ALH Jetta VERY long to pick up heat, thermostat tested good.

Sometimes the temperature is too low, a 90C thermostat should do well. Make sure it's quality and not a lower temperature thermostat.

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I just checked.

Rein is the brand of what I bought on rockauto. Already used their stuff and seemed great.
I hate to need to buy OEM things... they are way overpriced and most of the time, i consider them not even better... maybe I'll have to make an exception for thermostat!

Good info here:
http://forums.tdiclub.com/showthread.php?t=414545&highlight=thermostat
And here though it gets a little deep:
http://forums.tdiclub.com/showthread.php?t=306799

I tried various aftermarket brands. Some only reached 170ish, same as the old failed original one. Never ponied up for an OEM one, but have had good results with the Wahler brand.

I too have used Wahler with success. Just realize that with a bypass thermostat design (which nearly all diesels, including the TDI are) if the seal is not complete when closed the engine will be very slow to reach temperature when it's cold out.....

I was not aware of different cooling system types. What are they so I can google them... ? I can't find any straight answer to the different types... bypass type and..?

There are two basic designs (with some variations) -- "blocking" and "bypass."

Many gas engines are "blocking". The thermostat regulates flow to the radiator, and the water pump "dead heads" (ex any flow allowed to the heater core) when the thermostat is closed. If you look at this typical gas engine thermostat https://shop.advanceautoparts.com/p...Vs41bCh1LVQtxEAQYAiABEgKvzfD_BwE&gclsrc=aw.ds you'll see it has only one "valve" in it -- at the top.

Most diesel engines are designed as a "bypass" configuration. When the thermostat is closed coolant circulates in the engine but does not go through the radiator. When it is fully open the bypass passage is fully blocked (and the radiator passage is fully open.) There are also some diesel designs that have TWO thermostats (one for each bank in a "V" engine); 2-stroke Detroit "V" engines are typically set up this way.

If you look at this thermostat for the TDI engine https://www.autohausaz.com/pn/13090...MIv-6f9oXd3gIVBI9bCh2WgwLjEAQYBSABEgKK0PD_BwE you will see that it has a bottom plate along with the top coolant passage and two springs. When the thermostat is closed this design forces coolant through the bypass. When the thermostat begins to open the plate closes off the bypass passage and forces the coolant through the radiator. A blocking thermostat does not have the bottom plate, second spring or the bypass passage in the block at all.

In both cases the thermostat is usually partially open in normal operation. That is, SOME coolant goes through the radiator, but not all -- the rest is either bypassed for a diesel or simply not circulated at all in a gas engine. When the thermostat is fully open the system is operating at its maximum heat-rejection capacity and if more rejection of heat is required coolant temperature will rise and, unless the additional requirement is modest and can be met by the increase delta in temperature between ambient air and the coolant temperature, it will overheat.

(There are plenty of exceptions to the rule that gas engines are "blocking"; in particular I know the SkyActiv Mazda engines use a bypass configuration, and I'm sure there are others -- most-likely newer engines -- that do as well.)

the plate on the tstat does nothing in 4cyl TDIs, the circular boss it would 'seal' against is not even drilled through
also, everything has bypass flow, without it you'd boil the coolant in the head(s) and the tstat would never open
further, the only engine I can think of that has radiator-priority water pump plumbing is the gen 1 SBC, where without the restriction of a tstat the engine gets almost no water flow

Oh trust me on this, take the thermostat completely out of most vehicles and they will overheat badly. There are very few designs that flow enough coolant through the radiator without a thermostat in there to control the flow rate. The only reason a TDI might not is that under light load they reject so little heat that the heater core is enough to actually cool them down!

There is SOME bypass designed into single-valve systems (there has to be or as you noted you'd crack the head due to boiling the coolant in it) but it's not very significant -- the point of it is to circulate the hot coolant back to the thermostat so it can do its job.

Genesis said:

The only reason a TDI might not is that under light load they reject so little heat that the heater core is enough to actually cool them down!

Click to expand...

with no tstat in an ALH you get the same flow through the engine and rad as you would with the tstat full open, just no/low bypass flow through heater core, egr cooler, oil cooler and a/t cooler without the restriction of the tstat body

I’ve found that it’s tough to judge the proper functionality of a t-stat by doing the boiling water trick. I only use that method to double check if a thermostat actually functions, period. But OEM thermostats have never done me wrong.

I have bought three mkiv tdi alh's in the last year and bit and all three had heating issues.. One would no get above 70 ish but showed 90 most times on the gauge, another the gauge didn't even move it was so bad.
A new, good thermostat fixed it every time, heat up to 88-91c and maintained it well.