This is a 99 5.4 2 valves per cylinder engine. Rough idle when warm(not rough when first started cold) and poor acceleration. The codes are lean on both banks. Checked fuel pressure-OK. Changed fuel filter. Cleaned MAF and watched the MAF output on my scan tool-OK. Checked IAT-OK. Cleaned throttlebody. Checked voltage at fuel pump connector- >12Vdc. Suspect clogged cats. Did back pressure test. Both pre cat and post cat values are ZERO psi. I can understand the post cats being ZERO psi since the pre cats are ZERO psi, but I can't understand how the pre cats can be ZERO psi on a running engine. How is this possible and has anyone seen ZERO psi on the pre cat bungs?

 

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Rough idle,poor acceleration P0171 and P0174

Seems 9 times out of 10 when you get these codes on a 5.4L it's the PCV line sucking air, and most of the time it's the rubber 90% boot on the PCV line at the rear of the intake that has rotted away.

 

I checked for vacuum leak using a vacuum gauge. It's solid at 18 at idle. The needle is steady when the throttle is snapped open meaning it drops off and slowly comes back to 18. It doesn't jump or bounce erratically. I don't think this truck has that 90 degree elbow. The PCV feed directly into the throttlebody plenum. P0171 and P0174 both say vacuum leak so that's what I looked for first. I don't see a leak. If someone know how I can have ZERO psi at the pre cat bung(back pressure) this would point me in the right direction. I haven't figured out how a running engine can show ZERO psi at the pre cat bung. I know the post cat bungs are ZERO psi because the pre cats are ZERO, but how in the hell can the pre cats be ZERO psi on a running engine? Granted this engine isn't running good, but it is running.

 

Mine threw those codes and the tube from the inlet tube to the driver's valve cover was out. When both banks are to blame look for a common problem.

 

I hate to say it, but I'm going to do a compression and leak down. I've looked hard for a cracked hose and found none. Besides the Vacuum is rock solid. I could watch the STFTs while I have someone spray starter fluid on the top end rubber parts, but I don't think that will help at this point. I didn't mention it, but after I cleaned the inside of the throttlebody(intake) and started the engine some green slime dripped out of the tailpipe along with a lot of white smoke(steam?). I didn't want to believe it was a head gasket, but now it's starting to look like the heads might have to come off. I sprayed the intake a second time and the result was red slime(looked like ATF). This left me totally confused. I hope someone can save me in the nick of time and come up with an idea that doesn't require removal of the heads.

 

i had two small hoses, maybe 3/16 diameter and 8 inches long leak. they were on the drivers side valve cover area, back half. they set real lclose to the EGR line and got HOT and cracked. if i was you, i would pull off each of the small vacuum lines and check to see they are plyable and seal good.

 

You say it is rock steady but unstable idle?? See if this works:On Fire

 

I know all this doesn't make a lot a sense. The engine should have misfire codes the way it idles rough and shakes during acceleration, but there are no misfire codes. Doing a compression test on these COP engines is a real PIA with all the parts that have to be removed just to get the plugs out before you can even start the test. That's why I wanted it to be the cats, but it's starting to look more and more like I have a blown head gasket considering the green slime that came out of the tail pipe.

 

COPs can misfire without setting a code. They put out substandard voltage that gives a weak spark and a bad idle. I've replaced 3 already without a code ever being set.

 

BIG THANKS to Kaboom10! You hit the nail on the head. It was #1 COP even though I had NO misfire DTCs. I did have 2 other DTCs that sent me barking up the wrong tree. P0171 and P0174 sent me looking for something in common to both banks. Not enough fuel or too much air(hose leak). I found it when I had a helper put the truck in gear while holding the brake with the engine running. I started disconnecting the COP connectors one at a time. When #1 was disconnected there was no change in the engine. Every other cylinder caused the engine to idle rough when its' coil pack was disconnected. I hope someone else can learn from this post because I wasted a lot of time following the DTCs when those codes were total CRAP. That doesn't happen very often, but in this case I was led astray.

 

Guy's, new to this site. I have a 1997 f150 4.6, and keep getting the infamous P0171. I did clean the MAF, but after thinking about it since I didn't get a 0174 I don't think that the MAF was the issue. I have not checked for vacuum leaks, but the code just came back after about 3 days. Since Ron had stated that 9 out of 10 times he's seen it to be the 90 degree boot on the PCV, thats what I'll check first. Man, you guys sound like you know your poop !!! The thread was invaluable, and how ironic that it was like the second updated post on the site. Oh yeah..as an fyi....the magnaflow cat's and bosch O2 sensors are only a year old.......so I'm pretty sure there's nothing going on there

Thanks, Greg

 

I don't want to bad mouth DTCs, but this is one time I was led down the wrong path by them. That is by no means the norm. If you have just P0171 I would look for things that could make ONLY Bank 1 lean. What are those things? You have to ask yourself what is common to both banks and rule that stuff out. This doesn't mean the next time I see BOTH P0171 and P0174 together on a F150 I'm going to ignore that and start looking for a bad COP. Sometimes you get the impression that Satan lives inside of these PCMs. You have to know how to make use of the DTCs. I should have asked myself "Why is this engine misfiring and I don't have any misfire DTCs". I knew it was misfiring, but I ignored that and started troubleshoot the P0171 and P0174 DTCs.

 

Glad to help you find what I've gone thru. Didn't cost me a dime.

 

2001 Expedition.
Codes thrown. Changed a COP. ran good for about a day. more codes, bought a new MAF. No codes, still hesitating. Joined FTE. checked COPs as suggested above. Still hesitates. O2 sensors next?

any other help would be great...

 

If all your COPs are good and that's a big if you might want to check fuel pressure. It should be at least 32 and got up when the vacuum hose is disconnected from the regulator. Also try changing the fuel filter. What DTCs do you currently have? I'd be willing to bet they're NOT P0300 to P0308. I'd be really surprised if they are.