Normal idle then rough idle
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Laughing Gas
Joined: Oct 2005
Posts: 1,226
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From: The OC
Normal idle then rough idle
2000 Expo, 5.4, 113k miles, no codes, cleaned the IAC which didn't look too bad and had a little carbon, cleaned the MAF, replaced the PCV (yes it was bad) new enough fuel filter, Motorcraft plugs at 98K, new air filter. The last time it did this, I cleaned the IAC and that was 2-3 years ago and the idle problem was solved.
Question is has anyone experienced an IAC not throwing a code but your truck idled better after replacing it? I'm thinking the IAC is going out but not bad enough to present a code. Can that happen? I normally won't replace a part unless it's giving a code and I rule out other potential issues. I'm going to check the fuel pressure tomorrow, but there are no performance issues, only a sluggish idle 500-600 rpm sometimes.
Question is has anyone experienced an IAC not throwing a code but your truck idled better after replacing it? I'm thinking the IAC is going out but not bad enough to present a code. Can that happen? I normally won't replace a part unless it's giving a code and I rule out other potential issues. I'm going to check the fuel pressure tomorrow, but there are no performance issues, only a sluggish idle 500-600 rpm sometimes.
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Laughing Gas
Joined: Oct 2005
Posts: 1,226
Likes: 42
From: The OC
I believe my problem ended up being the PCV elbow at the PCV. 2 weeks ago I replaced the worn PCV (it was actually buzzing like a bumble bee) and cleaned the elbow with brake cleaner which I believe made it weak. Hearing a vacuum leak in the area of the PCV, I saw the rubber elbow was soft and collapsing, and twisting the new (not motorcraft) PCV in the grommet causing a vacuum leak. So I bought the PCV tube assy with PCV from Ford for $36, took 10 minutes to replace last night and so far so good. BTW, the rear elbow was in good shape. I'm not sure if the new IAC made a difference, but I'm not pulling it out to find out. 
Elder User
Joined: Mar 2011
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IAC is the problem i clean mines on my navigator and after i done cleaned it i put back in then when i was driving back home that same day it shut off on me x2!!! and NO CHECK ENGINE LIGHT! replaced it the next day and its been 2weeks since i replaced it and alot better now 
PS: when i had the bad IAC it sometimes howeled threw the intake wouldnt go away untell i pressed on the pedal hope this helps....
PS: when i had the bad IAC it sometimes howeled threw the intake wouldnt go away untell i pressed on the pedal hope this helps....
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Laughing Gas
Joined: Oct 2005
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Likes: 42
From: The OC
I'm back with the same intermittent problem. We got codes 0171 & 0174 again and my wife said it was idling rough sometimes. I drove it yesterday and could not get it to repeat after resetting the computer with the scanner. This morning I checked fuel pressure and this is what I got. After each sequence below I released fuel rail pressure for consistent test readings.
KOEO fuel pressure went up to about 6 psi.
KOER fuel pressure went up to 32 psi
KOER with fuel pressure regulator disconnected 42 psi
KOER and releasing the pressure on the gauge, psi slowly climbed to 32 psi, not a quick "snap" to 32 psi
KOER 32 psi, applied acceleration and it momentarily went to 30 psi then back to 32 psi
I've read that when I turn the key ON but not start, the fuel pressure should go straight to spec fuel pressure which should be 30-45 psi. I thought the pressure should "snap" straight to spec pressure, not rise slowly which was a 2 to 3 second count.
KOEO fuel pressure went up to about 6 psi.
KOER fuel pressure went up to 32 psi
KOER with fuel pressure regulator disconnected 42 psi
KOER and releasing the pressure on the gauge, psi slowly climbed to 32 psi, not a quick "snap" to 32 psi
KOER 32 psi, applied acceleration and it momentarily went to 30 psi then back to 32 psi
I've read that when I turn the key ON but not start, the fuel pressure should go straight to spec fuel pressure which should be 30-45 psi. I thought the pressure should "snap" straight to spec pressure, not rise slowly which was a 2 to 3 second count.
Logistics Pro
Joined: Jan 2001
Posts: 3,740
Likes: 37
From: Central NYS
With engine idleing and fuel pressure has stabalize. shut the engine off and monitor the fuel pressure. It shouldn't loose but a few pounds, unless the fuel pump check valve is leaking.
With your rough idle, did you try cleaning the throttle body?
And verify that the EGR valve isn't sticking every once in a while?
Logistics Pro
Joined: Jan 2001
Posts: 3,740
Likes: 37
From: Central NYS
With engine idleing and fuel pressure has stabalize. shut the engine off and monitor the fuel pressure. It shouldn't loose but a few pounds, unless the fuel pump check valve is leaking.
With your rough idle, did you try cleaning the throttle body?
And verify that the EGR valve isn't sticking every once in a while?
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Laughing Gas
Joined: Oct 2005
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From: The OC
I cleaned the throttle body within the last few months but I'll try it again. I haven't checked the EGR because of the 0171 & 0174 code fix recommendations didn't call out the EGR as a problem.
It will be fine for a few days after code reset, but then the codes return after the poor idle. Code indications are a vacuum leak or poor fuel pressure.
It will be fine for a few days after code reset, but then the codes return after the poor idle. Code indications are a vacuum leak or poor fuel pressure.
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Logistics Pro
Joined: Jan 2001
Posts: 3,740
Likes: 37
From: Central NYS
Right. I pulled my hair out as my Navi would throw a P0174 every few months. Finally figured it out. The complete fresh air side of the PCV system was MISSING on my truck! All this time I was searching for something that was broken, not for something that was MISSING!
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Laughing Gas
Joined: Oct 2005
Posts: 1,226
Likes: 42
From: The OC
2000, 5.4, approx 114k. I'm back with the intermittent 0171 and/or 0174 code. I just cleared the codes last week and it returned quicker then usual. It usually takes a few weeks to set the CEL, so I reset it every few weeks. I sprayed carb cleaner around the engine area, all hard plastic vacuum lines, soft vacuum lines, intake manifold areas and didn't get any idle speed change. The PCV assy is new as stated earlier. MAF was cleaned not to long ago. I'm starting to think intake manifold gasket after much reading here and elsewhere, but I don't want to do any unnecessary work. I'll do the fuel pressure testing again, but the last time it was good. Any other thoughts or suggestions?
Thanks,
John
Thanks,
John
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