2008 f150 4.6

I’ve replaced head gaskets, water pump, catalytic converter, spark plugs, fuel injectors, intake manifold, egr valve, fuel filter, and air filter. And Currently have a P0401 and P0420 codes. The truck idles above 1000 rpms then back down then back up back and forth but drives normally. I’ve taken it to mechanics and done work on it myself. Any idea what could possibly be the issue?

 

Were all of these replaced to resolve these two codes, or did these codes come up after all of this was replaced? . . . or something else?

What did the mechanics say?

Does it idle this way even completely warmed up or just when cold?

Have you done any other work on the EGR system?

How many miles on the catalytic converter?

How many miles on the vehicle?

How many miles have you owned it?

 

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I was away for training for 6 months I don’t know who had the truck when I was gone but they didn’t take care of it. My brother in law said he would put it in storage but not sure what happen. The day I came home before leave my brother in law said I needed new head gaskets. The mechanic working on it said he replaced the gaskets, water pump, fuel injectors, oil, oil filter, and spark plugs. I got it back the day before I had to report to my new duty station. The truck was running like **** but I was out of time I needed to get going. The mechanic said I just need to replace the catalytic converter‘s and it should be fine. I did before I left and the truck still continued to misfire and then the check engine light came on. Went to autozone to scan they said it was my #2 plug. So I replaced it everything was fine. Until 600 miles in. Check engine light and misfire. Had no choice drove the rest of the 950 miles like this. When I finally got the time I took it to a mechanic. He replaced all the plugs, 2 coil packs, all boots. Fuel filter, air filter. Said I had a lot of carbon build up in my #1 cylinder. No issue for 2 weeks. Mis firing comes back. I’ve already dropped $3500 so far. Get a second opinion mechanic says I have random mis fires. Says I have a cracked intake manifold. Could fix it for $1500. I do it myself. Old intake manifold is cracked by the thermostat. Replaced all the plugs as well. Now the issues I have are just the ones listed above.

it continues to idle like this even after it was warmed up.

155000 miles so far

owed it 59000 miles

 

MAF sensor is new
cleaned the mechanical portion of the throttle body. It’s mostly electronic. But I’ll definitely look into the tps and iat.
no luck on any vacuum leaks

 

OMG sounds like it got beaten for six months. Send the bills to whoever was supposed to take care of it and see what they say.

Yes, do what gDMJoe said ^^^^^^^

I think Mechanic 1 took you to the cleaners and doesn't know what they're talking about to some degree.

Mechanic 2 doesn't seem as bad, but I have some about of concern about him too.

If the catalytic converter is plugged because it melted while dealing with 1,000+ miles of misfires, it could cause EGR codes to be set due to backpressure. It would also cause the P0420 (catalyst efficiency below threshold). My suggestion: Take it to a better mechanic for a better diagnosis, get the catalytic converter replaced after any other issues are diagnosed and repaired.