My coolant gauge was starting to act up a few months ago so I went ahead an purchased a new coolant temp sender and put it on the parts shelf. The other day I got the "check engine light" illuminated on the cluster so I plugged in the Vag-Com when I get back home. Sure enough, it was a fault for the coolant temp sender. ETKA calls for p/n 059 919 501 A, 4 pin, green. This is also what I purchased a few months back. When I went to change the sender over the weekend, the one I removed was black, not green and the p/n on it was 078 919 501 C? It also has 12V - 140 deg MAX stamped on it? I wonder it this incorrect sender had anything to do with my occasional surge while in cruise and the high rpm start up that would go to about 2000 rpm or so but not indicate on the tach. (I was only going from the noise of the engine and 2000 rpm seems accurate as it needs to be) So after I changed the sender, I noticed that my fuel economy has gone up and my cruise has lost its rare surge and I no longer have that scary high rpm start up. I tried to search the ATP in ETKA for the 078 p/n and it came up with no results. Anyway, just thought I would share....
It didn't surge at start up; it would surge occasionally during cruise. At start up, it would sometimes act as if I tried to start it while having the accelerator peddle fully depressed and then suddenly go right back to idle and indicate just the correct idle on the tach. The high rpm at start up would never indicate on the tach. Might I have another problem?
I looked it up and they changed the part, the new one is green so it should fix the issue with the sensor.
My guess on the surging: sensor problem. Most funny RPM issues on the TDI are either a sensor problem causing the ECU to changing fueling (changing RPM), or a mechanical issue.
Things to check: EGR valve, you can apply vacuum to the can and see if the valve moves. If the solenoid or lines are not working as expected, then this could possibly cause a running issue.
Maybe the engine was running on a less efficient ecu map due to the dying temp sensor? There's no o2 sensor to meter fueling and control closed loop operation during warm up. I believe that the ecu has to use other sensors to choose the map so this is a possibility.
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