Hey guys,

My 01 4.6 Expy has started doing some strange things lately. Occassionally, when I approach a stop, and upon coming to a complete stop, the idle drops down below 500rpm and then pops back up to its normal range and stays there until the next stop. It does this (the drop) for a second or two and kind of feels like the engine is going to die but never does. I get no CEL and have taken the truck to the dealer. They suspected the IAC and replaced it but no luck. Still doing it.

Just wondering if anyone has any thoughts on this before I take it in again.

Also, is anyone is from my local area (Dunedin, FL) and has used Freedom Ford in the past for service, I'd like to hear your personal opinion on their service center. I've always used them for service due to convenience but am beginning to have mixed emotions on the quality of their work lately.

Thanks,

Anthony

 

they replaced the i.a.c valve?? are you familiar with the motor and where the i.a.c is?? did they really replace it or....??. if it wasnt that maybye you have a small vacumn leak somewhere.

 

Hi Keith,

Yes, I am familiar with the motor and the location of the IAC. They said they replaced the IAC so I don't think they would lie if this is what you're inferring. But, I guess you can never be too certain.

Vacuum leak does sounds like a possibilty but I think a vacuum leak would cause this to occur consistently. In my case, it only occurs after the engine has warmed and not consistently at ever stop.

Any other thoughts?

Thanks,

Anthony

 

I have heard of situations where the torque converter doesn't unlock when it is supposed to, and can result in symptoms like you're describing.

You may also want to check your EGR valve... I recall that the 5.0L Mustangs would develop a "rolling" idle (would constantly cycle between about 500 RPM and 1500 RPM) when the EGR valves weren't functioning correctly. Usually, cleaning the carbon/gunk out of them would make them idle smooth again.

 

you never know. that kind of stuff happens alot more than we think. sucks to think that a place would do that but.... hes right the e.g.r passages could be plugged up making it do that.

 

Couldn't a fuel filter do that?

 

I too was thinking more in line with the torque converter not unlocking but didn't know if that was really possible. As for the EGR valve and its passages, they are all clean. Fuel filter is also new.

Anthony

 

If the converter lockup was the problem you should be able to distingish that by the way it feels with the engine trying to pull the truck with no throttle applied.
When you brake, the converter disengages as a normal function from the contacts on the brake switch signalling the PCM.
On decelleration, the PCM shuts down fuel to a low level and closes the IAC as a normal action and updates tables but is supposed to recover the idle so stalling never happens.
Something is causing this action to fail.
The 500 rpm you see is the base idle throttle stop setting without the IAC open.
Dirty/corroded connector, sticky IAC, vacuum leak under high intake vacuum conditions or no speed feed back to the PCM is a cause.
Does it do it setting still and jazzing the throttle?

 

Hi Bluegrass,

I've been waiting for your reply because I knew you'd have some good input on this.

I too suspected the TC at first but tried letting off the brake when the issue occurs and didn't get a 'lurching' forward of the truck as one would expect. In fact, it was the exact opposite - the truck barely moves when this happens. So this probably eliminates the TC which is exactly as you mentioned.

As for the other items, the IAC is (or should be) new per my Dealer. So I suspect it's probably not the cause. Also, the connector does not appear to be corroded.

By 'jazzing' the throttle while setting still, I suspect you mean slightly pressing the accelerator and letting it go while holding the brakes, right? If so, I've tried this as well and the issue does not occur when I do this. Only happens when I approach a stop.

Any other advice / help is much appreciated!

Anthony

 

Yes, the 5.0 would do that. I got an EGR simulator and unplugged mine. As for the idle, my 05 aviator 4.6 does this every day. I have taken it back to Ford at least 15 times to no avail. They tell me there is nothing wrong with it everytime. This motor shouldn't idle at 500rpm, as mine does, causeing a shakey feeling, but the know it alls at Ford don't know what is wrong. They replaced the IAC, put in some kinda bypass (I have no idea what they did, but it didn't help) and rebuilt the tranny. Still idles like chit.

 

Unless you get a dealer tech that takes the time to apply knowledge of how the system operates and not just be lead by flow charts, you will be a long time in getting the problem solved.
A tech has to go outside their normal working boundaries on some of these problems.
Imagine if your doctor worked that way?
I outlined it for you but getting some one to look at it that way is another matter.
My truck doesn't act that way in the least.
Good luck.

 

Update....

So I take my Expy back to the dealer again yesterday to address the idle issue. The tech ran some additional diagnostics on it and somehow determined that the PCV hose, which runs from the PCV valve to the back of the throttle body, is collapsing under high vacuum. This in turn is causing a 'lean running' condition and that's what is causing the fluctuation in my idle. They actually call it a 'rolling idle' but whatever; it's symantics.

Anyway, the SOB's charged me $92 to diagnose it and wanted another $100 to fix the hose because my ESP doesn't cover this. No, they were not going to replace the entire hose unit, which costs about $30. Instead, they were going to cut and splice the soft areas that were collapsing. How freaking ridiculous. So, I of course told them no way and that I'd do it myself.

So, I'll stop by the parts dept tomorrow to pick up a new hose and replace the old one. I do have one question though:
How would a collapsing hose like this cause a 'lean' condition? Wouldn't it be the exact opposite - a rich condition? If the hose collapses, no air is passing through it. Thus, a reduction in air means more fuel in the normal mix. More fuel = rich. Am I missing something here? Bluegrass - any thought???

Anthony

 

I agree. The PVC vents air, (not fuel, haha) from the crankcase into the intake.

If the hose is collapsing, it could be because your PCV valve is clogged/malfunctioning.

 

cold air makes these IAC's stick but the torque converter locking in that low can cause it to

 

As much as I can get from what you report, the PCV subsystem is part of the calibrated air manegment so this makes the metered air into the engine sensitive to detection when a fault occurrs.
I suspect the faulty hose was admitting out side air at a variable rate vs intake vacuum. What happens that few ever include in the mix is the variable vacuum also changes the fuel regulator pressure and meters varying fuel at the same rolling idle rate to make matters worse yet.

Your thinking is trying to mix carbed systems with EFI.
With EFI, the whole system is no longer totally dependent on just vacuum to draw fuel because fuel is injected under pressure and amount is controlled by sensor reaction to the amount of oxygen present in the exhaust.
When unaccounted for air gets into the intake, the OX sensors do what they are designed to do,,,detect it and make the PCM meter more fuel as a response.
This can drive the fuel table matrix into an end value that sets a code and lights the CEL lamp.
In this situation the code discription says "lean" when it is actually a reaction to a rich condition that was initally caused by excess unaccounted for air.
This is a loop routine and the only way it stops is with the fuel table not being able to shift any farther for correction.
With a rolling idle, the condition drifts in and out of range enough that a code might not be set but could possibly be seen with a dealer level scanner if the tech takes the inititive to look..

There is a lot of very deep and involved actions going on in the PCM program overall that most don't realize. This makes solving problems difficult for many because one has to most often look beyond the code discriptions as it is much more involoved than just a few words of discription.
Glad your dealer tech did go the extra mile and find the real problem.
Good luck.