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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old December 20th, 2016, 18:03   #1
Roadblock
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Default Need Help Troubleshooting Horrible Fuel Economy

My mileage has started to hit the crapper since September. I used to get 1100+ KM on a full tank, now itís more like 600-700. Around the time the mileage started going down I noticed I had a huge loss of power so I scanned it and came up with an over boost. I looked at the MAF, actuator, and vacuum lines and found no issues but once I plugged my MAF back in my boost issue was gone so I assumed it was a corroded connector. The CEL hasnít returned but my mileage is getting worse. My temp gage is staying 1 notch below 90 and I canít get it up unless under HEAVY load for a while but as soon as I idle it starts to drop so a new thermostat and housing is on order. I just donít think that the thermostat would be the fuel mileage issue, especially since it started in September before it got cold. I am going to do some boost tests and test the MAF with VCDS but I am leaning towards a clogged intake since I have 370, 000km on the engine and it has probably never been done. Any other advice? I will post the VCDS results when I have them.

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Old December 20th, 2016, 18:16   #2
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with that high mileage... you probably need lots of things. intake manifold cleaning for sure. vnt vanes likely sooted up causing overboost code. need to be cleaned, but assuming original turbo, safer to get a new one at this point. your IP, if original, is also aging... could use a rebuild. injector nozzles worn.. certainly should be replaced with upgrades if original. etc etc
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Old December 20th, 2016, 18:22   #3
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Quote:
Originally Posted by burpod View Post
with that high mileage... you probably need lots of things. intake manifold cleaning for sure. vnt vanes likely sooted up causing overboost code. need to be cleaned, but assuming original turbo, safer to get a new one at this point. your IP, if original, is also aging... could use a rebuild. injector nozzles worn.. certainly should be replaced with upgrades if original. etc etc
The overboost has never returned after the MAF was unplugged and plugged back in. Not saying you're wrong, just giving the info.

Anything else on the list of things that need to be done? It had the timing done 40,000 km ago, and a new fuel filter about 20,000 ago.

Last edited by Roadblock; December 21st, 2016 at 09:16.
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Old December 21st, 2016, 21:27   #4
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If it suddenly dropped, my bet is the thermostat and yes, it will affect MPG that badly. Also replace coolant sensor with an OEM green one.

Also check the usual-dragging brakes, under inflated tires, dirty air filter etc.
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Old December 23rd, 2016, 19:00   #5
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thermostat would be easy to notice
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Old December 23rd, 2016, 19:04   #6
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Probably not the whole story, but as of September you've been on winter diesel... good for a 10-15% drop in fuel economy all by itself.
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Old December 25th, 2016, 09:04   #7
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Change the fuel filter. At 20,000km along with filling up at different stations could be part of your problem. Fill fuel canister, (not literally, allow for filter element) with a good fuel lube/cleaner and go for a fast drive. Check your VCDS for what the actual coolant temp is as a baseline ahead of changing to a new thermostat would be intersting..
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Old December 27th, 2016, 09:14   #8
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Thanks guys. I will be doing the thermostat and posting some VCDS findings soon. I've been wrapped up with the holidays and working. In the last month I've been doing some much needed maintenance on the VW. Changed drivers side lca bushings, ball joint, and axle and also did the front and rear brakes and the rear ABS sensor on the drivers side. Next up is thermostat and radiator (small leak in rad). We've had some bad winter roads lately and some parts that were ok on the car have since gotten bad so it needed to be done. Also will be doing lca bushings and ball joint on the passenger side and then alignment.

My brakes are not dragging. I checked them on a small hill from a stop and I rolled very easily once I let off the brake.

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Old January 1st, 2017, 13:58   #9
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Bad thermostat or temp sensor may cause ecu to enable fuel enrichment (similar to choke on in a gasser). Troubleshoot those systems.
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Old January 10th, 2017, 10:06   #10
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Changed the thermostat and fuel economy has increased dramatically.
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Old January 10th, 2017, 18:45   #11
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Quote:
Originally Posted by Roadblock View Post
Changed the thermostat and fuel economy has increased dramatically.
Change the CTS, would have been easier with the coolant drained.
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Old January 10th, 2017, 22:31   #12
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^^^ Did you have to be so harsh on the poor guy? You could have phrased your comment much more positively like,"It is a shame no one suggested you change the CTS while the coolant level was lowered." See how much nicer that would have sounded? LMAO!
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Old January 11th, 2017, 00:18   #13
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Quote:
Originally Posted by Nevada_TDI View Post
^^^ Did you have to be so harsh on the poor guy? You could have phrased your comment much more positively like,"It is a shame no one suggested you change the CTS while the coolant level was lowered." See how much nicer that would have sounded? LMAO!
It was suggested earlier. If he can't take the heat, change the thermostat,
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Old January 11th, 2017, 16:05   #14
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I still have to change the radiator. I had no time but wanted to get the most important issue done because it was fast. When I do the radiator I'll do the temp sensor.
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Old January 17th, 2017, 06:14   #15
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One can change the CTS with very little coolant loss, IF, that is, there are no problems encountered (stuck O-ring and such). One trick, seemed to work for me, is to loosen the cap on the expansion tank and squeeze the upper radiator hose; keep the hose squeezed and tighten the expansion tank cap back down; this helps create a vacuum in the system which reduces the coolant's desire to leap out of the CTS hole. I placed some cloth (sock?) under there and got it only mildly wet. Remove the air box and hose and you have an unobstructed view and more room to work.

No matter, if you're quick you should lose very little coolant. I'd say that it's so trivial as to not warrant waiting on some other major cooling system work to perform.

All said, I wouldn't bother swapping out the CTS unless I knew it were bad. A bad one tends to cause the gauge to be erratic (I've only witnessed it diving down and then coming back up to normal).
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