Is my operating temp correct?

My B4's temp gauge hovers right around 180F when warmed up, never above 185 (ambient temps -5 to -10C). The car is new to me and my first VW. Is that temperature reading correct, or is my thermostat stuck open, or is it working but at a lower temperature than OEM?

if your TDI had an open thermostat, you would know it no uncertain terms... your fingers would turn blue, there would be an icicle forming at the end of your nose, and your feet would possibly go numb.

there are alternative temperature thermostats available in the aftermarket, maybe someone thought it was a good idea to install one of those.

however, more likely, the engine coolant temp sensor is getting lazy. the coolant temp sensor is a pretty common failure on ALH powered TDIs. I have no idea if your B4 is among these, but it's not a bad place to start.

It heats up as fast as my gasser BMW and the heat is strong, so between what I've read of the TDI's thermal efficiency and your comment regarding blue fingers... I guess it's not stuck open. I'm a mechanic and know what a stuck t-stat is usually like, but this case was borderline.

It's a 1Z, not ALH. The gauge started at zero, ended up at ~179 after a few minutes of driving, and during a lengthy parkinglot/snow extraction maneuver might've gotten up to ~185, so if it's reading wrong it's at least being plausible.

What should that gauge show normally, assuming everything is working correctly?

oh I don't know about a 1Z, but the ALH is dead center of the gauge at 90 degrees celsius - 195F.

well, in your shoes I'd change out the thermostat anyways. like I said, there is the possibility of someone installing a low temp unit. that way at least you KNOW where you're at.

unless the 1Z has a separate gauge and ECU engine coolant temp sensor as some cars do, in which case you can compare the gauge reading to a VAGCOM live readout. to be honest I don't even know if the ALH uses a single sensor for both gauge and ECU, but I know my saturns have two separate ECTSs.

How are you getting those temp readings? If by a digital gauge connected to the OBD port, then the operating temp isn't too far off. If you are estimating the coolant temp by the cluster gauge and figure it's about 180 you aren't even close. All MklV cars I've owned (TDI and has versions) will creep up to 190 mark when the actual coolant temp is aprox 163-165. So, if you figure yours is running 180 according to the gauge, it's closer to 150.
Time for a new thermostat and coolant temp sensor probably too.
The coolant temp sensor is two part. One goes to the ECU, the other to the "buffered" cluster gauge.
If the 1Z is different from the ALH system, all bets are off. You would probably get some good info in the model specific forum area or a search. No doubt all those bases have been covered many times for the 1Z as they have been for the ALH models.

I'm reading the gauge itself. Looks like this (not my car):

Not familiar with the workings of the B4 gauge accuracy. Only way to know for sure is hook up the VCDS or something like the ScanGauge and see what sort of "real" numbers it generates. I run a ScanGauge in all the VWs. Sort of nice to be able to keep tabs on various things as well as a great mileage and trip computer.
I do like that gauge/display layout. Very nice.

I have a 96 B4, and the temp on the gauge is usually at least 190, or a bit higher than yours. Once it is up to operating temp it stays there. I also like that MPG display. Mine doesn't have that.

Much of what is stated about the ALH cars does not apply to the 1Z. Totally different engine, totally different car.

To the OP, it certainly would not "hurt" to replace the thermostat, but it is not "bad", just maybe not ideal. If it was stuck open, you'd know as the gauge would barely crawl up, would quickly go back down if the engine idled even after being driven a while, and your heat would be pretty poor.

FWIW, most of these cars' thermostats start to be less than ideal after a couple years. But unless they trigger a DTC for insufficient coolant temp, many never get replaced. I suspect a lot of Volkswagens have thermostats that could make an improvement if replaced.

I don't believe the thermostat is stuck open, but it may be a lower-temp one. Assuming that's the case, what are the downsides (other than emissions) to running perhaps 10 degrees cooler? I switched an 85C 'stat for a 95C in my BMW and picked up about 1mpg, but does the TDI system work the same way?

moroza said:

I don't believe the thermostat is stuck open, but it may be a lower-temp one. Assuming that's the case, what are the downsides (other than emissions) to running perhaps 10 degrees cooler? I switched an 85C 'stat for a 95C in my BMW and picked up about 1mpg, but does the TDI system work the same way?

Click to expand...

Diesels lose mpg when run colder, this applies to a TDI.

Yeah, lower mpg, and less than stellar heat production.

Heat works great.

As for MPG, I understand there's an inherent thermodynamic efficiency gain to be had from running as hot as the materials and fluids will allow, but is there also an electronic (ECU-related) reason for economy to be worse on a slightly colder TDI? Gasser EFI richens the fuel mixture below a certain temp, but diesel...? No idea; I have experience with EFI gas and 1-wire diesel, not much with EFI diesel.

Timing is changed with coolant and fuel temps, too. Always best to run at operating temp.

I have no faith in the dash gauge when it comes to water temperature. As a matter of fact, when people ask me what gauges they need in their vehicle, I say the first one you need is an operational temperature gauge that tells the TRUTH! An idiot light, I understand. But an idiot GAUGE??

I suggest those who have either a scangauge or VCDS strap the thing on and make a little test. Drive the car cold to warm and watch the water temperature reading and the VCDS/ Scangauge reading. On most of the dash gauges, beginning in 1996 to present day, the DASH GAUGE reads 190f (90c) when the temperature sensor reads 165f(72c). If you get it hot, the dash gauge will not move above 190 until the actual temperature is around 210f (100c).

I find this very disturbing, when many people with warped cylinder heads tell me, "It never overheated", when in truth, it was ALWAYS overheating!

Likewise, although between the two, I'd rather have a thermostat stuck open, when the temperature is as cool as 180, that is detrimental to fuel economy.

If you don't have a scan gauge or VCDS, accurately measuring water temperature is as simple as dropping a thermometer that will read boiling into the overflow tank. An engine that has just come in from a highway run should be between 185 and 195f. A little more or less is ok, but anything under 180 or above 205, I consider a problem.

Today was my first day DDing this car. Just below 180 except in traffic-like maneuvers or after a multi-gear hard pull, then more like 185. The gauge is labeled, and moves up and down in plausible fashion. It may not be as accurate as VAGCOM (which I have borrowing rights to from my previous employer) but it's a lot better than any other dash gauge I've seen.

Frank just told you what to do.....

Thermometer in the overflow tank, then you will know what you need to know

Hi guys, new to the forum and I've got a similar concern, my 01 has consistently reading less than 90 C on the temp gauge, I've got it hooked up to a scan gauge which confirms regular operating temp between 68-75, might hit 80 on a hot day after sitting in traffic.

no noticeable loss in MPG, although I've only had this car for about a month and its my first ALH... any suggestions?

I would change the thermostat and give the coolant a rinse and refresh