I've been working this problem a little at a time for a couple of months now. When the outside temp drops below 75 degrees or so my 97 F150 4.6 starts to spark knock under accelleration. The colder it is outside, the wrose it gets. The truck has 113K miles on it. I've changed the plugs, air filter, cleaned the MAF, cleaned the EGR ports which were almost completely clogged, and installed a new air temp sensor. There may have been some improvment as all of this has been done but not enough. Switching from 87 octane to 91 helped a little but didn't cure the problem. Do these engines have a known problem with carbon buildup in the cylinders?

 

Test the EGR operation to make sure it is not leaking exhaust gas into the intake.
Test the EGR valve seperate, test for vacuum getting by the EVR and test for false EVR action while driving under acceleration.
The EVR has a filter in it's body that may be plugged or have frozen moisture.
It is there to filter air that the EVR lets back into the EGR so it wil close up again once operated by vacuum.
The EGR is not supposed to function at idle, or at throttles above about 7/8th.
Only in a cruise condition, low load and between about 1600 rpm and about 2400 +/-.
To test actual operation, install a vacuum Tee and a vacuum gage to see while on the road. You should see about 3 to 5 inches of vacum depending on throttle opening while in a cruise condition. Vacuum at any other time is not correct.
Exhaust gas leans out the mix and has the same effect as low octane gas.
When the correct EGR operation is taking place the PCM make changes in the ignition timing and the amount of fuel injected to take care of the action.

 

BAD EGR. its probably stuck umm open? its stuck somewhere.

 

I have to bring this up again. It bugs me because my truck runs perfect. It doesn't surge, sputter, give any check engine light, no codes, .....nothing. But after about 10 minutes of driving when the engine temp is up to normal it spark knocks like crazy. This morning it was about 38F outside and when I pulled out of my driveway it sounded fine. About 10 miles down the road when I pulled onto the interstate and began to accellerate it sounded like there were marbles on top of the pistons. Surely I'm not the first person to have this problem. Is it possible to have something wrong in the EGR system without setting a code?

 

Clean your MAF sensor again, especially if you have a K&N air filter and have recently serviced it. Use brake cleaner to do the task.

 

You will have to look deeper into the places that would affect ignition timing.
Usually if there is a fault, the PCM sets a code and a light or pending code but since this is not constant, it may be cancelled within the software tables as a transitory event.
I would begin by investigating the Knock sensor resistance.
This sensor is supposed to detect detonation as bad as your indicating and retard the timing automaticly. The circuit may be marginal such that a permant code is not set.
Info: Knock sensor measures 4.38 to 5.35 megohms. (million ohms)
Measure leads back to the PCM at 10,000 ohms or greater.
Dealer OASIS NUMBER= 615x00 chart 9 info for more possibilities in other areas..

Do the testing right when the engine is acting up so you have the best chance of catching something faulty.
Good luck. Best I can steer you for now.

 

Thanks. If I get time I'll check the knock sensor this weekend. I had been focusing on the mixture vs. temp/air density side of things. Hadn't really thought about timing at all. The truck does seem a little weaker when the spark knock is occurring. Hmm.....