truck is 18 years old... Im guessing a vacuum hose is hard and brittle . Look at all the vacuum hoses, especially the smaller ( 1/4 inch size) around the exhaust manifold areas.

 

The system design is such that a mistake is virtually impossible 99% of the time.
Yes an issue can 'provoke' another code but on a secondary basis.
It depends often on who sensed the issue first, then the rest follow but only in a logical order.
The Program in the PCM has what is called 'executive' control such that only certain combinations are allowed.
When this happens, clearing to source fault clears the other codes automatically.
An example of this is the loss of power due to a blown fuse #23.
When this fuse opens for any reason, a long list of sympathy codes are set because a large number of systems cannot function, that the computer monitors and then sees are inoperative.
Restoring the fuse power clears all the fault codes providing the cause of the blown fuse has been cleared first..
Good luck.

 

Thank you for the help guys.

I changed out the coil pack this morning and it's running smoother now.

Before I changed the coil pack, the short and long term fuel trim was at about 18% for bank 1 (monitored with Torque for Android). After I changed the coil and confirmed that the problem cylinder was getting spark, the short term dropped to about 3 or 4% and after a few minutes, the long term dropped to 11%

Bank 2 was sitting at about 4-5% both times if i recall correctly.

Just to be sure, my brother and I sprayed some starting fluid on all the vacuum lines and around the intake gaskets to see if there would be any change in RPM's, which there was no change.

I still need to drive her around to see if the 171 code will return, but I'm pretty confident with the drop in fuel trim % that it was the misfire that was causing it

Thanks again for the help guys.

Daniel

 

Good analysis!