My friend recently bought a 1998 f250 light duty (f150 7700) it has a 4.6 v8 and when we were driving it home it started making a bad rattling sound under acceleration and felt low on power.

We checked the codes when we got home and it had a p0171 and p0174 indicating it was lean on both banks.

Once the engine had cooled off I started it and sprayed around the throttle body, intake manifold, PVC valve and hoses, and any vacuum lines under the hood with brake cleaner to check for vacuum leaks but we did not notice a change in idle at all. Although we found that the metal EVAP lines near the fuel tank and the frame rail that run to the engine were rusted through.

could this be causing a vacuum leak?

we put some seafoam in the tank in case the pinging was caused by dirty injectors or carbon buildup, also we replaced the fuel filter and fuel tank and filler neck because they were rusted and dirty. (The truck had been parked for a year and had some nasty fuel in it)

anyone have any other ideas on how to diagnose this? MAF sensor or maybe low fuel pressure caused by the fuel pump or fuel pressure regulator?

could a rusted through EVAP line cause a vacuum leak?

thank you for any suggestions on how to further diagnose this, we don’t have a smoke tester or a proper scan tool to check fuel trim so any help is really appreciated

 

If you had an EVAP leak you should have gotten some EVAP codes.....

The vacuum leak codes are on both sides so it's possibly from something common to both sides like the MAF. I always recommend checking the MAF before chasing vacuum leaks, especially on Fords.

A smoke detector is more reliable for finding possible leaks. You can make one. Lots of videos available on how to make one.

A scan tool is only about $20 and if you're going to work on vehicles you need to get one.

 

Vacuum leaks are usually worse at low RPM. Your symptoms sound more like low fuel supply. A fuel pressure check might tell you something.

 

I figured there would be as well because every metal evap line from the charcoal canisters all the way up the chassis to the VMV are rotted off. Just replaced another two today, only one more to go.

also where are you finding a scan tool for 20? I can’t even find a basic code reader for that cheap. I have a code reader but it isn’t capable of monitoring fuel trim or any live data so I’d really like to know…

 

yeah I’m thinking the pump could be getting weak. When we first got the truck running the fuel filter was so plugged it would stall whenever you gave it any throttle, and started really hard. I’m thinking that the fuel pump may have over-worked itself under all that restriction from the clogged filter

i plan on getting a fuel pressure tester soon. I think it’s something like 40+PSI when the fuel pressure regulator vacuum line is unplugged is within spec

 

That should be fine for basic diagnostics. Get the freeze frame data when the codes were set and see what it shows for fuel trims. I'm thinking even the most basic unit will do that much.