Hey guys, long time lurker, but first time poster. I was hoping to pick your brains on some data I pulled while trying to diagnose a check engine light. 2003 4.2 2WD Manual F-150. 135,000 miles.

Some background:
-I have changed the lower O2's fairly recently, but not the uppers
-Never changed MAF
-I checked for vacuum leaks in the last 18 months, none found

I pulled some freeze frame data after my hour long ride to work today, and this is what I have. Any ideas where I should go from here?? Truck seems to be running fine, but just "weak" when it comes to acceleration. There is smooth ideal and cruising. I am getting 20.2 mpg, mostly highway miles.

P0171
Fuelsys1 CL
Fuelsys2 --
Load Pct 87.5
ECT 188F
SHRTFT1 5.5%
LongFT1 28.9%
SHRTFT2 1.6%
LongFT2 28.1%
RPM 2115
VSS 50 mph


If there is other data you might need, I can pull it. I think I had a 0.01 lb/s reading on MAF when I did a separate test. That was likely at idle though.

Thanks in advance guys!

 

My SWAG is either fuel pressure, vacuum leak(s)' and/or MAF.

To narrow it down I'd like to see the fuel trims at idle.

Have you cleaned the MAF?
And do you have the stock air filter setup?

 

Thanks for the reply PDQ.

I have a K&N air filter, but stock intake housing. I have not cleaned the MAF before, and that is easy enough that I should do it anyways. the sensor is reading 0.01 lb/s at idle, and it goes up to about 0.06 when I am cruising about 50 mph.

As for idle numbers, I just went out and got these:
Short1 -1
Long1 -1.6
Short2 -1.6
Long2 3.1

One other tidbit that may lend more light on this...I moved from sea level to almost 8000 ft in altitude. While I cant be sure that caused the "check engine" light to come on, I can easily see how a dirty or faulty MAF could now be "confused" from that pretty drastic change.

 

Your idle trims are just about perfect so that rules out a vac leak. When was the last time the fuel filter was changed? A clogged filter will restrict fuel at higher load. The K&N could have contaminated the MAF with oil, I would clean it with MAF cleaner, reset the KAM, and drive it a little while to see what the long trims do at speed. If they still go high and the fuel filter is good, I would suspect a weak fuel pump. Can you get the BARO reading with your scanner?

 

The issue is starring you in the face.
Look at both LTFT tables.
They are shifted 28.9 and 28.1 %.
These set both codes.
Find the intake vacuum leak.
Leave everything else alone.
BTW, if you took the freeze frames with a fully hot engine right after shut down or while runing, the thermostat needs changing.
The engine is running to cold. Should be about 195 to 200 degrees.
Good luck.

 

Thanks for the replies! It has been 22,000 miles since I changed the fuel filter. I have never looked at the fuel pump. I will take Bluegrass7 suggestion and sweep through again looking for a vacuum leak first though. That's the fastest and cheapest, so I hope its there. I will report back for those interested.

 

I don't like to disagree with an expert like bluegrass, but I'm of the opinion that with those nice fuel trims at idle that we are not looking at a vacuum leak, as a vacuum leak would have a bigger impact at idle, when vacuum is at its highest, than it will have when at 2100 rpm under a laod as displayed by the freeze frame data.

The K&N may have impacted the MAF, but if the P0174 tripped under load, I'm leaning toward a fuel supply/pressure iissue. So I'd like to see what the fuel pressure looks like when under load at 2000+ rpm. (Double check to see that the fuel pressure regulator is plugged to intake vacuum and that the fuel pressure increases when you disconnect the FPR from its vacuum source.)

 

Concur.

The FF data, which appears to be incomplete, suggests either faulty MAF data or weak fuel supply.

 

+1 A vac leak shows up as high fuel trims at idle that drop at higher RPM when the leak accounts for a much smaller percentage of the total airflow.

 

The long term tables register peg counts over time intervals that can shift the table either way or not at all.
These tables do not move that fast like the short term tables do.
In the freeze frame data it shows the trans was not in OD by the high RPM and Vss speed of 50 mph.
These readings are dependent on which trans and what rear gear ratio is.
This will have an effect on the short trim readings as the throttle will be open farther changing the MAF reading that would normally be seen if this data was taken at idle.
My assumption is the freeze data was not the result is a trigger event but just a reading while running on the road.
This is a good exercise in relating readings to the trouble found and how just having a Scanner still does not tell everything unless the data registered is interpreted.
There are always more rare faults that can shift the tables as noted.
Good luck.

 

That would be an incorrect assumption. FF data is stored as a snapshot of the operating conditions WHEN the DTC is posted (can exclude misfire FF data, see PCED for discussion).

 

What are the correct percentages for LTFT1/LTFT2? I thought 28% was in the upper-middle of the acceptable range. The VSS of 50 would just be a particular screenshot of the readings at that particular time rather than an average, correct?

projectSHO89, where is the PCED for more discussion on this topic?

 

The PCED (Powertrain Control Emissions Diagnosis) manual is part of the factory service manual set.

 

Ideally, the fuel trims would hover around zero.

When the sum of the LTFT and STFT exceed ~ +30% or -30% the PCM will set a DTC code. When the PCM sets the FIRST DTC , it also grabs a set of values and stuffs them in the FF block.

If subsequent DTCs are set, the PCM does not change the original FF data. Often subsequent DTCs are the result of the first DTC set, so the original FF is more valueble in diagnosing the real fault.

When you look at live fuel trim data with a scan tool, as Greg did, the PCM actually has nine sets of fuel trim data and displays on the scan tool the fuel trim table that the engine is operating in at that time. (Think of a 3X3 grid with x-axis of low/moderate/high RPM and the y-axis of low/medium/high engine load.)
That way he was able to tell us what the fuel trims are at idle, and then run the RPMs and load to see what the trims are under the conditions when the first DTC was set.