Bucking. What should I start with?

[BTR]

2000 Explorer XLT 2WD 4.0L SOHC JOB1 5-Speed Automatic O/D 5R55E

Between 40-45 MPH (about 1700-1800 RPM) in 4th gear on a level road and maintaining a steady speed (very little gas pedal required), it's bucking. This is regardless of whether O/D is ON or OFF. Also, it bucks between 30-35 MPH (about 1500 RPM) under the same conditions. Typically, it doesn't shift into 5th gear until at least 50 MPH.

I'm sure there are several things that could be causing it to buck. What should I start with? Thanks.

 

[Turdle]

You might check the throttle position sensor. Check that it's output and resistance rise smoothly especially right off idle.

 

[donalds]

I'd look at the fuel trims if possible
Only time mine bucked was when I had fuel problems a pump and filter pump overheating

 

[BTR]

Fuel trims look AOK. I just checked them.

What do you think about the following diagnostics?

EGR Valve vac line unhooked / DPFE electrical connector connected = No Bucking
EGR Valve vac line unhooked / DPFE electrical connector disconnected = No Bucking
EGR Valve vac line connected / DPFE electrical connector disconnected = No Bucking
EGR Valve vac line connected / DPFE electrical connector connected = Bucking

This leads me to believe the DPFE is bad.

QUESTION: Regarding the vac line that connects to the EGR Valve, does the DPFE control the amount of vacuum and/or the ON/OFF of vacuum in this vac line?

 

[koda2000]

Sounds like it might be a bad DPFE to me...

How to diagnosis a bad DPFE

 

[donalds]

Sounds like it might be a bad DPFE to me...

How to diagnosis a bad DPFE

I agree or a clogged egr or egr tube

 

[BTR]

DANG!!! I wanted to remove the EGR Valve in order to see if it, and/or the pipes connected to it, were clogged. But, IT AIN'T COMING OFF!!! Even with plenty of PB Blaster. However, I was able to look into the valve and the pipe going to the intake manifold. Neither show any signs of clogs.

Given this, I'm now focused on the DPFE and the EGR Vacuum Control.

 

[410Fortune]

Any check engine light?

Does the engine run smoothly at idle?

How many miles?

 

[BTR]

140,000 miles. At idle, an occasional, minor judder every 8-10 seconds or so, otherwise idle behaves perfectly. At idle, when the vacuum hose to the EGR Valve is disconnected, there's no judder at all... it's as smooth as glass.

Replaced the DPFE about 30-days ago with a $10 aftermarket made by Labwork. Now, throwing P0401. My suspicion is that the DPFE has gone bad.

 

[410Fortune]

I recently replaced a 98 5.0 DPFE with a cheapo Amazon $14 unit just to do some testing. The "new" DPFE lasted exactly two weeks, back to a Ford Motorcraft part for this truck..........

 

[BTR]

What were your symptoms (when the DPFE was bad)? Did you get any bucking on a level road with very little gas pedal being applied?

 

[410Fortune]

yes the EGR will cause bucking as you describe, like a dead engine miss

The new DPFE I knew was bad because I got a check engine light and DPFE circuit failure code..... put the old aluminum OEM DPFE on and it was solved.
My original EGR problem ended up being a worn metal fitting on the exhaust manifold allowing leaks, the dpfe was replaced for testing before tearing the entire EGR system out and finding the leaky fitting (this is a 5.0 V8 Ranger I was working on)

 

[BTR]

Very helpful! Thanks!

I continue to want to understand these components. Question for you: When the vacuum hose is disconnected from the EGR Valve, does the valve allow NO exhaust, SOME exhaust, or FULL exhaust to flow into the intake manifold? Which applies... none, some, or full?

 

[410Fortune]

none
Without vacuum the EGR valve cannot open and stays shut = no flow

 

[BTR]

Using a hand-held vacuum pump, I applied 15 inHg vacuum to the EGR valve. It slowly bled down from 15 to 0.

QUESTION - One source I read said it should hold 15 inHg vacuum for at least 1 minute. Another said it should bleed down over about 20 seconds. Which one is correct? Hold... or slowly bleed down... which one?

 

[410Fortune]

Just thinking out loud here

it should not bleed down? its a diaphragm, vacuum to the backside = open. No vacuum = spring closes it. If it leaks down then the diaphragm is leaking = not good.

 

[koda2000]

Just thinking out loud here

it should not bleed down? its a diaphragm, vacuum to the backside = open. No vacuum = spring closes it. If it leaks down then the diaphragm is leaking = not good.

I agree.

 

[BTR]

Online research tells me: Some EGR valves DO leak-down when vacuum is applied. This is NORMAL. They are called Positive Backpressure EGR Valves.

Positive Backpressure EGR Valve
It uses exhaust pressure to regulate the EGR flow via a vacuum control solenoid. The stem of the EGR valve is hollow and allows exhaust backpressure to enter the diaphragm. When sufficient backpressure is present, the diaphragm moves and closes the valve, allowing the full vacuum signal to be applied to the upper portion of the EGR valve diaphragm. This opens the valve and allows recirculation to occur during heavy loads.

Be careful not to incorrectly diagnose this type of EGR valve. Because backpressure must be present to close the bleed hole, it’s not possible to test the valve with a vacuum pump at idle or with the engine off. The valve is working correctly when it refuses to move when vacuum is applied or it refuses to hold vacuum.

Question
Is the EGR valve on the 2000 4.0L SOHC JOB1 engine a Positive Backpressure EGR Valve? Yes? No?

 

[410Fortune]

no

 

[BTR]

BUCKING - PROBLEM SOLVED

I replaced the $10 Labwork DPFE with a $40 Standard Motor Products DPFE. This got rid of the bucking.

RECOMMENDATION FOR DIAGNOSING BUCKING

Bucking
In 4th gear, 30-35 MPH, about 1500 RPM, on a level road and maintaining a steady speed (very little gas pedal required) you'll feel repetitive jerking of the engine and drive train. It also occurs at 40-45 MPH, about 1700-1800 RPM. This is regardless of whether O/D is ON or OFF.

Easy Diagnostic
Before you get into complicated diagnostics and/or repairs like fuel filters, fuel pumps, fuel regulators, fuel injectors, spark plugs, plug wires, etc., simply unplug the electrical connection to the DPFE and the vacuum hose to the EGR valve. If the bucking goes away, then you've ruled out everything I listed in the previous sentence... and ruled in (as potential causes) five components: EGR Vacuum Control Solenoid, EGR Valve, DPFE, all associated vacuum hoses, and all associated electrical connections.