Help! Searched already, nothing anyone says has helped yet...

Vehicle: 2000 expo 5.4

Problem: rough idle, but not a constant rough idle. Happens periodically. Threw lean codes.

I have replaced plugs, coils, vacuum hoses, and now I'm stuck... Doesn't hesitate when I accelerate, just does it at idle, and not right away when it drops down to idle but shortly after.

No hissing from anywhere. I did notice the battery light comes on above 3000 rpm so I will be replacing the alternator. My biggest question is could voltage drops cause this? The truck did throw these codes once before but the battery was toast. New battery and the problem went away for almost a year til now.

 

If replacing the alternator doesn't solve the problem, I would next look at the EGR valve possibly sticking.

 

How can I test the EGR?

 

Im seriously at a loss. The egr opens fine, passages are obviously clear, I am thinking the EGR is sticking, or its a bad DPFE... No codes for the EGR, or DPFE, I replaced both hoses going to the DPFE and we will see how it acts now.

 

Can you be more specific? Exactly which code(s) ?

Is the check engine light currently "on"?

 

Codes are p0171 and p0174

Cleared the codes yesterday, hasnt come back on yet

 

Be sure your oil dip stick is fully seated in the dip stick tube and that the tube is seated in the engine block.

I'm assuming you found and verified that the infamous PCV elbow at the rear of the intake manifold is not questionable.

Also, be sure and check that the PCV fresh air line is properly seated in the valve cover and in the air box.

Wouldn't hurt to clean the MAF sensor wire if it has been a while.

 

ok, New MAF didnt help either.... all i can think of next is fuel press reg, pump, or filter...

 

I am in serious need of help with this!!! No vacuum leaks that we can find, fuel system tested good. Here is the list of stuff I have replaced.

IAC
MAF
EGR
DPFE
Plugs
Cam Sensor
All vacuum lines

I am at a serious loss here. This is an emergency response vehicle and i NEED it on the road asap

Only codes are P0171 and P0174

 

intake gaskets

 

Im doubting the intake mani gasket idea. Sprayed them with carb cleaner, no change in idle or fuel trim. Plus it isnt a constant issue, just hiccups every once in a while. Short term fuel trim is within tolerance on both banks. I did notice once the truck is pulled out of the shop, into the 14 degree temps, the intake air temp only drops to about 40, and the short term trim goes up to about 7-10. I also played with the pcv system tonight, capped the hole in the back of the mani and ran it to just see if that was a problem, ran fine. Cleaned out the carbon on the elbow since it looks to be in good shape still without holes, and it cleared up the idle quite a bit and only hiccuped twice tonight in about 6 hours of idle/drive time.

 

Figured you already changed the pcv valve there inexpensive, go with motorcraft tho the cheaper ones tend to whistle, did you measure you engine vacuum?

 

Its the sum of the short term fuel trim PLUS the long term fuel trim that trips the P0171/P0174 codes - when the sum is >28%

Did you check the dip stick and the PCV fresh air hose (see post #7)?
I'm not sure where the PCV f-r-e-s-h air hose is on the 2000 5.4, but on the 5.4 DOHC Navigator it connects to the rear of the driver's side valve cover, waaaay back under the windshield, and runs up to the fresh air box, downstream of the MAF. Be sure that the grommet is seated in the valve cover and not allowing an air leak.

Just to be sure what I am referring to, I'm not talking about the PCV hose that contains the PCV valve and the infamous elbow. I'm talking about the fresh air hose that supplies filtered air to the crankcase and replaces the air that the PCV valve evacuates.

The P0171/P0174 could also be caused by a fuel supply issue. Be sure the fuel filter is not restricted. And take a look at the freeze frame data to see under what condictions the codes were tripped. If they were set at low RPM and light throttle (i.e, high vacuum condiction), its more likely to be a vacuum leak. If at higher RPM and heavier throttle settings (low vacuum condictions) it might be a fuel supply issue.

 

It trips the code around 650 rpm. No throttle.

Also the fuel filter is new

 

At idle, with no throttle, that's when the vacuum is the highest. So a tiny vacuum leak (after the MAF sensor) will have the maximum effect. At higher RPM/throttle, the small vacuum leak will have a negligble effect.

Might try and disconnect the vacuum hose leading to the EVAP system and plug its source. It will probably set an EVAP code, but your looking to see if it helps the fuel trims. If it does, the EVAP purge valve might be causing the vacuum leak.