2002 ford f150 4.6L Codes 174, 171 and 353. I have not found any vacuume leaks, the maf is good cleaned injectors changed pluga coils are good. I get a s.t.f.t. reading of -9,7% on both banks and 29% on both banks for the l.t.f.t. I can't figure this out. Full pressure is good. I don't know where to go from here. I thought a burnt valve but no smoke at all from the exhaust. The truck runs very rough. Any advice

 

What kind of plugs are you running? I recommend Motorcraft.

 

It may be worth it to pay a mechanic to do a smoke test. If you have a shop vacuum, you can reverse the flow and blow up the tail pipe and use soapy water spray to find the leak. Chris Fix has a video on this. Or you can use an unlit propane torch to find a leak. There are lots of utube videos that show how to find a vacuum leak.

 

I am using autolite plugs .

 

I've done the propane test several times. Not only did I not find any, it would have to be a pretty good sized leak to make it run as rough as it is. I will try the reversed shop vac thing right now though.

 

No good. Known issues in Triton engines.

 

You may want to do a compression test.

 

There is a South Main Auto Repair video about smoke testing an F150 and the leak was a rusty brake booster!!!!

 

I've checked the converters they are fine. I've done the propane test very slowly and methodically and came up with nothing. I'm thinking maybe a valve

 

What should the fule pressure be?

 

My maf is jumping from mid 80s up to 115. It's all over the place at an idle.is this normal?

 

I disconnected a vacuum line and shot propane directly into it and it didn't even phase it. s.t.f.t. didn't change a bit.

 

I went and bought a compression test kit ans none of the fittings would screw into the head.

 

I highly recommend changing your plugs from Autolight to Motorcraft before going further.

Make sure they are gapped accordingly.

 

Fuel pressure on my truck runs around 30psig