1996 VW Jetta 2.0 is OVERCOOLING! Help!

My Jetta runs great in the summer and all seasons around town… the temp stays around 195 or so, the heater works ok, etc. When I get on the interstate however (now that the temp has dropped) , the story changes. The temp gauge falls below 150 and the heater gets cold. I assumed the thermostat was stuck open, so I replaced it but it didn’t solve the problem. I called a local dealer and on his recommendation, I replaced the Engine Coolant Temperature (ECT) Sensor. That also didn’t solve the problem. I have had cars overheat in the past, but never one that overcooled! Does anyone have any insight into why my car cools too well at highway speeds? It affects my gas mileage and makes for an uncomfortable ride on cold days!

Did you get a thermostat from the VW dealer or an aftermarket one?
Some of the aftermarket ones are crap.
Also make sure the right gaskets are used.

I can’t imagine how else an engine would lose too much heat.

oem from dealer!

Also, the one I took off tested ok once I got it into the house.

OK, first lets try to see if the temp gauge in your vehicle is actually telling you the truth… You can go to Harbor Freight or similar Chineseum emporium for auto parts/tools and the like… Get yourself a digital Infra Red thermometer… They run about $20 or less at Harbor Freight for a cheapie…they work well enough for this test. Then use it on your engine, you can aim it at an object and push the button and it will tell you via infra red the temp of the item…some even have a laser to guide you and tell you what you have aimed at.

You will be shooting the cylinder head, your Rad hoses, the engine block, your rad itself…(just the water carrying tubes), take temp readings all over but the ones listed are the most important. Tell us what you get…Does it jibe with the temp gauge in the car? I am thinking that it does not… There are a few reasons why this may happen. There is the gauge itself…obviously…the ECT was a good thing to swap out… and since its new I am placing my bets on the actual gauge in the car.

What temp is your thermostat in the car? How old is it? Is it a “safety thermostat”? Which are total crap…and if so…is it stuck in the open position?

HAVE YOU ALSO…BURPED your cooling system…the radiator etc… I dont want you to have ANY air bubbles or pockets in the system…they are extremely dangerous to your engine and need to be removed…You might even have a bleed nipple on the thermostat housing or near it…I do on my Honda Accord…or did rather. Make SURE you have no air pockets in there…VERY IMPORTANT’

Unless you installed a radiator from a Mack truck, this issue really doesn’t happen much…in the real world so methinks something is awry. So you have some homework to do…Let us know.


2 time-tested rules apply! Murphy’s Law and the KISS principle. It was quite obviously the thermostat, but that was ruled out when it was replaced with a new one… except -

The new thermostat was bad out-of-the box! Replaced with a NEW new one and all is fine!

I was re-reading your post and asked myself again why it wasnt the T-stat… ? It was the first thing most of us prob thought of. Good job though congrats…these things happen sometimes

Low coolant level could cause the no heat and if the level is below the level of the temp sensor it could cause a low reading.


Turn up your hearing aid. Two posts before, he found the problem was a bad replacement thermostat!

One question that has not been asked…where do you live? In sub zero climates (like the upper midwest has been this week) the engine can be very hard to keep up to temp on th ehighway. When I lived in North Dakota we used to put cardboard or hang blankets in front of our radiators.

Mountain bike has the answer…Sometimes, in severe cold, just the heater core can overcool a small engine. With the thermostat closed most of the time, the water in the radiator gets COLD! If and when the thermostat opens, this cold water is allowed into the engine cooling it below normal…That’s why big trucks have damper doors in front of their radiators to block off airflow in cold weather…

So yes, a sheet of cardboard in front of the radiator. Experiment to get the size right…

I’m with mountainbike here . . . . if it’s REALLY cold where you live, it will be hard for the engine to get or stay up to temp or the highway. Put your hand out the window and drive at 15 degrees for a few miles, you’ll see how cold it really is! The cardboard in front of the radiator is something I’ve done in subzero temps, but watch the temp indicator, you could actually overheat the car if you forget about it. Running too cold all of the time is bad for the engine . . . overheating the engine can be fatal. You’ll have to experiment with the size of the cardboard and the outside temp but it sounds like you’re a motorhead anyway, so it shouldn’t be a problem. Good luck! Rocketman

I suspect the OP is in Alabama (my Daddy’s home state); not that cold.