Monday, May 11, 2009

When is a TDI's 190º really 150º

When I sold my original Kungpao Jetta I was introduced to a Scangauge II. The fella who purchased my car brought along a TDI head who knew alot about TDI's and had one installed on his TDI Beetle. Basically it is a display that ties into the onboard computer (via VAG-COM connector) of the vehicle and gives you various information- such as instant MPG, RPMs, mile to fill up, etc, and the most important thing for vegheads - water temperature.

What I noticed immediately after installing one in my newly acquired 2002 TDI wagon was the discrepancy between the TDI's onboard water temperature gauge and the temperature the ScanGauge II was indicating. When the the TDI indicates 190º the SG2 shows 150º. The temperature will make it 190º+ but the initial 190º displayed on the dash is incorrect*. Now, to me this is very important if you have a manually controlled WVO system. My previous way of switching over to WVO was to wait until the water temp gauge of the TDI indicated 190º, then a would add an extra minute or two then throw the switch.

One can see the problem here - if 160º is our intended temp. of the oil and the water that is heating the oil is only at 150, then it ain't gonna be at 160 at injection. Not to mention that these temp readings are w/o a WVO system installed so you have to figure it will take longer to reach temp because the cool oil is going to remove heat from the water as the oil is heated (heat sink).

Even if the coolant is at 150º, I figure that the engine is at normal operating temperature and the oil is being injected into a hot engine (injecting oil into an engine that is not at operating temp is a HUGE NO-NO. Getting the oil to 160º before injection helps reduce viscosity, reducing strain on the IP and provides a better spray pattern in the combution chamber.

With an automated system (computer controled such as Plantdrive's or Frybrid's) this all may be moot as the automated systems generally include their own temperature sensor for the engine's coolant. Thereby not switching over to oil until it's temp sensor indicates the coolant is at the predetermined temp.

Of course everything is also relative to the ambient temperature. If it's cool or cold outside the engine will be cooler also.

Here is a video on how to do a clean Scangauge install on a TDI

*I have noticed that the time it takes to get to engine's H20 to 190º is directly related to the intensity with which one drives. Drive mellow and it takes awhile and in some istances never makes it to 190 - settling around 185 or so. Drive agressively or get on the freway and it makes it to 190 and higher much quicker. This is important because if you switch over and the H2O never makes it to 190 your oil might not be at 160.


  1. Hey, how does the ScanGauge's ability to monitor MPG work with a dual-tank conversion? I'd be really interested in getting one for various reasons, but I'd want the MPG to be accurate.


  2. No I don't think it would be able to do that