'03 Crown Vic, 4.6, approx 52,000 mi. Started noticing performance loss approx 2 yrs ago. No fault codes. Replaced plugs, all 8 coils.
Motorvac. New fuel filter every year. Runs ok when cold. Feels spongy when hot. Dealer has come up empty. Nothing jumps out on scan. Seems to be getting worse, not better. Starts fine, runs smooth, not using coolant or oil but coolant seems to have darkened somewhat in colour. Have also noticed same OCCASIONAL high pitched squeal or whistle that one of the F-150 owners mentioned, actually co-incidental with deteriorating performance. Haven't done compression or cooling pressure tests YET. Dealer says only other similar thing, MANY police cars with intake manifold gasket failures, but they have set trouble codes. Maybe early stages of head gasket failure or dying fuel pump?

 

Uhhhhhhh, I see this is your first post - are you aware what Forum your on, or did you just Read "Ford.." and start typing?

This is what you need btw:
http://www.crownvic.net/ubbthreads/ubbthreads.php

 

Hey try cleaning the MAF with MAF cleaning spray first. It's a common problem.

 

I would use some seafoam, or take it to a shop to do a 3 Step Fuel System Tune Up.

 

3 Step Fuel System Tune Up? What's involved in that?

 

Hey guys, sorry! But aren't we all Ford guys, not bowtie or japscrap. I've been all over the place on this one. A 4.6 is a 4.6. Tried cleaning MAF sensor. Dealer diagnostic guy's pretty good. He originally thought MAF but ruled it out. Who knows?
Could still be. thanks, grn65Gal

 

Have you checked the catalytic converter to see if it was plugged up?

 

3 step fuel system tuneup - probably the "motorvac" I referred to. Similar but different brand is terraclean. Disconnect fuel supply, connect machine at shop, cleans fuel system, throttlebody, de-carbonizes combustion chambers & supposedly "washes" O2 sensors & cat. Had it done just now & 2 years ago. My mind - jury's still out on effectiveness.
Some dealers & mechanics tout it heavily.

 

Yah. Thought's crossed my mind. Vacuum test would tend to rule it out but they say only sure way is removal & inspection. If so, becomes a chicken or egg question. if cat fails - why? Dirty injectors, lazy O2 sensors , weak spark etc - etc - etc. Fuel trims range plus / minus 5%
or so which should not contaminate a convertor. Read an article in local old car rag up here. They say wreckers full of 8 - 9 yr old vehicles. Nothing wrong but engine mgmt systems nobody can diagnose. Never thought I'd say it, but maybe time for full electric! This is why we all need to talk - share knowledge.