I have a 98 Expedition 5.4L with 111K. Engine seems to run smoothly at idle and all rpms except just under 1500 once engine is warmed up. It doesn't show up when cold. As you slowly approach 1500, the motor starts to surge a bit and drop out/miss and then occilates as it catches itself. Once you push past it, it smooths out again.

No codes popped. Have replaced the PCV valve, repaired a vacuum leak from a burned out elbow way in the back (which seemed to fix, but the problem is back), replaced a throttle position sensor, and checked every vacuum line I could find for defects. I now have a very smooth running truck except at 1500.

It sounds like another vacuum leak, but I already fixed the common culprit of a burned out elbow against the firewall and all the other lines appear good.

Any ideas or quirks I'm missing? Up to now, the truck has run like a champ. Except for the parts mentioned above, the truck is completely stock.

Thanks.

 

My thought would be maybe the MAF clean/check. After that maybe start checking EGR stuff.
But I'm sure there are alot of guys on here Way smarter than me that can come up with some more specific checks/tests.

 

I forgot to mention above that I cleaned the MAF as well. Now it runs REALLY smooth except at 1500. Thanks for the suggestion.

 

Possible COP on it's way out....

 

The fact you state it does not show the condition when cold indicates to me it could be EGR related. The EGR is disabled until the engine is warmed up. The EGR is also not supposed to function at idle. The EGR is controlled by the EGR solenoid. The DPFE is only a feedback to the computer to tell the EGR is working or not. It does not do any active control for the EGR, it is a sensor.

 

Thanks 454! You were right on. After metering everything out, it turns out the EGR valve was good but the DPFE was not working properly. It started at 1.25V and climbed to 1.6V when fully warm which is a far cry from the .5 - 5V it should have been...thus why I didn't get a code because the computer couldn't tell that it wasn't working. Thanks for your help!

 

You're welcome. Been fighting EGR issues on mine and so I have tried to learn how the system works and your symptoms agreed with my thoughts. Mine had the EGR staying open and idling rough or stalling. I knew the EGR was holding open, but the solution was hard to figure out. I ended up having a sticking EGR solenoid control valve on mine that drove me crazy. Checked the part and it measured correct resistance for the solenoid coil. Checked voltages at connector and it all seemed to be good. I decided to try replacing the solenoid control valve, as that is the only thing that I could figure out could cause the problems besides a wiring problem. Not having a lot of time on the fix so far it seems to have worked. If the new part is not the problem then it is serious wiring investigation time. Which means beyond me since I don't have the wiring diagram or dedicated computer testing tools.

 

All,

I'm revisiting this post since I still have the problem. Here's the cruxt of the problem. For some reason, after the engine warms up and rpm increases to 1500 rpm, the computer is commanding the EGR to open prematurely/excessively causing the engine to stumble and surge until out of that 1500 +- 200 rpm band. The DPFE, EGR and EGR control solenoid have all been replaced. The engine idles just fine either cold or warm. Also, the problem has never popped any codes whatsoever. I'm going to start looking into an O2 sensor problem since it's the only thing I can think of that would give the computer 'bad gouge' and commanding the EGR valve to open up too much. Many others have had this problem, but I haven't found a smoking gun on this issue yet. Thanks.

 

How do you know that? Did you pull the green vac off the EGR? Well, if you do that, it would narrow down the problem to EVR and PCM. Yea how do you know the PCM is commanding that?

Sounds like a coil IMO.

 

I think most egr problem repairs has to include cleaning the egr passages.

 

He'll be back in another 4 years, look at the date of his first post lol.

The EGR passages plug up on the 4.6L's, but not on the 5.4's. What happens more, EGR related is they can stick over time. Thing is, he doesn't have any of the problems that are EGR related lol. ALL EGR problems that effect the way it runs happens at idle.

He has to go thru fuel diagnostics first, then ignition first. Specially if there's no DTC's.

 

jbrew/roadie,

I appreciate the help. To clarify a few things mentioned above. I also read about EGR problems that effected the idle, not 1500 rpm. Here's where I beg to differ and offer this additional info. I DO KNOW that the PCM is commanding the EGR valve to open. I just don't know why. Yes...I did disconnect the EVR from the computer and the problem immediately went away and then popped a code for EVR. Prior to that I hooked in a vacuum guage in-line with the EGR valve using a T-fitting and watched the vacuum pressure build and cycle as the EVR opened the EGR valve and quickly closed once the engine stumbled at 1500. The EGR valve is new and not sticking and it holds vacuum pressure just fine. I also put a multimeter on the EVR valve and watched the voltage change from the PCM commanding the valve to open just prior to the engine stumbling. Just in case, I ordered a new EVR valve (because it was a cheap fix/shot) and it did exactly the same thing as the original. I also operated the EGR valve with manual suction and duplicated the same engine response. If I plug the EGR line and left everything else hooked up as normal, the engine runs perfectly fine...but then will eventually pop a code for faulty EGR as expected. So...yes, I think it's PCM/EGR system related. I just can't figure out why the computer is commanding the system to open with good valves and sensors.

Roadie brought up a great point that I often wondered about. What if there was a carbon clog/restriction inbetween the two sensor hoses on the EGR tube? The DPFE is new, but if there's a blocked tube and the sensor is not reading the correct differential pressure, maybe it's not sending the right data to the PCM? However it's a big tube and I'm not sure how narrow that EGR orifice is.

Yes, it's a 4 year old thread, but I'm tired of putting up with it.

 

So, did you pull the green line like I said ? It's very easy to do. Take a picture of the way you have it connected. Worse comes to worse, just keep the line disconnected until you can replace the PCM. That way it'll run right, not hurting anything. You'll get a little less mileage, maybe. Although I don't see it with the way the PCM is forcing it to run as it stands.

Do you understand?

The valve ONLY is suppose to open momentarily @ cruising speeds.. That's it!

 

Tell you what, look here, - http://www.fordscorpio.co.uk/egrmonitor.htm

I think it may help.

 

Thanks, jbrew. Greatly appreciate the EGR tutorial. I understood the concepts, but I guess I didn't understand to what extent the EGR is supposed to open and when. I had some time to work and experiment on it this weekend. Here's what I got. I disconnected the green vacuum line to the EVR as instructed and plugged the white vacuum source back into the EVR (kind of turned the over-under boot to a 10 oclock position). After letting the engine warm up, I revved it slowly up to 1500. The engine 'stumbling' went away and the RPMs held super steady. However, this is the first time I've noticed without the EGR aggravating the situation that there IS a slight feeling of a miss at that same RPM. It doesn't change the RPM one bit, but I can feel small/fast/light 'nudges' at that RPM. Again, there are no codes, but maybe that's what the PCM is trying to compensate for? I'll go back to your original suggestion and start doing some fuel system diagnostics and looking into COPs. The fuel filter is new (less than 15K on it) but the fuel pump in the tank has never been replaced. Truck has 129K on it, but I've had it under heavy load several times on cross country drives without indications of fuel starvation.