2000 F150 XLT 4.6 V8 Automatic w/overdrive 642,000+ miles;
Last week while driving down a highway doing about 65 mph using the cruise control, my truck suddenly started running sluggish. As I described in the description, it also shifts between 2nd and 3rd very slowly and weakly. During the last couple of weeks I repaired several items that had thrown TDC's. One was the EGR system was clogged. I tore it down and cleaned the two ports out in the bottem of the throttle base plate. One was the speed sensor in which I replaced. One was the multi purpose switch (turn signal/windshield wiper etc.) which I rep[laced. The codes were cleared and have not returned. I suspected a clogged Catalytic Converter. I checked them all and replaced one weak one. Still the same performance issues.
Do any of you have any ideas as to what this could be?
Thank you!
Chuck

 

Take it to a muffler shop, they have the equipment to test for excess backpressure. The slow shift may be pointing to a transmission on its last legs.

EDIT: You can do this to check the EGR valve itself. Disconnect the vacuum line at the valve and plug it. Connect a line to the valve and do a "suck test" with it idling. When you suck on the line, the idle should start to stumble, and it may even stall. Then, leaving the line disconnected and plugged, drive it and see if it drives any differently. If it runs a lot better, there's still an EGR issue.

 

The EGR test worked like a dream. I can now write that off of my list of possable culprets. Using a vacuum gauge I see no problem with back pressure at this point. I will write another reply to give more information. Thank you for trying!

 

When I let it idle, it runs fine. When I start out driving very slowly it seems to run fine. If I try to gain speed quickly it starts to stumble at about 45 mph. The problem seems to be when I accelerate. By building up speed slowly it seems to do fine. When it hits about 55 mph, it goes into overdrive just fine. When I stomp down to downshift it stumbles. It seems as if the transmission is in between gears. I'm not ruling out the transmission but there is no slippage or delay at all when taking off from a standstill. While sitting in park, when I rev up the engine it starts sounding sluggish at about 3000 rpms.... This is sitting in park... Thats why I tend to think I have a motor performance problem. Any ideas?
Thanks again!

 

Weak spark? A compression test may be an idea with that kind of mileage?

 

Does it run any better with the vacuum line OFF the EGR and plugged? Do you have COP or coil pack ignition? How many miles on the plugs and cops/plugs, wires, and coil packs? Have you tested the fuel pressure? Put some Techron in the gas to clean the injectors. Agreed, a compression test would be a good idea with that many miles.

 

The motor has about 340,000 miles on it and burns no oil. I realize that is a lot of miles and a compression test may be coming up quick. The plugs have about 50,000 miles on them and are Motorcraft. It is a COP system and everytime I have had a problem with any of them the computer threw a DTC. This time, nothing. You can tell no difference with the EGR disconnected and plugged and it nearly died when I gave it the suck test. What really throws me on this thing is how fast it showed up. Driving down the highway just fine running on cruise control and it immediatly started acting the way it is right now. I can't help but think it is a sensor or switch of some kind. Could it be the PCM messing up? It is the original with 642,000+ miles on it... Unless I come up with something quick tomorrow it will go to Firestone to be diagnosed. If that happens I will come back on here and give you guys the results... If you think of something else please let me know. I can't thank you guys enough for trying to help me!

 

How old is the fuel filter? It's worth borrowing a fuel pressure gauge from a parts store and checking it - should be between 28 to 40 psi, disconnecting the vacuum line from the FPR should cause it to rise 5 to 10 psi. When you disconnect the vacuum line (engine off) there should not be any liquid fuel or odor in the vacuum line.

 

"Did you happen to put gas in the tank just before this happened?"

If So... add a few small jugs of water dehidrator stuff (I forget the actual name of the stuff) to the gas and one or two jugs of injector cleaner into the gas tank. Drive around for 25 miles or so and see if it clears up.

Try to do the most simple checks/fixes first.

 

It's called Iso-Heet. The Techron I recommended is injector cleaner.

 

OK Guys, I have not abandond this. I took it to a muffler shop and got their opinion. They said I had no problem with the exhaust. Looking the situation over I had them replace the rear left cat It had the original 642,000 miles on it. The rest I have replaced over the years. I took it to Firestone and had it diagnosed. They said both rear cats needed to be replaced.They said it was extreme back pressure. As you can imagine that hacked me off seeing that the left rear cat was only one day old... I took it home and pulled both rear O2 sensors out. I drove it and it ran awesome. I started focusing on my muffler. It was original as well. The muffler shop said it was fine but I had them replace it anyway...only $100.00 compared to $350.00+ to replace another cat. It sounds better but no change in performance. I pulled the left O2 sensor out and still no change. I put that sensor back in and pulled the right one out...BINGO! It ran awesome. Since there was nothing loose in the cat a gentleman recommended I run a bottle of Cataclean through it. The bottle recommends adding the bottle to less than 1 quarter of a tank of gas (4 gallons). I had to drive it about 50 miles to run the gas down. I left that right O2 sensor out and drove it down the highway. I ran it kind of hard stomping on it making it downshift and hitting speeds up to 80 mph just making sure the transmission was shifting ok. I put the O2 sensor back in and dumped in the Cataclean. Believe it or not it was running better already. The only thing I can think of is blowing some of the crud out of it with that O2 sensor out. It is now running much better but still slugs out when trying to downshift passing someone at about 65 mph. So, now the question is, Do I run another bottle of Cataclean through it or just bite the bullet and replace that right rear cat? What would you guys recommend?
Thanks again!
Chuck

 

How are the FRONT cats?

If the right rear cat is original, replace it and gut the front cats.

 

WOW! Interesting! I've heard the front cats are useless. Please educate me on this subject.
Thanks!

 

Not useless, but they can plug up just like the rear cats. Gutted front cats and new rear cats should pass emissions with no CEL.

 

I have read online several personal experiences with using: Lacquer Thinner in the gas tank.

The suggested amount is 1 gallon Lacquer Thinner to 10 gallons of gas, then drive it until you run all the gas out and get the Low Fuel light to come on Before you refill the tank with gas. Dont let the Lacquer Thinner sit in the gas tank, Run it All Out.

MOST... of the vids and write-ups have said that it: "Worked for them and the P0402 code went out on its own".

I was have some engine running problems and thought it could not hurt to try this so, I put a half gallon into about 5 gallons of gas that was left in the tank. My engine running problem was Not the Cats but, the Lacquer Thinner hasnt caused any other running problems in my engine either.

Do some research of your own and use at your own discretion!! (see below)