Rough idle? P0171, P0174? Here's what you need to check first!!!

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Lane24

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Very common symptoms we all have at one time or another with our 5.4L is a rough idle where we will have high idle for a few seconds when slowing or coming to a stop followed by a drop off to a very low rough idle after stopped for a few seconds sometimes even to the point of stalling out.

I had this problem for a few weeks, and then the weather turned cold, and it got even worse to the point that my ‘ check engine light’ was even coming on. I took it to the dealer, and they replaced my IAC. No change. Still idled rough. Still had the CEL. The only change was the weight of my wallet!! So I turned to the internet!!!

By doing some ( a lot) of research on the subject from other Ford sites and forums, this is a culmination of what I’ve found out. The fix is a little rubber ’boot’ or elbow with a crack or a hole in it causing a vacuum leak.

The Ford dealer will run a diagnostic and indicate that it returned an error code P0171 and P0174, which indicates the engine is running lean. They will most likely recommended the following service:
Replace EGR valve - Replace PCV valve - Replace differential feedback EGR sensor -Replace mass airflow sensor -clean fuel injectors
all at a cost of almost $800. Yikes!!!!

The first thing to check is your vacuum assembly. You most likely have crack or hole in the little rubber elbow that connects to the intake from the PCV valve on the right hand side of engine. Even the slightest crack will allow more air to be sucked into the engine causing a too lean mixture.

The PCV valve is in the right valve cover... which is on the left side when your facing the engine (passenger side). It's under a lot of other hoses so it is very hard to see.. The hose goes back from the valve, then toward the drivers side to attach to the back of the intake way back between the engine and the firewall very close to the top of the engine. The common faulty part is the rubber boot that connects the hose to the back of the intake.

The way I found it was to follow the hose back from the valve cover. I sat right on the engine shroud, and felt for the tube sticking out of the top rear of the valve cover on the passenger side of the engine. With the engine off, it pretty much just pulled out and went back in real easy. Then I just followed it with my hand the best I could ( with the motor now running so be careful!!) until I got back to the firewall, and it actually comes up from underneath the rear of the intake. . You can kinda reach your finger up under there to where it's connected, and feel and hear a definate shoosh sound when your finger goes over the hole or crack in the elbow.. You can't see it, but you can definatley trace it back by following that tube. You'll know it when you find it because your finger will plug the hole, and your truck will start running smooth again!!!

The part number for the entire vacuum assembly with the correct boot on it is - F85Z-6C324-AA probably around $30 -$40. Or you might be able to just replace the elbow itself. Either way, make sure if you do have the leak, to make sure you have the replacement part handy before you take off the old one, because it will crumble in your fingers!!

Hopefully you just saved a few $100, and a lot of frustration. I know I was pretty happy when I found this easy fix!! Good luck!!!

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firefighter407

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Thanks for the post it came in very handy and saved me a lot of searching.

Just a quick question, is that part number in the picture is that just the elbow or for the entire assembly with the plastic tube too?
 
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TxOutlaw

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Wish my P0171 was that easy on my Jeep... I chase that lean bank 2 all over the engine bay... Turned out to me a bunch of wires melted under a plastic covering!!!

Nice write up!
 
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Lane24

Lane24

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Thanks for the pot it came in very handy and saved me a lot of searching.

Just a quick question, is that part number in the picture is that just the elbow or for the entire assembly with the plastic tube too?

The pic is just the part number for the elbow.. The entire hose assembly part # is above it in bold. It is F85Z-6C324-AA .
 

trdragons44

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i was wondering if this would cause the rpm to drop to 300-400 after putting it in park?
 

Blackwater

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2nd this. I had the same issues. Turned out to be the same spot plus cracks just over the PCV
 

KWT2000

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hey thanks for this great post... on mine it wore a hole where it expands over the fitting heres the updated part number F75Z-6C324-JA....its the elbow / hard line and PCV valve it was $39 after tax five minute install
 

panda24619

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i had this problem about a year or so ago. mine wasnt the valve. it was the elbow in the back. i guess it was rubbing againest the firewall or something and waa laa a hole. went to take it all apart, broke the hard line of the hole. now what? well i went to a napa and the guy had no clue what he was talking about. went into another one. the lady there had the same problem with her 02 150. she gave us about a 4 inch piece of fuel line. let it soak and put the two pieces together. got a new valve too just because and no more problem.
 

OK98EXP

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Thanks for the post...Helped me out a lot! I have a 1998 Expedition Eddie Baur 4x4 with a 5.4 ltr engine. After hearing a whistling sound coming from the engine area and a considerable rough idle I recognized it was a vacuum leak of some kind. I traced the sound to a ruptured boot attached to a hard rubber line- very much described in this post.

My truck required a replacement Ford Part # F75E-6C324-JB (see photo) The part is also identified as F75Z-6324-JB. The part comes in two versions - heated and non heated which is identified by the "JB" code at the end of the part number. Lucky for me I needed the non-heated version which was about $29.99 from the Ford Dealer...$20 cheaper than the heated version.

I have attached photos of the non-heated hose. From what I hear is a 10 minute or less install...I called Pepboys and they said it was an approximate 1/2 hour repair and quoted be $35 to put it on FYI. Thanks again

UPDATE: Pepboys mechanic said the Ford parts guy was wrong and I needed the heated hose which actually has a second hose connected at a Y-fitting. Oh well...also after the repair the idle still was rough and the engine dies when a/c is turned on. Next step is the idle air control valve (IACV) which was a $40 part. This is not difficult to repair however is located in the same tricky spot as the PCV hose which is toward the back of the engine. Just gonna have my local mechanic put it on for $25. Will update after install.

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UPDATE: I replaced the vacuum hose and the truck actually improved a bit however the idle remains rough and when gearing up to about 15 to 30 mph the idle gets jerky. I had the mechanics do a system check for codes and sure enough the #2 and #6 cylinder coil pack needs replaced along with a new spark plug. So the sage continues. Will update...
 
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corvettej

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Again, thanks Lane24 for the info, I had the followings codes on the 4.6l also, P0171, P0174, P2195, P2197 I took the cover off and reach behind the throttle body feeling for the hose end when I felt a dimple and placed my finger on top, I felt the suction and the truck idle smooth out I knew I had found the problem after replacing the IAC. The part from the dealer was only $29.00 where as the elbow was $15.00, so I got the whole hose and only had to push it on to install. Thanks , again for saving me some money.
 

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