The image below shows temperatures I recorded while troubleshooting. Specifically, note the ambient temperature. The readings were taken just prior to the 129 fault code. What is missing from when I originally did this image is the reading where the output port meets the pump body, when it reached 80°F the 129 fault would occur, even when it is really cold out. Note that the perspective of the photo is confusing and the exhaust pipe was nowhere near as close to the pump as it looks but was still part of my problem. Both are in different locations now.
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OK. Here's the latest and it's not good. At this point I can't say I was very optimistic but went ahead and made the changes anyway.You may have the wrong version of the fuel pump if you have the larger inlet port.
The incorrect replacement pump I got from (and returned to) Esparparts is 22.4519.01.xxxx which had the larger inlet side and a different electrical connector than my original pump and harness has. Apparently this is the pump for the original B4 heater version.
I got the proper fuel pump from Heatso. The M2 B4L fuel pump is part number 22.4552.03.xxxx.
Your pump is mounted exactly how mine was when I was experiencing the my problem with the P000129. My thinking was that it is a nice protected area. The new location for the pump below the frame rail resolved my issue.
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Our symptoms are not identical, so all this info may not be useful. Hopefully, your new sensor resolved this.
Dropped the fuel tank and put in new 2mmID fuel line from the dip tube to the pump. Made sure the lines were butted up tight end to end and clamped good. Moved the pump while I was at it to a very similar location as this one in the picture.AND TA DA........NO CHANGE in behavior. Solid fuel into pump, bubbles coming out, same as before. And that's from start up when cold. No possible heat related issues. Same as before. What's left? I have now replaced everything from the fuel tank to heater with the "correct" parts, "corrected" pump position, flashed ECU, replaced combo sensor, replaced burner screen, "corrected" combustion intake. I'm assuming the fuel pump generating bubbles is the issue so what else is there to do? Do all these heater installations' pumps generate bubbles?