I have a '99 5.0 LTD with 135k on it. Startup, cold starts, hot starts, idle, acceleration from a start are all fine. Even my mileage is good for a V8 AWD.
But I have this nagging pinging & lack of power when going up hills at highway speeds. Minivans passing me with soccer moms driving them, okay, but yesterday, an ambulance passed me....not on a call. My '91 4.0 with 165k would have smoked my '99 going up hills @ highway speeds.
I run regular Exxon or Shell gas, and I have done the following:

Installed new plugs & high quality wires
Replaced the air cleaner with a K&N
Replaced the fuel filter
Replaced the thermostat
Run a few tanks with Chevron System Cleaner
Cleaned the MAF
Reset the PCM
Run 1/2 pint of Seafoam through the system

The most noticeable improvement was the Seafoam. Good stuff. My all-around mileage went up dramatically....and it wasn't horrible to start with. I can get 19 to 20 highway now at 70 mph. But still, the pinging/bogging up hills!
What is left?
One thing that I'm thinking, is some kind of EGR problem. Although I haven't seen the CEL in two years, it did come on shortly after I bought the truck (two years ago). The P.O. offered to make good on the transaction, and took it to a Precision Tune. They ran the codes, and "Insufficient Air Flow - EGR" came up. They replaced the EGR valve, reset the CEL, but the light came on about 300 +/- miles later. It turned out to be the DPFE, which I replaced. No more CEL since then.
My question at this point:
Could there be some sort of corrosion/deposits in the EGR passages that is creating this pinging & loss of power? How would I find out? Are there EGR-related problems that wouldn't throw a code? Do '99 5.0's have knock sensors? How do you test a knock sensor?
Thanks for any help.

J.P.

You can rule out a pcm problem by running race gas or very high octane like 93 or higher to see if its real knock (dont use octane booster).

I would suspect you could have plugs that are too hot causing pre detonation (Bosche are know to be hotter that advertised).

You could have a bad injector.

Programming can go bad in your PCM causing bad timing.

try resetting the PCM by disconnecting the batery for about 2 hours.

Run some more of that sea foam through your motor. I hear it takes a couple of doses for everything to flush out.

get it scanned!

get it scanned. the CEL may not come on. FYI- i drove my X to work and back one day with NO MASS AIR METER OR INTAKE TUBING ON IT and it didnt throw a CEL. it was bare-assed throttle body with nothing in front of it. CEL means nothing. ;)

Thanks for the posts.
Smiley- I installed Autolites (didn't vary the heat range...to my knowledge) Aren't Autolites virtually the same as Motorcraft? If not, maybe there's a problem there.
I agree with another shot of Seafoam. If there's a carbon problem that's bad enough to cause pinging & power loss like this, I can't imagine 8 oz. of the stuff running in your engine for a few minutes making much more than a moderate improvement.
Another 8 oz. couldn't hurt.
James t.-
I had it scanned a few days ago....no codes. I reset the PCM recently.....after installing the K&N; which was only a few months back.
One thing I'm thinking.....
Since K&N filters have a coating to help to trap particles, could some of that coating become airborne in a vapor & end up coating the MAF?
Another look at my MAF couldn't hurt, I guess.

If you still have the box the plugs came in, you may want to run the part numbers to make sure they are the same heat range as the stockers.

hope everything works out...