Fluctuating RPM

MichST4

Member
Hi all,
Here's a little head scratcher and a bit of a tail that I'm hoping the combined knowledge base of the group may have an answer for, or at least will have experienced it before.
OK, just for info I'm running a lovely little breathed on TDI 75. The problem started a short time back with a the engine spluttering on acceleration or a need to maintain speed up hill with the Cruise Control set. Dirty fuel was my immediate thought, however on my way North about a week ago (house move on-going) a dual situation arose where it was experiencing the 'splutter' and a fluctuating RPM. This continued until I had the inevitable EML indication, though the car would continue to run.
Anyway, having made our interim accommodation, I broke out my VCDS (luckily I had it in the car) which pointed to the ECU and an associated msg of 17964 Charge Pressure Control! Searching the forums the Injector Harness was pointed at loud and clear regarding the spluttering, however could find nothing concrete garding the fluctuating RPM. So being where I am and having had a think on it all and and really good look around the car, I decided to replace the Harness and also the Throttle Valve. Long and short of it is that I appear to have cured the spluttering but not the fluctuating RPM issue??
Does anyone on the forum have any suggestions/experience of this issue??
IMG_4710.JPG
 
I would start by checking for air leaks in the turbo/ vacuum pipes followed by a potential fault in the N75.
 
As above.
Carefully check the small rubber hoses that run to and from the turbo for splits, holes and looseness on fittings. Looking at your scan results you have a 2004 car so a BHC engine. On these cars the N75 is inside a box near the right inner wing IIRC. Again check all the small pipes for leaks and damage as they enter the box. It only takes a very small air leak for the system to shut off. Does the fault temporarily clear by switching the engine off and restarting? The other problem could be a leaking intercooler due to cracked or split end cap ( usually the lower one ).
These little pipes on the turbo are pressure and not vacuum although they are mistakenly called vacuum pipes.
 
And another schematic inside the box......


 
I would start by checking for air leaks in the turbo/ vacuum pipes followed by a potential fault in the N75.
Many thanks for the heads up. You mention the N75 sensor, is this a replaceable item or does it come as part of the assembly. If it's replaceable as a separate item, would you by any chance have the part number? In the meantime I'll be checking those pressure/vacuum pipes.
 
Further to my earlier posts. Here is another picture showing a failed end cap with the seal pushed out. Could not remember where the photo was to post earlier...

1608389306818.jpeg
 
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