I have a 1997 F-150 4.6L (VIN engine code 6) with 159100 miles on it. For quite some time I have had to run higher octane fuel to keep from getting spark knock at medium to high acceleration. I am sure it has something to do with carbon build up, but I have done all that I can without spending A LOT of money to clean it out. I managed to go thru some TSB's and found one that fit my problem (article No. 98-23-10). Now before I go into all the data that I have to help diagnose this problem let me explain the symptoms in more detail. If i run 87 octane fuel I get spark knock at medium to high acceleration, the knock is even worse the colder the ambient temperature (which makes no sense to me at all). In fact when it is cold outside the knock seems to happen even at light acceleration. Today I hooked up my truck to a scan tool and pulled some sensor readings, hopefully this will help any of you that have more knowledge than myself. KOER self test indicated codes P1127 HO2S Downstream sensors not tested, and P1116 ECT Sensor out of range. I think the ECT sensor was just a one time thing because I found no indication that it was not working properly.

At idle:
LFTRIM 1 - -2
LFTRIM 2 - -3

MAF (gm/s) - 4.51
MAF (v) - 0.78

Spark Advance - 17 degrees

Baro (kPa) - 95
Baro (Hz) - 153
(Live in Houston)

ECT (v) - .66
ECT (F) - 196

At 3000 RPM:
MAF (gm/s) - 21.10

Spark Advance - 30 degrees

Baro - SAME

2000 RPM:
LFTRIM 1 - +7
LFTRIM 2 - +3
SFTRIM 1 - -4
SFTRIM 2 - +8

The TSB that I pulled was for MAF contamination, and from what I can gather it is working properly, but unfortunately I cannot do a road test with the scan tool, so I cannot test with engine under a load.

I have heard many other fords have the same problem, my fathers Thunderbird does the same thing, and I heard a ranger knocking on the freeway today. Any information will help a lot, thankyou in advance.

 

Baro (Hz) - 153
That's the first indication to me of what a large portion of your problem is. Houston is around sea level if my memory serves me right. You should have a Baro reading of 156 at the low side to 160 at the high. Baro is not a true reading as you might think. It's not a measurement thats picked up by any sensor, but is more or less calculated by the pcm through other sensor inputs, chiefly- mass air flow sensor (maf) and intake air temp sensor (iat). With sensor contamination, the MAF doesn't measure correctly so the pcm is calculating the wrong atmospheric pressure and applying that to it's fuel trim to keep 14.7 to 1 air/fuel ratio. Disconnect the MAF sensor, drive it around and see if its better. This forces the pcm to run on precalibrated tables for your fuel trims. The MAF sensor is in the air filter housing and the connector for it should be under the housing. If it drives better, replace the sensor, disconnect the battery for 10 minutes (reset KAM). If you have a K&N type filter, get rid of it because the oil in them ruins your MAF over time. Go back to stock.

Good luck