Coolant warmup graph, ALH engine


Veteran Member
Jun 28, 2007
Kirkland, WA
2001 Golf TDI
I thought I'd share a little graph with the TDI community. The data geeks in our midst will appreciate this.

When I did my timing belt last fall, I installed a new thermostat. With 360K on the original, it was just time for a new one.
After doing the belt, I went for a short test drive to get the coolant up to temp so I could set the pump timing correctly.
I noticed that after a short run, the coolant temp was in the 90C range. In the past, that same short run would have only resulted in temps in the 80's or so.
Sure enough, I gained about 2mpg afterwards. Clearly, the old thermostat was opening too early and keeping the coolant too cold, which hurt mileage.

Later, I was using VCDS to log starting rpms. Car seemed to crank a bit too slow.

While doing this, I decided to make a graph of the coolant temp as she warmed up. I thought this might be of use to someone using VCDS to diagnose
a suspected thermostat issue. Comparing their results to those of a new thermostat might be of use.

Test parameters:

2001 Golf TDI. Stock except for EGR delete (which might slightly slow down the warmup time a bit).
Thermostat is genuine VW, marked as 87C (same as the original).
Outside temp 8C or 46F.
Log was started immediately after engine start. I have a 1 mile drive to a stop light, about 2 minutes idling there, then onto the freeway where I
accelerated to 65mph and parked it on cruise control.
Total time was about 20-25 minutes. There are about 4300 samples taken, so about 200 samples per minute. Each hash mark is then is about 1 minute of time.

The yellow line is coolant temp. The other lines are fuel and air intake temp and can be ignored.

Interestingly, the Bentley says the thermostat should begin opening at 87C and be fully open by 105C.

Notice the coolant rises steadily until it hits about 74C. At that point, it kinda looks like the thermostat first cracks open. The temp holds there as all the cold coolant in the
radiator starts getting added to the hotter coolant in the engine. Rises to about 80C, holds there for a bit, then rises to just above 90c as the thermostat
opens more and system reaches a state of equilibrium. The temp then stays fairly steady from there on.

The rate of temp rise would depend on the ambient temp and the load being placed on the engine, but the general shape of the graph would be similar.



Top Post Dawg
Oct 20, 2005
Golf,2005,anthracite blue
For comparison purposes, I drove ~ two miles to the highway and my SGII steadily rises to 194F(90C) while cruising @ 60mph and gradually comes down to 190F and settles @ 188F(86.66C). My Behr, 87C t-stat, is also new. This was from this morning with temperatures in the mid 70's. Too many temperature fluctuations in stop-n-go traffic to know precisely when the t-stat starts to open.