I have had a pretty consistent P1151 code being tripped on my '99.

Some background: I just replaced a seized engine (at 220k miles) with a used 5.4 out of a '99 NASCAR F150 with ~140,000 miles.

The pickup runs astonishingly well, IMO, except for the code that is being thrown. It definitely isn't missing, stalling, or low on horsepower.

I have been monitoring the Ultragauge whenever I drive and it seems that, under ~70% load the LTFT, Bank 2 jumps to up to 20% (bank 1 stays around acceptable levels). Under light load/cruise the LTFT on both banks stays between 0% and 5%.

The code is tripped at approximately the same load level (70%) with the engine at operating temp (185°F) according to Freeze Frame data.

Some searching on the internet shows that the Bank 2 Sensor 1 O2 sensor isn't switching quickly enough "during a test" -- without explaining what this test is looking for. I read on a Chevy forum that the ECU deliberately oscillates between very rich and very lean in order to verify that the O2's respond to the mixture changes, then finds the average time it takes for the O2 to respond and compares that to an acceptable range of values. Unfortunately I don't know if this is applicable on a Ford.

The possible causes range from a leaky intake manifold or bad EGR valve (both are doubtful, IMO, since it is tripped under moderate-to-heavy load), to loose wires or bad 02 sensors. Since I don't like to simply throw money at a problem I figured I'd confer with those more knowledgeable on the forum.

What, say you, is the most common problem in a '99 F150 with a 5.4L that would trip a P1151 code? What other tests can I perform in order to determine the cause?

 

If they haven't been changed in the last 100k, I'd change both front O2 sensors on general principle.

 

The very first thing is to replace the thermostat to get the operating temp up to 195-200 degree range where it is supposed to be for all other systems to reference on..
The lower operating temperature is running fuel injection too rich.
Then if the issue does not clear the OX sensor or it's harness could be at fault since one bank is being affected as your seeing with your monitor.
The trims only change based on what the sensors feeds back.
No good signal, the fuel runs even richer the same as if the plug were not on the Ox sensor
The short term trim is the one that most controls fuel.
The long term table is a background table that changes over longer term from the short term averages.
The code is one from a test of switch timing that found the sensor indicating a lean condition for to long a period.
It does not means the bank is running lean from an air leak.
Check that the plug is set fully on the sensor.
Replace the sensor or swap with the rear or other side
Good luck.

 

The PO must've installed a colder thermostat. I'll have to pick one up. Yay, more money to spend.

I'm thinking it may be a harness issue, as I also monitored the O2 voltages on the way in to work today and whenever the FT was ~25 the O2 volts were close to if not right on 0.00v.

So-called limp-home mode right?

Dang it. I'll change the gauge and monitor it on the way home today. I don't suspect it to react too much differently, however, but we'll see. This is from my trip to work this morning.



That's reassuring; I was hoping it wouldn't be a hard-to-track-down vacuum leak.

Thanks!

 

Still been meaning to check mine on the 4.2, but unlike yours, its inconsistent. Sometimes it will do 2-3 times a week, sometimes not for 2-3 months.

 

Unfortunately we've had a cold snap and ice/sleet so I haven't taken the time to change the O2's (I don't have an indoor shop). Hopefully I'll get them swapped either tonight or tomorrow and will be able to report back.

 

Not to thread jack, but I found out an interesting thing about mine. Its been turning on more as of late, but only when the weather is about 65º or cooler. So it turns on only in cold weather... hmm.

 

I got both O2 sensors change last night and it didn't throw a code this morning on the way in to work (~35 miles) which is a good start. If it doesn't code out tonight than I'll consider it fixed.