Former VWDieselParts.com Forum

This is the first time that I have personally replaced the coolant and thermostat on my 1991 MK2 NA diesel.
Temps here in Vancouver, WA are in the 90s.

Will replacing an 80C thermostat with an 87C unit cause the engine to run hotter after it has warmed up to operating temp??
I understand that the 87C thermostat will open at a higher temperature, but thought that the operating temp would be about the same.
My gauge reading has gone from the half way point (about vertical) to almost the next white line. Did not top out the gauge.

How can I be sure that it is full of coolant? I have more coolant left over that I expected. I can't get any more in the system.
About 3 years ago, all hoses, the radiator, heater core and pump were replaced. It is "spitting" very nicely into the filler bottle.
I've squeezed the hose and can feel fluid in them.

Comments, please.

Seems like some aftermarket thermostats these days run hotter than their rated termp. I've had to return them before because they ran way too hot for me.
It's worth noting when the electric fan is coming on because that is also a set temp. If that thing is coming on all the time, it's running too hot (or the thermal switch screwed in the radiator is made for a lower temp range. The thermostat and thermal switch should match.

A higher temperature thermostat will make the engine run hotter.

Most of the gauges on my rigs indicate 1/2 to 5/8 on the scale when fully warmed, 3/4 when pulling up long hills.

A MK2 will self-purge air, provided the small hose is not obstructed. The stream should be steady, not spitting/sputtering.

A handheld laser thermometer is handy for cooling system work and much more.

Thanks, TylerDurden.

"Spitting" was just a figure a speech. It is running pretty steady. Good the hear the MK2 will self purge.

At 75F ambient and a run of about 8 miles, the gauge reads about 3/4. After idling in my driveway for about a minute, the fan comes on and runs for a short time.
With my laser thermometer the front of the block read 214F and the head near the radiator hose about 195F.

I'm thinking about re-installing an 80C thermostat. With the 80C, I would almost always see 1/2 gauge deflection. What do you think? Are my temps excessive???

I'd probably put in the 80C t-stat.

This past week has been in the seventies here and my fan hasn't come on, around town or after highway runs.

The temps might not be excessive, but there is half the headroom for hard operation.

Nice if the temp gauge rides near the middle,.. or maybe a little above but not much.
I don't like them running lower than that.

IMO the only thermostats I'd use would be the German units, and even then they'd all get tested by thermometer in boiling water, before installation. Once tested a batch of 10 new-from-the-box brand S---- thermostats for a wholesale customer: 3 were defective!

Have also found much more effective cooling by merely cleaning up any casting flash around the water pump inlet & outlet. Obviously, nothing should be done to alter the pump's actual impeller-to-wall dimensions. Some pumps have cast impellers, while most others are of stamped design. Some folks say that a plate (disk) on the rear of the impellers improves pumping, but I don't recall seeing such a design for our diesels, or if such a unit would even fit inside the mounting flange.

With an engine that combines an iron block, an aluminum head, pistons of a different alloy of aluminum, iron rings & inconel prechambers, it needs to keep fairly close to the working temps the designers & metallurgists specified to attain best fuel utilization for power, economy & lowest emissions, as well as longest operating life span. My personal favorite is to use the 87F stat & as was said, match that up with a similarly-rated thermo-fan switch, but it all has to be working perfectly, including pump flow.