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Discussion Starter #1
I have been getting a regular P0401 "low EGR flow" code. This is a 2003 ZX5 with a 2.0L DOHC Zetec engine, 235k+ miles on it - I've replaced the EGR once and the vacuum solenoid once in the life of the vehicle, but never the DPFE... So I lazily figured maybe it's the DPFE and replaced it (with a new Motorcraft DPFE15). However my issue still remains. Watching several parameters with Forscan while driving, I can see that my EGR duty cycle seems to operate correctly (until the fault is thrown and then the computer just quits trying). But my DPFE voltage is very constant at 1VDC. I'm pretty sure it is supposed to be bouncing around in response to the EGR opening/closing? Does anyone know what the correct voltage range should be here? I can provide some Forscan graphs that I saved.

When I got to work this morning I thought maybe the DPFE connector isn't seated well, so I unplugged it and re-plugged it while running/idling. I did see that it looked like the DPFE signal spiked up to 5V when I did that, and it immediately set the EGR fault. I think maybe it was 5V when the connector was unplugged, because it settled back down to 1V like before when I was back in the car looking at the graph (although I forgot to save that data set). So maybe it's operating correctly? I like to think that it isn't defective out of the box, but I guess that's possible...

Guess I still have left to troubleshoot:
-> Vacuum solenoid maybe isn't responding to the computer's *seemingly correct* duty cycle signal
-> EGR isn't responding to the varying vacuum signal
-> blocked tube/hose (which ones are common culprits here?)
-> hole in a vacuum hose

Any ideas/insight on what else to check or what to look for? I know I could just replace the EGR and vacuum solenoid - they're not terribly expensive - but I'd like to avoid throwing parts at it, especially if it could be just a clogged tube/hose or something...
 

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Turns out it was a small hole/leak in the vacuum hose that supplies the solenoid valve. I taped it up and it should be working fine now (I'm getting response on my DPFE signal now). Still haven't cleared the readiness flags yet on my drive into work this morning, but I'm pretty sure the issue is solved (No P0401 yet anyways). So my old DPFE was probably fine. Glad I didn't go and purchase a new EGR and solenoid valve. Together they would've only been like $100, but it wouldn't have fixed anything.

Now, I wonder if I can purchase a replacement vacuum line. I can't pinpoint that exact part number anywhere - anyone know it? It's the vacuum line that comes off the vacuum manifold port and goes over to the EGR vacuum solenoid supply. Nothing really special about it except that it has the boot molded on to connect to the manifold, and I believe the solenoid end is molded into the little 2-port connector that fits onto the solenoid port.

I reset the PCM so it would relearn now that the vacuum level should be fixed (so this could be just be a result of that) but I found that a hesitation that I had been seeing (for years) at low speed under hard acceleration may have also gone away. I'm wondering now if I've had this vacuum leak for a loooong time, and it just recently got bad enough to throw a code. Oddly enough, I just passed emissions in July (and have passed every year prior). Maybe I'll get better mileage too...
 

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oh also: when I was investigating, I noticed that the nut at the bottom of the EGR valve was completely loosened and just held captive on that rigid tube below the EGR (just kind of sitting there where the tube starts to turn). Thought maybe since that wasn't connected, the EGR was essentially leaking "fresh" air and causing a problem. After connecting that though, I still didn't have DPFE signal so I kept investigating.

A manual vacuum pump/gauge was extremely valuable for the troubleshooting process here, just FYI. I was able to confirm that the EGR worked properly, as well as test vacuum line integrity, leading me to find that the leak (ultimately, the vacuum solenoid, even though it was getting a correctly duty-cycled signal from the computer, did not have good vacuum on its supply side, so it wasn't able to operate the EGR diaphragm appropriately.
 
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