I have an annoying skip (which I think is pre ignition) that appears after about 4 minutes of running time from cold start. Before 4 minutes, everything runs flawlessly...no skip under load. After the engine has been running, is shut off and then started again, even a half hour later, it will skip right away under load and also if you goose it while in park you can tell it skips a small amount during rev up. If you drive gingerly and dont load the engine...you will never know it has a problem. If you push the pedal, in any gear, past about 28% power level, it will start to skip.
I had a code come up as cylinder misfire on number 6, but to get the code to come up I really had to tromp on the gas pedal and get the skip to be real bad...dont really want to do that anymore...quite violent under really heavy load at low rpm. Replaced spark plug, injector, and coil on number 6. Then changed all the same on 5 and 7. No difference. Plugs looked normal, nice light tan. They are fairly new (20k) Bosche Iridiums.
I get a Lean condition Bank 2 also, but it is difficult to make it come up.
Only bank 2 is coming up lean, so this rules out MAF? no? If the 02 sensors are not working isnt there a separate code for that? Could the 02 sensor be causing the problem?
The vehicle is my 1997 Expedition with 120k 5.4 Triton
-already sprayed the MAF...looks very clean.
-new air filter
-battery voltage good
-alternator good
-unsure of 02 sensors age or condition
-all vacuum hoses look good
I think it could be an intake leak...but I hope not. Any other ideas?
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