2003 Navigator 32v 5.4L165K

Steadily loses power going uphill. Every once in a while, it will throw a PO174 code (bank 2 too lean) which leads me to believe a vacuum or exhaust leak on the drivers side.
I replaced the PCV hoses and elbows. New plugs and coils all around. Used propane and starting fluid to find any other leaks. Ran all lines and can’t find the culprit.
The engine idles and revs just fine in park or neutral but if you power brake it, it will exhibit same symptoms and max out at 2000 rpm. It almost seems like a clogged cat but when I took it to the shop, they said it looks like it’s blowing just fine. They removed the rear cats and piped them for me but there was no change. They don’t think the front cats are clogged and they do this for a living. They used a point and shoot temp gauge right when I got there after an hour’s drive.
It almost looks like the PCM may be going into limp mode but only when it’s in gear.
When I accelerate slowly, it will get to 75mph on level ground. If I stomp it or climb hills, it stumbles and slows almost like it’s not advancing the timing properly. This engine doesn’t have VCT, so it’s not a cam phaser issue.
Ford has tested the fuel pump, cleaned the injectors and recommended new cats. I’ll go back to them if I have to but I don’t want to keep changing parts that aren’t the problem.
Would the speed sensor or coolant temp sensor only cause the engine to go into limp mode while in gear? Wouldn’t the PCM record this if this was the case?
I’m going to find out if fuel delivery rate was also measured along with fuel pressure when they tested the pump. I’m stumped and out of ideas.
New: Air filter, plugs, coils, fuse box (to correct bad fuel pump relay), cleaned MAF and throttle body.
Thanks all! This site has helped a lot over the years!!!

 

Does it lose power on every hill, and how much fuel is in the tank when this power loss occurs?

 

The steeper the hill, the quicker it loses power. I could drive for 12 hours on mostly level ground with no issues except for acceleration power. The transmission shifts properly and fluid is normal.


Thanks for posting Alloro! I 've read and used your posts several times!

 

Not familiar with the '03 Navigator with the 32v engine, but if it has a similar PCV setup as the '05 Expy 3v 5.4 engine, the PCV feeds into bank 2 on the intake, and if it's stuck open you might get a lean code on that side as it's basically a massive vacuum leak if it's not working as it should. I see you state that you replaced the hoses and such, but you didn't mention the valve itself so that might be a point to look at. Depending on how it's built into your engine, it might be replaceable and it might be an integrated part of the valve cover.

 

You didn't answer the second half of my question regarding the fuel level in the tank.

 

I suppose I should have listed that on items done. I checked the PCV valve and cleaned it. I also checked the vacuum to PCV by paper test on PCV intake.


Thanks!

 

OOPS!


Does it regardless of fuel level. No change when full or at 1/4. 93 octane.


I did have the fuel relay problem last year where the truck would shut down for no apparent reason. I changed out the smart junction box and never had the issue again. Don't know why they had those relays soldered in but now I have a lifetime supply of fuses!

 

All I can suggest at this point is to connect a fuel pressure gauge and go for an uphill drive. The pressure might be fine at idle but fall off under a heavier load.

 

165k ! What's the fuel filter like?

 

Guess I for got to mention that too. I started with that before cleaning the injectors. Thanks.

 

I'll give it a try!


This project is going on the back burner for awhile. Gotta travel for a funeral. Ugh...

 

I had the same problem. I fixed it. you need new fuel pressure regulator and new fuel filter. Then replace 1 or 2 little hose that came out from the EGR valve tube. I bet you either 1 of that hose is crack just a bit that cause your hassitage when acelerating. I have 04 Navigator 170k on it. Took me over a year to research and find the problem. I glad I did finally.

 

Yeah those multiple things wrong problems can be extra hard to track down.

 

I will be glad to give it a try! Thanks!!! I'm traveling, so will let you know in a week or so...