99-04 have that spark plug issue. A 99 probably has non-PI heads which will have less power than a 2000+ 5.4. It will say PI on the exhaust side of the head if it's a PI engine and I believe the intake ports are square on the PI vs round on the non-PI. Someone correct me if I'm wrong.
The plug issue really isn't a big deal of you torque them down right. I've had three of them without issue.
I have bought 2 5.4 2v's that ran pretty poorly when I test drove them and all it took was a set of spark plugs and a fuel filter to get them running right. It's surprising to me how many people are out there that just refuse to maintain their vehicles.
No guarantees that's the same problem that the one your looking at has but it's quite likely.
i had a 1998 5.4 auto trans, manual shift transfer case, mine had the huge rear end in it though and it was not a 7700 version. very solid trucks. I ran mine from 60k or so till 180 k never had any plug issues. I did change them and got factory plugs and torqued them to 18ft lbs, I think that was the spec but look in service manuals. some people will crank them in so tight for some reason.
mine was not a pi head truck and it had plenty of power and got good (realative) fuel mileage if not driven hard. at one time I was getting 17-18 with it driving mix of freeway (65mph) and some city.
my 2001 V10 (also with the spark plug problems) went 150k and I changed the plugs and they were the orginal ones, the gap was almost 1/4 inch new plugs and fuel filter and back on road with no issues.
if the truck feels good and drives right i dont think you will have problems with it. just have it looked over
The spark plug issue is due to lack of threads in the head. Do not let this scare you off. A miss can be as simple as a bad coil or need a plug change and filter change as previously mentioned. Unfortunately the coils can be difficult to diagnose with out the proper diagnostic tool showing live data.
The 2004 2V and later build 2003 2V engines have the extra thread length in the heads. My dads 03' was an earlier build date and he does not have the extra thread length. I have done all the plugs changes on his and no issues thus far.
A lot of used car dealers will wash the engines with water or steam clean them. That ends up putting some moisture down in the plug recess. If the boots are not so limber or the plugs a little old you can get a missfire. A friend of mine was looking at F-150s with modulars at a local used dealer. He test drove 2 and both misfired. But he said the engines were shiny and clean. Guess what they did? Most people don't want a truck that misfires, though the fix can be a simple fix.(plugs and boots)